development of precipitation retrievals at millimeter and sub

Development of precipitation retrievals at millimeter and

sub-millimeter wavelengths for geostationary satellites

E. Defer,1 C. Prigent,1 F. Aires,2 J. R. Pardo,3 C. J. Walden,4 O.-Z. Zanife,5

J.-P. Chaboureau,6 and J.-P. Pinty6

Received 19 March 2007; revised 19 September 2007; accepted 14 December 2007; published 23 April 2008.

[1] We study the potential of millimeter and sub-millimeter wavelengths for precipitationretrieval from geostationary sensors based on mesoscale cloud modeling and radiativetransfer computation. Hydrometeor profiles simulated with the Meso-NH cloud resolvingmodel for five European midlatitude situations are used to compute the brightnesstemperatures at frequencies from 23.8 to 875 GHz with the Atmospheric Transmission atMicrowaves (ATM). Performances of both rain detection and rain rate retrieval areanalyzed for different frequency sets, over ocean and land separately, and compared to theuser requirements. The performances of a frequency set such as that already plannedfor geostationary satellites (with channels in the O2 lines at 50, 118, and 424 GHz, and inthe H2O lines at 183, 325, 380 GHz) satisfy the requirements for Numerical WeatherPrediction and NoWCasting in terms of rain detection as well as for rain rate retrievalabove 1 mm/h. Suppressing the 50 GHz O2 channels does not seriously degrade theperformances, except for rain rate below 1 mm/h, and, in addition, limits thespatial resolution problem from a geostationary orbit. Adding the thermal infraredobservations has a limited impact. The retrieval of other hydrometeor quantities (cloud,ice) is also tested as well as the possibility to retrieve rain and the other hydrometeorprofiles. These theoretical results are evaluated at close-to-millimeter wavelengths withcoincident AMSU-B and radar observations (BALTEX and CAMRa). The results aredegraded with respect to the theory, as expected, but are consistent with the observations.

Citation: Defer, E., C. Prigent, F. Aires, J. R. Pardo, C. J. Walden, O.-Z. Zanife, J.-P. Chaboureau, and J.-P. Pinty (2008),

Development of precipitation retrievals at millimeter and sub-millimeter wavelengths for geostationary satellites, J. Geophys. Res.,

113, D08111, doi:10.1029/2007JD008673.

1. Introduction

[2] Precipitation is characterized by a high variability inspace and time and, as a consequence, both high spatialresolution and temporal sampling are required to obtain arealistic estimate of the rainfall intensity and distribution ata global scale. Rain gauge measurements are relativelyscarce and are not well distributed over the globe, especiallyover oceans. Increasing national and international radar

networks provide high temporal sampling of rain ratesessentially over land but the spatial coverage in terms ofglobal precipitation estimates is still poor. Space-borneremote sensing of precipitation is a solution, provided thatboth high spatial resolution and very rapid revisiting timecan be achieved. The needs for precipitation observationswere recently assessed by a EUMETSAT expert group[Rizzi et al., 2006] and are presented in Table 1 (anotherdocument [Golding et al., 2001] specifies the user require-ments for nowcasting and very short range forecasting but itis less detailed in terms of precipitation and the require-ments are all less stringent.) They cover meteorologicalapplications, both Numerical Weather Prediction (NWP)and NoWCasting (NWC), as well as hydrology and climatefor 2020 onwards. Currently, precipitation observations arenot systematically assimilated in NWP systems and thereport underlines that precipitation information will becomeincreasingly important to improve forecast in NWP and inNWC applications related to severe weather events.[3] Contrarily to infrared and visible observations, mi-

crowave measurements show a more direct relation withprecipitation, especially at frequencies below 30 GHz. Thesuccess of the Tropical Rainfall Measuring Mission(TRMM), equipped with both precipitation radar at

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1Laboratoire d’Etudes du Rayonnement et de la Matiere en Astro-physique, Centre National de la Recherche Scientifique, Observatoire deParis, Paris, France.

2Laboratoire de Meteorologie Dynamique, Institut Pierre-Simon La-place/Centre National de la Recherche Scientifique, Universite de Paris VI/Jussieu, Paris, France.

3Instituto de Estructura de la Materia - Departamento de AstrofisicaMolecular e Infrarroja, CSIC, Madrid, Spain.

4Space Science and Technology Department, Rutherford AppletonLaboratory, Chilton, Didcot, UK.

5Collecte Localisation Satellite, Ramonville Saint-Agne, Toulouse,France.

6Laboratoire d’Aerologie, Universite de Toulouse, Centre National de laRecherche Scientifique, Toulouse, France.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JD008673$09.00

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http://dx.doi.org/10.1029/2007JD008673

13 GHz and passive microwave radiometer between 10 and85 GHz demonstrates the ability of this frequency range forprecipitation estimation [e.g., Olson et al., 2006; Yang et al.,2006]. However, from low orbits the revisiting time is poorand even a constellation of low orbit satellites such as theGlobal Precipitation Measurement (GPM) project providesa limited number of overpasses of an area per day (8 over-passes per day in the GPM project) [Mugnai, 2003].Geostationary satellites offer the advantage of a quasi-continuous observation of a large portion of the globe thatis adequate for nowcasting or short range forecasting ofsevere events and for climatological analysis of the fulldiurnal cycle of precipitation patterns. The major limitationwith passive microwave instruments on board geostationarysatellites is the difficulty in reaching acceptable spatialresolution with reasonable antenna sizes. The use of higherfrequency makes it possible to reduce the antenna aperturefor a given size. The NASA-NOAA Geosynchronous Mi-crowave Sounder Working Group developed the project of asub-millimeter wavelength GEosynchronous Microwavesounder and imager (GEM) equipped with a 2 m scanningantenna, with channels in the oxygen bands (54, 118, and425 GHz) and in the water vapor lines (183 and 340/380 GHz) [Staelin et al., 1998]. The Geostationary Obser-vatory for Microwave Atmospheric Sounding (GOMAS)project was also proposed recently to ESA as a Next EarthExplorer core mission with similar channels but a 3-mfilled-aperture antenna, providing a spatial resolution of37 km at 118 GHz and 12 km at 380 GHz [Bizzari et al.,2005]. The Geostationary Synthetic Thinned ApertureRadiometer (GeoSTAR) on the upcoming GOES-R mission[Lainbrigtsen et al., 2004] will use an aperture synthesisapproach to sound the atmosphere at 50 and 183 GHz and toprovide data products such as rain rate with a 25-km spatialresolution. Both GOMAS and GEM satellite projects rec-ognize the importance of millimeter and sub-millimeterwavelength observations for cloud and precipitation char-acterization from geosynchronous orbits. However, therehave been very few studies dedicated to precipitationretrieval in this wavelength range. It is the goal of thisstudy to analyze the potential of millimeter and sub-milli-meter observations for precipitation detection and retrieval.

[4] Meteorological satellites today, in low or high orbits,do not provide observations at frequencies above 190 GHz.Although aircraft measurement campaigns have been con-ducted at millimeter frequencies above ice clouds, precip-itating cases were not specifically explored [Evans et al.,2005]. As a consequence, the feasibility of rain retrieval atmillimeter wavelengths can only be tested from radiativetransfer simulations. Measurements in the millimeter wave-length range are not expected to directly sense the rainprofile close to the ground. At these frequencies, cloudopacity in the precipitating areas will drastically limit theamount of upwelling radiation emitted or scattered by rain.Potential correlations between precipitation and millimeterwavelength observations are expected to arise from indirectrelationships between the cloud microphysics and the pre-cipitation down below. It is thus very important that thesimulated atmospheric and hydrometeor profiles reproduceas accurately as possible the whole atmospheric column inthe precipitating situations. The potential of the millimeterwavelength observations for precipitation retrieval will beanalyzed using statistical methods. The quality of thestatistical data set is essential: The variability described inthe data set should represent as correctly as possible thedifferent regimes of precipitation in the region of interest.This study focuses on European meteorological situations,with special attention to the user requirements specified byEUMETSAT. The simulations are generated from realisticmesoscale atmospheric outputs representative of Europeansituations, from stratiform to convective precipitations. Themesoscale cloud model outputs are coupled to an up-to-dateradiative transfer model to provide a realistic simulated dataset, generated for a large range of frequencies, from micro-wave to sub-millimeter wavelengths. It complements theprevious work by Mech et al. [2007], with special emphasison the rain detection, development and comparison ofdifferent retrieval schemes for integrated quantities as wellas profiles, and comparisons between radar rain estimatesand satellite retrievals from close-to-millimeter observa-tions. The methodology is described in section 2. Theresults of the precipitation detection and rain rate retrievalare presented in section 3 for the various frequency sets,along with results about cloud characterization and rainprofiling. Systematic comparisons with IR simulations are

Table 1. User Requirement Table Related to Liquid Precipitation in Terms of Accuracy, Based on the EUMETSAT Position Paper [Rizzi

et al., 2006]

Parameter Application

Accuracy, RMS

Unit Threshold Breakthrough Objective

Precipitation profile (liquid) NWP global % 100 50 20NWP regional % 100 50 20

Precipitation rate at surface (liquid) NWP global % 100 50 20NWP regional % 100 50 20climate mm/h 10 5 2hydrology (>10 mm/h) % 20 10 5hydrology (1–10 mm/h) % 40 20 10hydrology (<1 mm/h) % 80 40 20

Precipitation detection (liquid) NWP global HR/FAR 50/50 95/10 99/2NWP regional HR/FAR 50/50 95/10 99/2NWC HR/FAR 50/50 70/40 85/20

Precipitation Type NWP global classes 3 4 6NWP regional, NWC classes 3 4 6climate classes 3 4 6

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also performed to assess how the two wavelength rangescomplement one another. In section 4, the rain detection andretrieval algorithms are tested at close-to-millimeter wave-lengths, using collocated AMSU-B and radar observationsover the south of England (the Chilbolton CAMRa radar)and the Baltic region (BALTEX radars).

2. The Methodology

[5] The present article synthesizes the results of the laststage of a three-step study. The first step consists inperforming realistic cloud simulations with a cloud model[Chaboureau et al., 2008]. The second step described byMeirold-Mautner et al. [2007] deals with the computation ofbrightness temperatures at different frequencies with thehelp of a radiative transfer model. Meirold-Mautner et al.[2007] present results of a detailed investigation of theimportance of the representation of the microphysics forthe radiative transfer calculations. Meirold-Mautner et al.[2007] also show that the simulated brightness temperaturesas computed from the outputs of the cloud model showconsistent behaviors compared to concurrent observations atfrequencies below 190 GHz. The present article assesses theability of different frequency sets to detect rain and quantifythe rain rate from the merging of the microphysics data setand the associated brightness temperature database. In thefollowing section, we describe the microphysics/brightnesstemperature database and both classification and retrievalmethods while results are discussed in the next section.

2.1. The Database of Atmospheric Profiles andthe Associated Tbs

2.1.1. Mesoscale Cloud Simulations[6] In order to study midlatitude cases, a database of five

European meteorological situations are selected that repre-sent a large variety not only in rain rate distribution but also interms of precipitation nature: Cyclonic systems, stratiformcases, as well as the millenium storm in the UK with veryhigh rain rates. The cloud-resolving model Meso-NH is usedto provide a detailed description of the atmosphere includingthe hydrometeor distribution of five hydrometeor types (rain,cloud, ice, graupel, and snow) [Lafore et al., 1998].[7] Chaboureau et al. [2008] describe the setup of the

cloud simulations as well as the different processes imple-mented in the cloud model, detail cloud and precipitationvariability of the cloud simulations and show that themidlatitude database is complementary to the GoddardPROFiling (GPROF) database [Kummerow et al., 2001;Olson et al., 2006]. (The GPROF database, that is used todevelop the retrieval algorithms for TRMM, focuses onTropical situations, with outputs from 7 cases [Kummerowet al., 2001].) The cloud simulations were performed with atwo-way grid nesting technique [Stein et al., 2000] with a50-layer configuration and with a depth of the vertical gridranging from 60 to 600 m. Each meteorological situationcovers roughly a region of 1600 km � 1600 km and foreach situation two time steps representative of the evolutionof the situations are available.[8] The cloud simulations are performed with an explicit

microphysics scheme while hydrometeor size distributionsfor the 5 species are assumed to follow a generalized g-law asdetailed by Chaboureau et al. [2008]. Different studies have

been carried out to assess the Meso-NH cloud model schemesuch as comparisons with METEOSAT [Chaboureau et al.,2000, 2002], TRMM Microwave Imager [Wiedner et al.,2004], SSMI and AMSU [Meirold-Mautner et al., 2007] orradar [Richard et al., 2003] observations. Those studies atvarious wavelengths inmany investigations show that neitherstrong nor systematic deficiencies in the microphysicalscheme on the prediction of the precipitating hydrometeorcontents are present. Recently, a model intercomparisonexercise has further shown that Meso-NH as well as otherstate-of-the-art non-hydrostatic models perform better inforecasting rain than operational models [Richard et al.,2007]. The large spread in precipitation found between thenon-hydrostatic models recalls however the uncertainty stillinherent in cloud microphysics schemes.2.1.2. The Radiative Transfer Code[9] Simulations from the cloud model are used as inputs

to the radiative transfer model Atmospheric Transmission atMicrowaves (ATM) developed by Pardo et al. [2001]. Thismodel includes an up-to-date gaseous absorption schemevalidated up to frequencies of 1.6 THz [Pardo et al., 2005]and radiation scattering by oblate or prolate spheroids iscalculated with T-matrix algorithms [Mishchenko et al.,2000]. The emissivity of the wind-roughened ocean surfaceis calculated using Guillou et al. [1996]. The land surfaceemissivity is estimated from atlases derived from SSM/Iobservations [Prigent et al., 2008]. Multiple scattering cal-culations are based on the adding-doubling method andoriginate from Evans and Stephens [1995]. Interpretation ofthe Meso-NH outputs in terms of radiative transfer propertiesis not always straightforward and sources of uncertaintiesinclude the estimation of particle density (consequently therefractive index) and shape.2.1.3. Sensitivity of the Radiative Transfer Simulationsto the Hydrometeor Characteristics[10] Meirold-Mautner et al. [2007] study the sensitivity of

the microwave simulations to the hydrometeor propertiesfor the present database and discuss the importance of acorrect representation of the snow in midlatitude clouds in aradiative point of view. Meirold-Mautner et al. [2007] alsodetail the potential sources of uncertainty associated withthe microphysical scheme and the different properties of theparticles (density, refractive index, shape).[11] The radiative computation performed in the present

study uses the cloud content provided by the cloud modelon a 10-km grid. It is assumed that the microphysics isspatially uniformly distributed over each grid point. In orderto sense the impact of the non-uniform beam filling effecton the retrieval performances further studies should beperformed at higher cloud resolution convoluted with thefootprints at the specified frequencies.2.1.4. Evaluation of Meso-NH / ATM CombinationWith Satellite Observations up to 190 GHz[12] Since the quality of statistical retrieval algorithms is

directly related to the realism of the simulated database, it iscrucial to evaluate the quality of the simulations. Given thatthe Meso-NH calculations correspond to real atmosphericsituations, comparisons have been performed betweenMeso-NH / ATM simulations and satellite observations atfrequencies up to 190 GHz using the Special SensorMicrowave /Imager (between 19 and 85 GHz) and AMSU


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(between 22 and 190 GHz) [Meirold-Mautner et al., 2007].Such a comparison for a large range of frequencies is verychallenging: Each frequency is sensitive to a large set ofatmospheric and surface parameters, imposing very strongconstraints on the quality of both the Meso-NH simulationsand the radiative transfer calculations. SSM/I frequenciesare more sensitive to the lower part of the atmosphericcolumn, including the surface contribution. The AMSU-Bwindow channel at 150 GHz is particularly responsive toscattering by frozen particles. In addition, IR responses havebeen simulated with RTTOV87 [Saunders et al., 2005] andcompared to the METEOSAT images [Chaboureau et al.,2008]: This helps in distinguishing the errors of the radia-tive transfer model from those of the cloud model becausethe radiative transfer model used for the IR has beendeveloped independently from the one used at microwaves.The comparisons show an overall good agreement betweenthe simulations and the observations but first revealed anunderestimation of the scattering by frozen particles atmicrowaves above 80 GHz (especially at 150 GHz). Acareful analysis made it possible to overcome the discrep-ancy, by defining more accurately the scattering propertiesof snow in the cloud [Meirold-Mautner et al., 2007].2.1.5. Frequency Selection[13] Satellite projects in the millimeter and sub-millimeter

wavelengths such as GEM and GOMAS on geostationaryorbits or Cloud Ice Water Sub-millimeter Imaging Radiom-eter (CIWSIR) on a polar orbit [Buehler et al., 2005]propose frequency channels that have similarities but arenot identical, mainly because their objectives are not exactlythe same: The first two (GEMS and GOMAS) beingoriginally focused on water vapor and temperature profilingand more recently on precipitation, and the last one (CIW-SIR) on cirrus characterization. The relationships betweenthe frequency responses and the hydrometeor profiles arecomplex, non-linear, and regime dependent. In the presentstudy simulations are performed at a large number offrequencies, in window channels as well as in water vaporand oxygen lines (Table 2), covering the frequency rangefrom 20 GHz to 900 GHz. Channels around the 183, 325,and 380 GHz water vapor lines can provide water vaporprofile information, whereas the channels around 50, 118,and 424 GHz in the O2 lines give access to the atmospherictemperature profile [Klein and Gasiewski, 2000; Prigent etal., 2006]. In addition to their water vapor and temperatureprofiling capabilities, these sounding channels are expectedto constrain the hydrometeor profiles [Bauer and Mugnai,2003]. A total of 37 frequencies, including the AMSU and

the 11-mm thermal IR channels, have been simulated for allthe profiles in the database (Table 2). Simulations at SSMIfrequencies were also performed [Meirold-Mautner et al.,2007] but are not used in the present study. Various channelcombinations are tested and compared for their potential todetect rain and estimate the rain rate. Table 3 lists thefrequency sets investigated in this study. The F07 set is ofspecial importance in this study as it is close to thefrequencies suggested for the GEOstationary projectGOMAS. The F09 is close to the channel selection forCIWSIR in Low Earth Orbit.2.1.6. The Resulting Database[14] Simulations for each frequency are performed for 9

different incidence angles (0, 9, 19, 30, 40, 52, 63, 74, and85�). The two orthogonal linear polarizations are calculatedseparately and are combined as in the AMSU cross-trackscanner [Saunders, 1993]. From a geostationary orbit, thepolarization combination will also depend upon the scan-ning angle and the areas in satellite coverage. In this study,polarization emanates essentially from the surface as thehydrometeors are assumed to be spherical. Scattering-induced polarization can be generated by oriented non-spherical hydrometeors [Prigent et al., 2005] but giventhe lack of information concerning the particle shapes andorientations, this effect is not considered here. The initialatmospheric profile database consists of a pixel populationof 48% over land, 40% over ocean and 12% over the coasts.Note that in this work, coastal pixels are ignored. The entiredatabase consists of 10 situations (2 outputs from thesimulation of 5 meteorological situations) with 157(6 parameters (rain, cloud, ice, graupel, snow and watervapor) � 25 layers + 6 integrated quantities (rain, cloud, ice,graupel, snow and water vapor) + rain rate) cloud and rainparameters and 325 (9 angles � 36 microwave channels + 1IR channel) brightness temperatures for each profile.[15] For a careful comparison of the retrieval potential

over land and ocean and to double the database, twodifferent simulations are performed for each frequencyand atmospheric profile, one with the surface emissivitycorresponding to the exact location, and the other one withan emissivity corresponding to the opposite surface type(land becoming ocean and vice versa). In the secondsimulation, the realism is maintained by using the sameMeso-NH surface temperature, the Meso-NH surface windspeed (in case of ocean calculation) and a land emissivityestimate in a nearby location at the same latitude for theland cases. Figure 1 presents the distribution of the rain rate

Table 2. List of the Simulated Frequencies and Associated Noise Level

Frequencies (GHz) or Wavelengths (mm) [noise (in K)] Characteristics

23.8 [0.2], 31.4 [0.27] low frequency windows50.3 [0.22], 52.8 [0.15], 53.596 ± 0.115 [0.15], 54.40 [0.13], 54.94 [0.14], 55.50 [0.14] O2 line 50 GHz89.0 [0.37], 150.0 [0.84] medium frequency windows183.31 ± 1 [1.06], 3 [0.7], 7 [0.6] H2O line 183 GHz220.5 [1.0], 301.0 [1.0], 462.5 [1.0], 684.0 [1.0], 875.0 [1.0] high frequency windows118.75 ± 0.2 [0.5], 0.4 [0.5], 0.7 [0.5], 1.1 [0.4], 1.5 [0.4], 2 [0.3], 3 [0.2] O2 line 118 GHz325.15 ± 0.5 [1.0], 2 [1.0], 7 [0.9], 25 [0.9] H2O line 325 GHz380.19 ± 0.5 [0.5] H2O line 380 GHz424.76 ± 0.3 [0.5], 0.6 [0.5], 1 [0.5], 1.5 [0.4], 2 [0.4], 4 [1] O2 line 424 GHz11 thermal IR


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for both sea and land pixels, both pixel categories exhibitingnow the same rain rate distribution.[16] The first tests showed that the strong rain rates were

not properly retrieved because they were under-representedin the database. To solve this problem, the histogram of therain rate (Figure 1) was modified to be uniform. For eachrain rate (RR) bin of the histogram, randomly selectedpixels are duplicated so that the number of samples in eachbin becomes identical. Such an operation is a way to forcethe different retrieval algorithms to be equally sensitive tothe entire RR range. In both rain detection and retrievalalgorithms, a threshold of 0.1 mm/h is set on the rain rate:Below this value, the pixel is rain-free.

2.2. Classification and Retrieval Methods

[17] Figure 2 presents the flowcharts of the methodology,that is to build the statistical relationships between rain rateand brightness temperatures (Figure 2a) and then to apply itto observations (Figure 2b). The idea is to design first a raindetection tool and then algorithms to retrieve the rain rateand the hydrometeor distribution along the vertical profile.The rain detection tool, a two-class classification with a0.1 mm/h threshold, identifies the rainy pixels based ontheir brightness temperatures. The retrieval algorithms aredeveloped for rainy pixels and their associated brightnesstemperatures. As a consequence, the inversion problem isbetter constrained: The variability of the brightness temper-atures for rainy pixels only is limited as compared to the fullextent of their values observable under clear, cloudy andrainy conditions. Two statistical algorithms are tested: Amultilinear (ML) regression and a neural network inversion.The problem being complex, non-linear, with a largenumber of degrees of freedom, the neural network (NN)technique is a natural candidate. Although this work con-centrates on precipitation, the objective is not only toretrieve the rain rate but also the full hydrometeor column,including the ice phase.[18] Situations over land and ocean are treated separately:

The surface contributions to the brightness temperatures inthe window channels below 150 GHz can be significant and

the development of algorithms dedicated to each surfacetype usually improves the retrieval accuracy. For frequencysets limited to the millimeter and sub-millimeter ranges, thesensitivity to the surface is reduced and the same algorithmscould be used, but with restrictions for very dry atmosphereand/or regions with high elevations. The full scheme,including the classification step, is derived for each fre-quency set (Table 3), each incidence angle, and each surfacetype, and their performances are compared, for both raindetection and hydrometeor quantification. For the fivemeteorological situations considered in this study, twomodel outputs are available [Chaboureau et al., 2008]:The first one is used for the training of the algorithms andthe second one is used for testing the algorithms.2.2.1. Rain Classification[19] The development of a precipitation flag for a given

observation is often the first step in a precipitation retrieval.Instead of applying thresholds on selected channels, wesuggest the use of a supervised classification method, basedon the data set of simulated brightness temperatures andatmospheric profiles used in this work. It allows for thecombination of all investigated channels at once to benefitfrom their synergy, contrarily to threshold methods [e.g.,Staelin and Chen, 2000; Chen and Staelin, 2003] that needto discriminate channel by channel. In addition, classifica-

Figure 1. Histogram of the rain rate in the trainingdatabase for both sea and land pixels.

Table 3. Frequency Combinations Investigated in the Present Study

FrequencySet

LowFrequencyWindow O2 50

MediumFrequencyWindow O2 118 H2O 183

HighFrequencyWindow

H2O325 + 380 O2 424 Mission

F01 X + 89 X AMSU-AF02 X X X X AMSU-ABF03 X X AMSU-BF04 X 89 & 150F05 X X XF06 XF07 X X X X X GEO likeF08 X X X X HF-GEO likeF09 X X X LEO likeF10 X X X X LEO like + 118F11 XF12 X XF13 X X XF14 X XF15 X X XF16 X X XF17 X X XF18 XF19 X X


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tions are not limited to the binary precipitation / non-precipitation flag: Distinction between different precipitationregimes can be derived in order to develop a regime-dependent retrieval scheme afterward. Both two-class andmulticlass classifications have been tested but the simplebinary flag has finally been adopted.[20] The rain classification is based on a linear method, a

standard statistical discriminant analysis tool designed toperform supervised classification [Duda and Hart, 1973]. Itreproduces the classification observed in an a priori classi-fied data set of samples. The linear classification estimatesthe decision boundaries (hyperplanes in the data space) that

are optimal, in the least squares sense, for the data set ofsamples. Contrarily to other discriminant analysis thatworks with one data dimension at a time (e.g., decisiontrees), the linear classification is performed in the multivar-iate data space, which makes it relatively robust to thepresence of noise in the data.[21] To evaluate the classification, the Hit Rate (HR) (also

called the Probability Of Detection POD) and the FalseAlarm Ratio (FAR) are calculated. Table 4 presents thecontingency table as used here.

HR ¼ A

Aþ Cð1Þ

FAR ¼ B

Aþ Bð2Þ

2.2.2. The Rain Retrieval Algorithms[22] The database has been carefully analyzed to design

an adequate pre-processing step. A Principal ComponentAnalysis (PCA) of the atmospheric profiles is performed inorder to compress the information, to reduce its dimension(from the 157 initial variables in each profile) and toregularize the inversion process, which is important inparticular for the retrieval of the atmospheric profiles.PCA is a powerful tool to extract uncorrelated pieces ofinformation. Fifty PCA components are considered, repre-senting 99% of the variance of the training database.[23] The database of the selected PCA components and

their associated brightness temperatures is then used to traintwo statistical (Multi-Linear (ML) and Neural Network(NN)) algorithms (Figure 2a). The standard ML method isa particular case of the NN regression: It is compared hereto illustrate the advantages brought by the non-linearscheme (e.g., better treatment of the saturation effects andthe interactions between the satellite observations). The NNmodel that is used is a Multi-Layer Perceptron [Rumelhartet al., 1986] trained by the back-propagation gradientdescent algorithm.2.2.3. Evaluation of the Retrieval Schemes[24] The next step consists of testing the algorithms and

determining their performances. The testing database (i.e.,the portion of the database that has not been used for thetraining) is used to evaluate the inversion. For a given set ofbrightness temperatures, the corresponding algorithm isapplied to estimate the hydrometeor quantities in the PCAbasis, which are then projected into the initial atmosphericdomain by applying the inverse PCA function. For physicalparameters showing a large variability such as the rain rate,the sensitivity of the results to the error metric is large andrequires a clear definition. In order to quantify the errorbetween the true value xi and the retrieved value ri for eachgeophysical parameter, we compute the mean absoluterelative error em as defined in equation (3). A second metricto measure the error is used for comparison with thethresholds discussed in the EUMETSAT position paper(and listed in Table 1). The metric erms (equation (4))corresponds to the RMS of the relative error and includesboth the random component and the bias error: This is the

Figure 2. Flowcharts of (a) the building of the relation-ships and (b) the retrieval methodology.


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accuracy criterion usually used in operational meteorolog-ical applications. Both em and erms are computed per RRrange in order to assess the changes in the retrieval perform-ances as a function of the rain rate.

em ¼ 1

N

XNi¼1

ri � xij jxi

ð3Þ

erms ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N

XNi¼1

ri � xi

xi

� �2

vuut ð4Þ

To summarize, for each frequency set, each observingincidence angle and each surface type (land and ocean), arain detection scheme is developed, along with twostatistical retrieval schemes (multilinear and neural networkalgorithms) that estimate simultaneously the integratedcontents of the five studied hydrometeor classes, as wellas their vertical distribution. In the following sections, therain detection and the rain rate retrievals are evaluated andcompared for the various situations. The characterization ofthe derived ice microphysics is briefly discussed as well asthe capability of retrieving rain profiles.[25] Given the large range of tested configurations (fre-

quency combinations, angles, surface types, retrieval meth-ods, retrieved variables) all the results will not besystematically presented. In the following, we try to illus-trate the major conclusions and to provide a syntheticanalysis of these results, focusing on the rain estimates.2.2.4. Impact of the Noise on the Classificationand the Rain Rate Retrieval[26] Realistic instrument noises have been added to the

simulated brightness temperatures. The applied noises fol-low Gaussian laws with standard deviations that dependupon the frequency (Table 2). For AMSU frequencies, thenoise figures are taken from Saunders [1993], while thenoise levels for frequencies related to GOMAS are derivedfrom the required values listed by Bizzari et al. [2002],which are in most cases higher than the expected noisevalues. Three noise configurations have been tested: Onewithout any noise, one with the standard deviation set to thevalues of Table 2, and finally one with twice the noise fromTable 2.

3. Classification Results

[27] First, the rain classification is evaluated, for thedifferent frequency combinations, over land and ocean.Figure 3 presents the results of the classification for twooriginal scenes (a cold front (Figure 3a) and a storm overEngland (Figure 3b)) in the database using AMSU-ABfrequencies (F02) for a fixed incidence angle (19�), withno noise added to the simulations. For both scenes, thespatial structures of the rainy regions are well captured, and

although two different classifications are used above oceanand land, there is no discontinuity at the transition betweenthe two surface types. For the case that is mainly locatedover land (Figure 3a), most of the misclassified pixels aresituated at the edges of the system. For the second casewhere most of the pixels are located over the Atlantic Ocean(Figure 3b), the misclassified pixels are predominantlylocated on the north-eastern part of the scene while nearbythe main front the classification performs well, althoughsome non-rainy pixels on the front of the system have beenmisclassified as rainy pixels.[28] For the 19 different frequency sets, Figure 4 presents

the scores of the classifications for the same incidence angle(19�). Frequency sets presented in Figure 4a are identical tothe ones listed in Table 3. The HR and FAR breakthroughthresholds are indicated, for NWP regional and globalapplications (95% and 10% respectively) and for nowcast-ing (70% and 40%). Results are plotted with overlaid bars inblack for noise-free, in gray for 1-sigma noise and white for2-sigma noise. As expected the classifications are lessperformant (lower HR, higher FAR) when the noiseincreases. For nowcasting applications that are of primaryconcern from a geostationary orbit, the FAR is usually wellbelow the breakthrough threshold and the HR is close to it

Table 4. Definition of the Contingency Table

Rainy Pixels Non-Rainy Pixels

Classified as rainy pixels A BClassified as non-rainy pixels C D

Figure 3. Results of the classification for two scenes usingthe frequency range F02 (AMSU-AB) for 19� incidenceangle. Categories are the same as in Table 2.


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for most combinations without noise added. In the presenceof noise, only the F18 frequency combination (only118 GHz channels) exhibits significant changes in bothHR and FAR while for the other frequency sets thevariations of HR and FAR are relatively limited. For mostfrequency sets one can observe a dissymmetry of HR andFAR above sea and land. The classification at AMSU-Afrequencies (F01) performs better over sea than over landaccording to its HR for the three noise configurations. Thisfrequency combination has the highest HR over sea for thenoise-free configuration and is slightly less performant thanthe GOMAS configuration (F07) when the 2-sigma noiseconfiguration is applied: The low frequency window chan-nels provide the sensitivity to the rain emission over theoceanic cold background (related to the low ocean emissiv-ity). Over land, the HR is higher when higher frequenciesare used due to their sensitivity to scattering effects:Scattering by the hydrometeors at high frequencies isobserved over the high emissivity background. The fre-

quency set corresponding to the high frequency windowchannels alone (F06) exhibits the lowest HR scores of allfrequency ranges for all noise configurations as well as thehighest FAR, for both sea and land pixels. When profilingcapabilities are added (observations in O2 and/or H2O linessuch as in the combination F09), the performances areimproved. The frequency combination planned for geosta-tionary missions (F07) can provide a HR of rain eventslarger than 70% with a FAR of the order of 20%, accordingto the present theoretical work for the three noise levelsconsidered. This is within the breakthrough thresholdsrequested for nowcasting. Even if the lower frequencychannels are suppressed for spatial resolution considerations(F08), the HR remains close to the HR breakthroughthreshold and the FAR stays below the FAR breakthroughthreshold. Suppressing the O2 channels at 424 GHz does notsignificantly affect the results, as seen by the comparisonsof the F08 and F13 sets, but suppressing the O2 118 GHzchannels does (compare F08 and F15). All the sets that donot have frequencies below 150 GHz (F06, F09, F11, F14,F15, F17) exhibit poor scores, even poorer at higher noiselevel, over land and ocean. It appears that the role of thesounding channel in O2 lines (around 50 and/or 118 GHz) issignificant, due to their potential to sound at differentaltitudes in the atmospheric column. However, it is alsodue to the fact that these frequencies are rather lowcompared to the other millimeter and sub-millimeter chan-nels and penetrate deeper into the clouds. Discussions aboutthe respective role of the H2O and O2 sounding channels[e.g., Pinori et al., 2006] should make clear that part of themerit of the O2 sounding channels as compared to the H2Ochannels is due to the fact that the O2 lines at 50 and118 GHz are located at lower frequencies than the H2Olower frequency line for sounding (183 GHz), thus provid-ing a deeper penetration within the cloud structure. Notethat the 118 GHz channels alone (F18) perform almostwithin the thresholds in the absence of noise but exhibits theworst performances when realistic noises are considered.Adding the 183 GHz set to the 118 GHz one (F12) doesimprove the results of the 118 GHz alone. The 118 O2 and325/380 H2O combination (F19) yields acceptable perform-ances, better than the 118 O2 / 183 H2O combination (F12).Adding the high window frequencies also helps the detec-tion slightly (compare F19 and F16). The performanceshave been tested for the other incidence angles and therelative changes are insignificant.[29] Figure 5 presents the initial rain rate distribution as

well as the rain rate distributions of pixels misclassified asnon-rainy for different frequency sets, for sea and landseparately and for the three noise configurations. For bothsurface types, the high rain rates (>5 mm/h) are welldetected regardless of the frequency combination and thenoise configuration suggesting that the detection of intenserain can easily be achieved even at high frequencies, i.e.,with a configuration such as HF-GOMAS (F08). The lowerthe rain rate, the more difficult the detection of the rainespecially for RR < 0.3 mm/h over sea and RR < 2mm/h overland. Over land, rain is essentially detected through thescattering signatures of the ice phase, and, as a consequence,the low rain rates that are associated with stratiform situa-tions with a limited ice phase or without it cannot be easilydetected.

Figure 4. Performances of the classification for the 19frequency sets defined in (a), at 19� incidence angle asdetermined from the Hit Rate (HR, (b)) and the False AlarmRate (FAR, (c)). See text for the definition of HR and FAR.The breakthrough values defined in the EUMETSATposition paper for NWP and NWC are indicated with solidand dashed lines, respectively.


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[30] In conclusion, the use of channels at rather lowfrequencies is necessary to provide a good rain detectionover ocean. The addition of frequency channels below100 GHz improves the results, especially over ocean, butgiven the spatial resolution problem from geostationaryplatforms and the rather limited improvement they provide,it might not be recommended. Observations in high fre-quency window channels only are clearly not enough andsounding channels have to be added. For precipitationdetection purposes, the 424 GHz observations are notnecessary. A frequency set such as planned for GOMASshould provide precipitation detection within the thresholdvalues, and close to them if the channels in the O2 50 GHzlines are suppressed. Frequencies below 150 GHz (such asthe 118 GHz sounding channels) will have to be added to aninstrument like CIWSIR to detect precipitation with asuitable accuracy.

4. Retrieval Results

4.1. Rain Rate

[31] RR retrieval is applied to the rainy pixels. The resultsfrom both ML and NN approaches over sea are plotted inFigures 6 and 7 for eleven different frequency combinationsat 19� incidence angle for the noise-free configuration. Forselected frequency ranges, the initial (from Meso-NH) andretrieved RRs are plotted in terms of the density of initial-

retrieved RR pairs per bin of 0.1 (in logarithmic scale ofmm/h). Comparisons between the ML and NN methodsclearly show that for all frequency combinations the MLscheme undergoes saturation effects at low and/or high rainrates and exhibits larger dispersion at low rain rates: TheNN inversion makes it possible to account for the non-linearrelationships between the rain rate and the brightnesstemperatures and saturation effects are limited. The NNresults (Figure 7) show that the RR retrievals for frequencyranges containing frequencies below 100 GHz (i.e., F01,F02 and F07) present less dispersion. This is also obviouswhen comparing the results for the geostationary projectwith the O2 channels around 50 GHz (F07) and in theirabsence (F08). The 50 GHz frequencies included in the setplanned for geostationary application (F07) as well as inAMSU-A and AMSU-AB improve significantly the resultsof the RR retrieval over sea. Using only window frequen-cies above 200 GHz (F06) does not appear suitable forretrieving accurate RR over the entire RR range but haspotential for the detection of significant rain events. Thedispersion is usually close to symmetry around the diagonalsuggesting the absence of significant biases. Similar resultsare observed over land (not shown) but with a largerdispersion for low rain rates.[32] For each RR bin, the errors erms and em as defined in

section 2 are documented in Figure 8, along with thestandard deviation around the error em (vertical segments),for the NN inversion over ocean at 19� incidence angle forthe noise-free configuration. The errors usually decreasewhen RR increases. Note that erms and em can be signifi-cantly different, explaining in part the differences in theresults obtained from one study to the other when the errordefinitions are not identical. At AMSU-A (F01), AMSU-AB (F02), or at frequencies selected for geostationaryprojects (F07), the errors are within 50% for RR above0.5 mm/h, i.e., are well within the threshold values of 100%requested for NWP application and below the breakthroughvalues of 50% (Table 1). Suppressing the 118 GHz O2

channels (F09) degrades the results below 1 mm/h but notabove. However, all these measurement combinations areclearly not suitable for land surface hydrology applicationsgiven the accuracy specified in Table 1. Strong precipitationevents (RR > 10 mm/h) can be quantified with errors around50% for all frequency configurations. A frequency combi-nation similar to CIWSIR (F09) does not provide therequested accuracy but the addition of the 118 GHz channeldoes improve the results. A simple instrument combiningthe 118 GHz O2 and the 325/380 GHz H2O (F19) sets yieldsperformances close to the specifications for RR > 1 mm/h.The other incidence angles have also been examined.Figure 9 shows the results for the F07 frequency set overocean and land. The results are comparable for all incidenceangles.[33] Figure 10 synthesizes the r.m.s. errors as a function

of rain rate for five selected frequency combinations andcompares the performances over sea and land (results arepresented at 19� incidence angle). The addition of the IRchannel is also examined. Over land, the low frequenciesplay a much weaker role and suppressing the frequenciesbelow 100 GHz from F07 for instance (i.e., set F08) doesnot change the results significantly, contrarily to what isobserved over ocean especially below 1 mm/h. Regardless

Figure 5. Histograms of the mis-classified rainy pixels forsea (a) and land (b) (i.e., pixels classified non-rainy thatwere rainy) for different noise level. The black line indicatesthe initial rain rate for each surface type, and each color linecorresponds to a frequency set.


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of the frequency combination, the retrieval of low rain rate(<0.5 mm/h) over land presents errors larger than 100%.Adding the thermal infrared has almost no impact, exceptfor the configurations that originally suffered from largeerrors. Over land and ocean, the RMS errors with ageostationary type configuration (F07) are below or closeto 50% for rain rates above 1 mm/h, similar to what can beobtained with AMSU-A or AMSU-AB instruments (F01and F02). Between 0.5 and 1 mm/h, it is still true over oceanbut not over land where it can reach the 100% thresholdvalue (Table 1). Below 0.5 mm/h and regardless of thesurface type, the rain rate retrievals are too much uncertainthat they bring no value in a NWP framework.[34] Figure 11 presents the RMS for two frequency sets

(GOMAS and HF-GOMAS) over ocean and land separatelyand for the three noise configurations. Introducing noise inthe treatment induces small variations in the retrievalperformances except for the F08 frequency set (HF-GOMAS) for low rain rates over ocean. Above 1 mm/h theimpact of the noise is very limited: The rain and relatedclouds above induce changes in the brightness temperatures

that are well above the specified instrumental noise levels.Similar sensitivity to the noise has been obtained byMech etal. [2007].

4.2. Integrated Ice

[35] So far we have concentrated on the retrieval of therain rate. Identically, the other hydrometeor quantities canbe retrieved with the same method and we systematicallyretrieved the integrated quantities for all the hydrometeorspecies (cloud water, rain, ice, snow, and graupel). Becausethe methodology was first developed to investigate RRretrieval and starts by a classification of the rainy pixels,the other hydrometeor species are only retrieved for precip-itating cells in this study. Following the same method, wecould derive algorithms for all cloudy pixels based on apreliminary classification of clear and cloudy scenes but thisis beyond the scope of this study. The retrieval of ice andcloud parameters is not our main focus here: The potentialof the millimeter and sub-millimeter ranges for ice charac-terization has already been explored by Evans et al. [2002]or by Buehler et al. [2005, 2007]. We briefly show some

Figure 6. Density of initial-retrieved rain rate pairs per bin of 0.1. The results are presented over sea at19� incidence angle for the multilinear inversion scheme. The density is expressed in % relative to thetotal number of initial samples within a given 0.1 bin.

Figure 7. Same as in Figure 6 except the results are for the neural network inversion scheme.


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results about the total ice content obtained with the set offrequencies considered here for geostationary applications.[36] Similar to Figure 7 (case F07), Figure 12a presents

the results for the integrated total ice retrieval (ice + snow +graupel), for the NN method at 19� incidence angle overocean in the absence of noise. The densities are plotted foreach bin of 0.4 kg/m2. The retrievals are distributed veryclose to the diagonal. The average error for total-iceretrieval is below 40% and for most of the bins close orbelow 20% (Figure 12b). This is within the threshold of50% specified by the user requirements for this quantity andclose to the 20% breakthrough value [Rizzi et al., 2006].The performances we obtain are similar to what is expectedfrom CIWSIR that also contains sounding channels aroundthe H2O lines at 183, 325, and 448 GHz along with windowchannels at 243, 664, and 874 GHz [Buehler et al., 2007].

4.3. Profiling Capability

[37] In addition to the estimation of the integrated hydro-meteor quantities, the various frequency combinations aretested for the retrieval of the hydrometeor profiles. Weconcentrate here on the rain profiling capability.[38] Figure 13 presents the results obtained with the NN

inversion for the rain profile over ocean, for a 19� incidenceangle in the absence of noise. The parameter plotted herecorresponds to the mixing ratio of rain (kg/kg), that is theratio of rainwater mass over dry air mass directly provided

by the cloud model. The RMS error (erms section II) iscomputed per bin of mixing ratio of rain and per level. Thegrey scale follows the error thresholds defined by the userrequirements (Table 1). As already observed for the rainrate, the errors decrease with increasing rain contents.Frequency sets having low frequencies as well as soundingfrequencies (F02 and F07) provide better retrievals for lowrain contents. For a given rain content, the accuracy isusually better at higher altitude than close to the surface, asexpected due to the increased attenuation when soundingcloser to the surface and due to the surface radiationcontribution. Except for the very low rain content, theRMS error is below the breakthrough threshold of 50%and for the sets that include low frequencies (F02 and F07)the RMS error is larger than 100% only for the very lowrain contents. Over land, the results are slightly worse,especially at low altitudes. This profiling capability is partlydue to the correlation between the integrated contents andthe vertical distribution. These vertical distribution shapesare captured by the PCA pre-processing step that facilitatesthe profile retrievals.

Figure 8. Mean absolute relative error em (solid lines) ± the standard deviations (vertical segment) andthe RMS errors erms (dashed line) computed per 0.1 bin for pixels over ocean at 19� incidence angle withthe neural network (NN) method for different frequency sets (see the text for the definition of the errors).

Figure 9. Variations of the RMS error erms as a function ofthe incidence angle for the frequency set F07 with the neuralnetwork (NN) method over ocean (a) and land (b).

Figure 10. Average RMS error erms per rain rate bin fordifferent frequency combinations over sea (a) and land (b)without (solid lines) and with (dashed lines) IR observa-tions. The incidence angle is 19� and the neural network(NN) retrieval is used.


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5. Testing the Method at AMSU-B FrequenciesAgainst Radar Observations

[39] In order to evaluate the rain rate inversion method, itis applied to close-to-millimeter wavelength observations(AMSU-B) and the results are compared to radar measure-ments. Coincident AMSU-B and radar observations havebeen collected during the BALTic sea EXperiment (BAL-TEX) campaign and specific measurements for this studyhave been performed with the CAMRa radar.[40] The two-step method is applied to the AMSU-B

observations (89, 150, 183 ± 1, 183 ± 3, 183 ± 7 GHz).First, the rain classification selects the rainy pixels then theinversion algorithm is applied to these pixels. The AMSUinstrument is a cross-track scanner and for each beamposition the algorithm corresponding to the incident angleis adopted. In the previous analysis, it was shown that thefrequency combination corresponding to AMSU-B is notoptimum and shows poorer performances than what is

expected for the recommended channel sets. As a conse-quence, it is likely to underestimate the performances ascompared to the recommended frequency combination. TheAMSU-AB combination has not been tested due to thepoorer spatial resolution at AMSU-A frequencies thatwould seriously hamper the comparison with radar scenesof limited extent (especially for the comparison with theCAMRa radar).

5.1. Comparisons With the BALTEX Radars

[41] During BALTEX, the Swedish Meteorological andHydrological Institute (SMHI) was responsible for thecollection and distribution of the hydrological and radardata sets. The BALTEX radar network consists of 29 radars(most of them in C-band) located in Northern Europe[Michelson and Koistinen, 2000]. The radar products areadjusted using rain gauge measurements, at 0600 and 1800UTC each day. The radar reflectivity factors are convertedto rain rate using a Z-R relation that depends on the season.[42] In addition to our own algorithm, for comparison

purposes two rain detection algorithms developed forAMSU-B measurement are applied to the satellite observa-tions. Grody et al. [1999] provide a rain flag for eachAMSU-B pixel. Bennartz et al. [2002] characterize eachAMSU pixel with a 4-class rain classification (CL1 : RR <0.1, CL2 : 0.1 < RR < 0.5, CL3 : 0.5 < RR < 5, CL4 : 5 <RR in mm/h) that was derived from the analysis ofcoincident radar and AMSU observations during BALTEX(as a consequence, the performances of this algorithm arebest for the comparisons with the BALTEX observations).Figure 14 presents an example of retrieval maps for onecase recorded during the BALTEX experiment (12 July2000, at 15UTC). Most of AMSU-B brightness temper-atures exhibit a decrease in amplitude where the radarobservations indicate the presence of rain (Figures 14a–14d) showing the consistency of the space-borne andground-based measurements. The classification from Ben-nartz et al. [2002] (Figure 14e) identifies several regions withsignificant rain, located where the brightness temperatures

Figure 11. Variations of the RMS error erms for F07 andF08 frequency sets at different noise configurations with theneural network (NN) method over ocean and land.

Figure 12. (a) Density of initial-retrieved total (Ice +Snow + Graupel) ice content pairs per bin of 0.1 normalizedto its maximum value for sea pixels with the NN method;(b) RMS error erms (solid line) ± standard deviation (verticalsegment) of total (Snow + Ice + Graupel) ice retrieval forsea pixels and with neural network (NN) method per bin of0.1.

Figure 13. Example of vertical profiles of rain rate overocean for different frequency ranges at 19� incidence anglewith the neural network (NN) method. The RMS error iscomputed per bin of rain and per level. The grey scalefollows the error thresholds defined by the user require-ments (Table 1).


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are significantly depressed. The Grody et al. [1999] classifi-cation (Figure 14f) delineates rainy regions similar to the onesidentified by Bennartz et al. [2002]. The rain classificationdeveloped in the present study provides quite similar loca-tions for rainy regions (Figure 14g), both over land and sea.The spatial structures of the three satellite-based rain detec-tion algorithms are very similar and, compared to the radarrain detection, they show the same differences, with a regionof rain missed in northern Norway and slightly displacedstructures in the Gulf of Bothnia.[43] Our rain rate estimates are presented on Figure 14h:

Although a specific algorithm is developed for each inci-dence angle range and surface type, there is no discontinuityin the rain rate field. Compared to the radar measurements,the RR derived from AMSU-B observations are clearlyoverestimated. This is also the case for the Bennartz et al.[2002] algorithm although it has been specifically developedduring this measurement campaign. This can be due to (1) anunderestimation of the radar rain rate for this particularsituation (2) the inability of the AMSU-B frequencies toquantify this type of rain event. The similar results obtainedbyBennartz et al. [2002] and our algorithms tend to show thatour retrieval method cannot be directly incriminated.[44] A total of 120 overpasses covering totally or partially

the BALTEX domain during the entire year 2000 wereinvestigated to perform a statistically representative study.Rain rates derived from radar observations and convoluted

according to the AMSU-B beam patterns ranged from 0.1 to14 mm/h. In that database, about 12% of the rainy pixelsexhibited a rain rate exceeding 1 mm/h. AMSU overpassesoccurred roughly at 06 and 18 UTC and radar observationswere collected in a temporal range of 10 min. HR and FARwere calculated for three rain detection methods separatingthe results by surface types and by 6-month period. In thepresent data set, Bennartz et al.’s [2002] rain detection toolperforms better both over sea and land compared to the twoother tools which is expected considering the fact that thisalgorithm was built from the BALTEX observations. Alldetection tools performed better during the warm season(May to October) than during the cold season: With ouralgorithm, the mean HR over sea is 0.69 during the summerand 0.56 during the winter. During summer, the rain is morelikely to be convective and as a consequence easier to detectwith high frequencies.

5.2. Comparisons With the CAMRa Radar

[45] The Chilbolton Facility for Atmospheric and RadioResearch established a set of ground validation test cases,by performing radar scans during AMSU-B overpasses forboth stratiform and convective cases. The Chilbolton Ad-vanced Meteorological Radar (CAMRa) [Goddard et al.,1994] is unique in incorporating a fully steerable 25 mantenna. This provides an extremely narrow (0.25�) beam,giving a high spatial resolution (0.4 km at 100 km range).Rain rates were determined using reflectivity (Z) and differ-

Figure 14. A case during BALTEX (12 July 2000, 15 UTC). Panels (a) to (c): Observed AMSU-Bbrightness temperatures; (d): Coincident radar rain rate (mm/h); (e): Bennartz et al. [2002] rainclassification composed of 4 classes; (f): Grody et al. [1999] rain detection; (g): Rain detection based onour methodology; (h): Rain rate (RR) retrieval from our method. RR (mm/h) is plotted in logarithmicscale.


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ential reflectivity (ZDR) data via the formula of Illingworthand Blackman [2002] for ZDR > 0.5 dB and via a standardpower law Z-R relation otherwise. Bright-band correctionswere applied using the method proposed by Smyth andIllingworth [1998]. To enable comparison with AMSU-Bretrievals the radar-derived data were transformed using aCressman filter to a fine-scale coordinate grid interpolatedfrom the AMSU-B swath, and then convolved with a Gauss-ian approximation of the AMSU-B antenna pattern to estab-lish a single ground-truth value for each satellite footprint.[46] Figure 15 presents a case on 21 April 2004, at 14:10

UTC. The NOAA-16 overpass was at approximately14:07:30 and the radar scan took place between 14:08:13and 14:13:59, suggesting a relatively good simultaneity ofboth ground-based and space-borne observations. Com-pared to the previous case (Figure 14), the AMSU-Bbrightness temperatures (Figures 15a, 15b, and 15c) arevery homogeneous and do not show significant effects ofscattering by the cloud structure, although the rain ratemeasured by the radar reaches 5.2 mm/h locally (Figure15d). The rain detection methods (Figures 15e, 15f, and15g) all miss large parts of the rain area and they all havevery similar structures (coastal pixels have not been treated).As a consequence, over land the FAR is very good but theHR is rather low for all methods. The estimated rain rates byour method (Figure 15h) are very close to the radarestimates in the region where rain has been correctlydetected.[47] A total number of 1727 AMSU beams colocated

within the CAMRa domain (60% over land, 12% over sea,

28% on coastlines) from 45 different AMSU overpasseswas analyzed to determine the performances of the raindetection tool. Time intervals between the radar observa-tions and the satellite measurements ranged from a fewseconds up to 30 min while rain rate (convoluted accordingto AMSU beam patterns) varies up to 7 mm/h. Over sea, ourrain detection tool detects correctly the rain pixels with anHR above 0.7 (NWC breakthrough threshold, Table 1) andthe rain detection tool from Bennartz et al. [2002] performseven better. The FAR for pixels over sea is almost of thesame order of magnitude for the three rain detection toolsand remains below the NWC breakthrough threshold of 0.4.Over land the rain detection performances exhibit HR belowthe NWC breakthrough threshold whatever the rain detec-tion tool while the FAR is well below the NWC break-through threshold. Study of the HR as a function of the rainrate reveals that over sea (land), HR reaches 0.9 (0.5), 1(0.5), and 0.7 (0.5) for a rain rate above 1 mm/h suggestingthat highly precipitating events can be well detected oversea while over land only 50% of the rainy pixels aredetected regardless the rain detection tool. In terms of RRretrieval performance, the RMS varies similarly to the oneobtained theoretically: The higher the rain rate, the lowerthe RMS with a value around 90% for rain rate rangingfrom 1 to 6 mm/h.[48] These comparisons with the BALTEX and CAMRa

radars show that our method, only based on simulations, hasperformances similar to the other methodologies developeddirectly from comparisons between radar and satelliteobservations [Bennartz et al., 2002]. This confirms the

Figure 15. Same as Figure 14 but for a comparison with the Chilbolton radar estimates on 21 April2004, 14:10 UTC.


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validity of the method for this frequency range and givesconfidence in its extrapolation to higher frequencies. As theAMSU-B frequency combination (F03) shows poorer per-formances than the recommended channel sets such as F08(see Figures 4 and 8), both in terms of detection andquantification of the rain rate, these comparison results areclearly an underestimation of what could be obtained withoptimized frequency sets.

6. Conclusion

[49] The importance of millimeter and sub-millimeterwavelength observations for cloud and precipitation charac-terization from geosynchronous orbits has been recognizedfor nowcasting and short range forecasting. The potential ofthis wavelength range for precipitation retrieval is analyzedin this paper in order to prepare future geostationary mis-sions. The study is based on simulated observations thatcorrespond to five real meteorological situations overEurope. Outputs from the Meso-NH cloud model are cou-pled to the ATM radiative transfer code and a large range offrequency sets is simulated and tested. For each frequencycombination, both rain detection and the rain rate quantifi-cation are evaluated and compared to the user requirementsrecently specified by EUMETSAT. The impact of adding theIR information is also systematically evaluated. The resultsshown here are very promising.[50] The theoretical results are evaluated at close-to-

millimeter wavelengths, using collocated AMSU-B andradar observations in the Baltic region (BALTEX radars)and over the south of England (the CAMRa radar). Asexpected, the comparison results are not as good as thetheoretical ones but are consistent with them. As therecommended frequency sets for a geostationary missionhave better theoretical performances than the AMSU-Bchannel combination, the radar comparison results withAMSU-B show the lower limit of what can be obtained.[51] The frequency channels below 100 GHz improve the

results for rain rates below 1 mm/h over ocean. Over landthe role of these frequencies is less important, as expected.For a given antenna size, sounding in the 50 GHz O2 lineprovides less than half the spatial resolution obtained at118 GHz. For high rain rates such as observed during severeweather events, high frequency observations alone (i.e., nochannels below 100 GHz) can provide result accuracies thatare close to the user requirements. With a frequency setsimilar to GOMAS, the performances are within the break-through values specified by Rizzi et al. [2006] for the raindetection for NWC applications and for the rain rate forboth NWC and NWP applications, except for the ratesbelow 1 mm/h. The impact of the 424 GHz channels isonly marginal for rain retrieval. The role of the O2 soundingchannels around 50 and/or at 118 GHz appears importantfor rain analysis, whereas sounding in the H2O lines doesnot add much information. However, we cannot concludethat the better ability of the O2 lines compared to the H2Olines is only due to the nature of the lines and not to theirhigher penetration into the cloud structure related to thelower opacities at lower frequencies. A key aspect of therain detection and quantification from a geostationary orbitlies in the spatial resolution of the measurement. In thisanalysis, it is assumed that the rain cell fills all fields of

view for a given frequency set. This is a limitation that willhave to be accounted for but that will be lessened if lowfrequency channels such as the O2 50 GHz are avoided. Forthe detection and quantification of significant rain events(rain rate above 1 mm/h), the combination of soundingchannels in the O2 line at 118 GHz and in the H2O lines at325 and 380 GHz should provide results close to thespecifications; a mission with frequencies similar to CIW-SIR with the addition of the 118 GHz O2 channels wouldalso have similar performances. Addition of IR observationsdoes not improve the results, except for frequency combi-nations that initially showed very poor performances. Re-alistic instrumental noise levels have been tested, and theretrieval performances are rather stable.[52] In the meteorological community over the last de-

cade, the purposes of the geostationary missions at micro-wave to sub-millimeter wavelengths focused first on watervapor and temperature profile retrievals to shift morerecently toward nowcasting of precipitation and extremeweather events. The objectives of these missions have to beclearly stated and defined in order to optimize the instru-ment specifications and design a realistic mission that willfully benefit from the high temporal sampling rate providedby the geostationary orbit.

[53] Acknowledgments. This study was supported by ESA undercontract 18054/04/NL/FF. Partial support was also provided by EUMET-SAT under contract EUM/CO/04/1311/KJG and by CNES under TOSCAcontract. We are especially grateful to Ulf Klein (ESA) and Chung-Chi Lin(ESA) who critically reviewed the successive steps of the present work andgave insightful comments. We thank D. Michelson (SMHI) who kindlyprovided the BALTEX radar data. We also thank Suzane Crewell and MarioMech for discussions. We are grateful to the three reviewers for their carefulreading of the manuscript and constructive comments. Computer resourcesfor Meso-NH simulations were allocated by IDRIS. AMSU data come fromthe NOAA Satellite Active Archive.

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Bizzari, B., A. Gasiewski, and D. Staelin (2002), Frequent rain observationfrom geostationary stallite by millimeter submillimeter-wave sounding,Proceedings of the 1st IPWG workshop, Madrid, 23–27 September.

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��F. Aires, Laboratoire de Meteorologie Dynamique, Institut Pierre-Simon

Laplace/Centre National de la Recherche Scientifique, Universite de ParisVI/Jussieu, Paris, France.J.-P. Chaboureau and J.-P. Pinty, Laboratoire d’Aerologie, Universite de

Toulouse, Centre National de la Recherche Scientifique, Toulouse, France.E. Defer and C. Prigent, Laboratoire d’Etudes du Rayonnement et de la

Matiere en Astrophysique, Centre National de la Recherche Scientifique,Observatoire de Paris, 61, avenue de l’Observatoire, 75014 Paris, France.([email protected])J. R. Pardo, Instituto de Estructura de la Materia - Departamento de

Astrofisica Molecular e Infrarroja, CSIC, Serrano 121, E-28006 Madrid,Spain.C. J. Walden, Space Science and Technology Department, Rutherford

Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK.O.-Z. Zanife, Collecte Localisation Satellite, Ramonville Saint-Agne,

Toulouse, France.


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