VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Elevation and Climatological Dependence of the SSI Capabilities to Discriminate Atmospheric

Propagation Conditions

Ada V. Bosisio 1, Ermanno Fionda 2, Piero Ciotti 3, Antonio Martellucci 4

1) CNR\IEIIT, Milano, Italy; bosisio@elet.Polimi.It2) Fondazione Ugo Bordoni (FUB), Roma, Italy; efionda@fub.It

3) Dept. Of Elect. And Info. Eng., Univ. Of L’aquila, L’aquila, Italy; piero.ciotti@univaq.It4) ESA-ESTEC, TEC-EEP, Noordwijk (NL); antonio.martellucci@esa.Int

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VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 20122

Microwave Propagation Phenomena Key parameter: Frequency (f)

O2, Water Vapor emissions: (Clear Sky) Cloud emissions: relevant at f > 15 GHz Rain events: relevant at f > 10 GHz Snow and Ice, not relevant below f = 30 GHz

SCENARIO

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

MOTIVATION

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Rain events can be source of strong scatter processes extremely

severe for space communications at K/Q/W band

1. For satellite communications purposes, the knowledge in real

time of rain events, in the volume of the operative beacon, could

suggest the adoption of dynamic fading mitigation techniques

to overcome the lost of signal quality

2. For Water Vapor and Cloud Liquid retrieval, the availability of a

user-friendly tool (able to detect microwave radiometric

observations affected by rain) is welcomed in case of massive

measurements (radiometric-networks) or for analyzing large

database

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Candidate Indicator (SSI)

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• Indicator should be generated from ground-based brightness temperature values Tb(f)

• It could benefit from the different sensitivity of the microwave radiometric channels to the various atmospheric constituents

Candidate:• The Sky Status Indicator (SSI) issues from the ratio

between available Tb(f) @ 23.8, 31.4 GHz

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

outline

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• Sky Status Indicator – SSI features

– SSI computation

– Clear sky Tb(f) and calibration check

• Sensitivity analysis on SSI coefficients (synthetic data)– elevation angle and site (climatology & latitude) dependence

– erroneous calibration procedure and/or apparatus uncertainty

• Sensitivity analysis on SSI coefficients (measured data)– elevation angle dependence

• SSI classification results• Final remarks

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

• ASSUMPTIONSThe ratio T31/T23 of concurrent ground-based radiometric data depends on the thermodynamic state of the atmosphere.

• This ratio detects the status of the sky along the path as it neutralizes the contribution of the water vapor plateau (continuum) and of the dry gases by defining a modified brightness temperature T31 value:

• The coefficient c0 is frequency-, location-, and elevation-dependent.• It is computed either from measurements or from simulation data.• Specifically, c0 is the intercept of the straight line that relates the couple

(T31,T23) of values under clear sky condition

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SSI features

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

SSI computation

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The key point is c0 calculation, i.e. the identification of the linear fit between T23 and T31 under clear sky conditions from simulated Tb(f):

from measured Tb(f):

Forward RT

Forward RT RAOBsRAOBs Tb(f),IWV, LWPTb(f),IWV, LWP

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 20128

Clear sky Tb and calibration check

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Sensitivity analysis on ssi coefficients

• RAOBs database + forward RT model including:– Rosenkranz absorption model (water vapor)– Mattioli et al. (cloud model)

• Classification criteria• 3 sites (De Bilt, Roma, Milano)• 5 elevation angles = 27.6°, 35.5°,40.2°, 69.6°, 90°

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The sensitivity analysis aims at assessing the dependence of ci coefficients on the elevation angle and on the climatological region.

INPUT

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

RAOBs and simulated Tb(f) database

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Period # samples Lat, Long % clear skyconditions

De Bilt 2002 -2008 3618 (52.1N,5.18E) 29

Milano 2002 -2008 6518 (45.43N,9.28E) 41

Roma 2002 -2008 5984 (41.65N,12.48E) 50

Classification criteria according to the outcome of cloud liquid model

•Clear sky conditions: LWP <0.001 cm

•Cloudy sky conditions: 0.001< LWP< 0.07 cm

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Sensitivity analysis on ssi coefficients

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Observed variability @ 90° : 8%

Climatology-latitude

joint effect

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Noise and improper calibration effects

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Tbn(f)=Tb(f) +N(0,

N is AWGN with = 0.5 K.

It accounts for the instrument radiometric resolution.

Tbne(f)=Tbn(f) ± i where i=1,2 K

The bias reproduces measurements under improper calibration periods

Robustness is evaluated on simulated Tb(f)

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Noise and improper calibration effects

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Sensitivity due to ± 1 and ± 2 of about ±6% and ±12 %

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Experimental field in Cabauw (NL)

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Antenna beam parameters

Frequency [GHz] 15.3 22-31 90

Side lobe level [dBc] <- 30 <- 30 <- 40

Directivity [dB] 28.4 33.2 33.0

HPBW [°] 6.2 3.3 -3.7 3.0

ESA ATPROP DataSet Julian time # Samples Elevation [°]

A 18/05/09 - 12/07/09

3395161 90.0

B

12/07/09 - 31/07/09

about 6x106 69.6 1/10/09 - 30/11/09

ESA ATPROP

RAOB

22 km

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Sensitivity analysis on ssi coefficient

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• the T23 domain ranging from its minimum value to 50 K is divided in 200 bins

• for each bin the minimum T31 value is selected

• linear fit over the selected couples T23 - T31

discrepancy from 6% to 9% for co and of about 1% for c1

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

SSI Probability density functions

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VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

SSI classification results: Cabauw(NL)

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SSI range variability and boundary threshold values with respect to the clear, cloudy and rainy sky conditions.

The SSI values are referred to brightness temperature values measured in Cabauw during 2009.

VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Tb(15.3) as possible rain detector tool

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VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Tb(15.3) during rain events

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VII Riunione annuale CeTeM-AIT – Bari – 4,5 Dicembre 2012

Final remarks: results and outlook

•SSI has easy software implementation and online performance capability•Clear/cloudy and cloudy/rainy sky conditions were discriminated by two SSI boundary threshold values: 0.39 (at equal to 90° and 69.6°) and 0.86 or 0.88 (at equal to 90° and 69.6°). •A robustness analysis on SSI considering simulated Tb AWGN with = 0.5 K and a bias of ±1 and ±2 K:

– Sensitivity of about ±6% and ±12 %

•Precipitation prediction through plane parallel rain slab to extend classification criteria to rainy sky conditions•Database of meteorological information such as rain gauges and/or radar data for experimental validation

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