Climate applications of Ground-Based GPS KNMI 1.12 2003

Professor Lennart Bengtsson

ESSC, University of Reading

MPI for Meteorology, Hamburg

Why do we need to monitor water vapour?

Water vapour is the dominant greenhouse gas and

enhances climate warming significantly

Substantial change (+40%) is expected during this century

The role of the water cycle in the climate system Precipitation is crucial for life on the planet

The largest warming factor of the atmosphere is through the relaease of latent heat amounting to 80-90 WM-2

The net transport of water from ocean to the land surfaces amounts to some 40000 km3/year

Precipitation over land is about 3 times as high

Water vapour is the dominating greenhouse gas. Removing the effect of water vapour in long wave radiation reduces climate warming at 2 x

CO2 by a factor of more than 3. (For the GFDL model from 3.38 K to 1.05 K).

Annual mean global values of relative humidity f (in %) vertically averaged for 850-300 hPa and vertically integrated absolute humidity q (in kg/m2).

Onsala, Sweden E= 0.13 mm

Elgered

Summer Winter1995-2000 Trends

Integrated Water vapour 1978-1999

ECHAM5: T106/L31 using AMIP2 boundary conditions

Preliminary results:

Globally averaged results vary between 25.10 mm (1985) and 26.42 mm (1998)

Mean value for the 1990s is 1% higher than in the 1980s

Interannual variations are similar as in ERA-40

Variations follow broadly temperature observations from MSU (tropospheric channel) under unchanged relative humidity (1°C is

equivalent to some 6%).

How has atm. water vapour varied over the last 50 years?

Objective: to extend estimates of IWV for longer time periods

(Re-analyses exists now for some 55 years)

1. How well can we determine IWV from GCM forced by

observed SST?

2. How well can we determine it from analyses with

observations typical of the pre-satellite period?

3. How well can we determine it from the present

observing and assimilation systems?

Questions:

Methodology

We have mimicked earlier observing systems by redoing the ERA-40 assimilation for limited periods.

This has been done at the ECMWF computer system from ESSC at Reading University

IWV Decadal trend1979-2001ERA-40: + 0362 mm

IWV/TLT: 3.15 mm/C

( presumably only half as much)

ECHAM 5 (1979-1999): + 0.290 mm

IWV/TLT: 1.54 mm/C

NCEP/NCAR: - 0.056 mm

We have done four main experiments

1. ERA 40 - all humidity observations

2. Exp 1 - all space observations

3. Exp 2 - all upper air observations

4. Exp 1 - all upper air observations

Experimental periods

DJF 1990/1991

JJA 2000

DJF 2000/2001

We call the experiments:

1. ERA-40 dry (b02d)

2. No space observations (b03v)

(NOSAT)

3. Surface observations only (b046)

4. Space observations only (b040)

Integrated water vaporDJF 1990/91

IWV Decadal trends1958-2001

ERA-40 IWV/TLT : 2.85 mm/C ( 4.05 mm/C)

ERA 40 (corr) IWV/TLT : 1.55 mm/C

IWV Decadal trends1958-2001

ERA-40: + 0.405 mm

ERA 40 (corr) : + 0.155 mm

( ERA-40(24.9mm) - NOSAT (23.8mm) = -4.3%)

NCEP/NCAR: - 0.238 mm

GPS data from global IGS network

1997 - present

The IGS network of ground-based GPS stations

125˚ 130˚ 135˚ 140˚ 145˚

25˚

30˚

35˚

40˚

45˚

How well does the ECMWF operational forecasting system analyse IWV as compared to those retrieved from GPS measurements?

To what extent is it possible to separate errors in model analysed IWV from IWV obtained from GPS measurements? What are the most likely sources of errors?

What are the long term requirements fora GPS-based water vapour monitoring system?

Comparing operationally analysed IWV and IWV calculated from surface based GPS

measurements

1. Calculate IWV from GPS signal delay (temperatures from ECMWF operational analyses and pressure from the GPS station or if not available interpolate from met stations)

(Care must be taken in the vertical interpolation of moisture due to the fact that the model height differs from station height)

Determination of integrated water vapour, IWV

Methodology

Zenith path delay, ZPD, from GPS measurements

Obtain temperature from operational weather prediction model

Surface pressure from met. observations

Calculate IWV

Error estimates

Measurement error: co-located GPS stations (mid-latitudes) indicate very small errors (<0.7 mm IWV)

Horizontal interpolation error: (<100 km) and small vertical interpolation (<200 m) also indicate small errors (<0.6mm)

Results

We undertook intercomparison between IWV analyses and GPS calculated GPS for Jan. and Jul. 2000 and 2001.

For all stations we calculated the standard deviation of the daily differences (SD) and the monthly average difference (bias) .

1. When both SD and bias were small we concluded that both GPS and the analyzed fields were reliable with an error of the same order as the difference between the estimates.

Results

2. When both SD and bias are large it is generally not possible to conclude whether the GPS measurements or the analyzed values are incorrect or both. However, a detailed inspection of the daily differences can be helpful.

3. When SD is large and the bias is small then we may conclude that it is more likely that GPS is more reliable than the analyzed value.

4. When SD is small and the bias large then we may conclude that it is more likely that the analyzed values are more accurate then the GPS.

January 2001: IWV [mm] for station METS (Finland)

July 2000: IWV [mm] for station LPGS (Argentina)

July 2000: IWV [mm] for station CEDU (South Australia)

January 2001: IWV [mm] for station Gough Island (South Atlantic)

July 2000: IWV [mm] for station Diego Garcia (Indian Ocean)

July 2000: IWV [mm] for station Guam (West Pacific)

July 2000: IWV [mm] for station MALI (Kenya)

July 2000: IWV [mm] for station NKLG (Gabon)

July 2001: IWV [mm] for station NLIB (Iowa, USA)

January 2001: IWV [mm] for station NLIB (Iowa, USA)

January 2001: Vertical humidity profile from radiosonde measurements and OA at station Quad City (Iowa, USA)

OA

July 2001: Vertical humidity profile from radiosonde measurements and OA at station Quad City (Iowa, USA)

OA

January 2001: IWV [mm] for station HOFN (Iceland)

July 2001: IWV [mm] for station HOFN (Iceland)

January 2000: IWV [mm] for station Ascension (Trop. Atlantic)

July 2000: IWV [mm] for station Ascension (Tropical Atlantic)

Regional station averages of normal. bias (GPS- OA) (IWV bias divided by GPS derived IWV) in %

NStat Bias [%] NStat Bias [%]North America 30°N-90°N 170°W-50°W 25 -3.02 22 -0.92Central USA/Canada 35°N-65°N 120°W-70°W 12 -13.74 12 0.26Central America 5°N-30°N 115°W-55°W 4 -0.91 4 -1.44South America 55°S-10°N 85°W-30°W 3 -2.27 3 -3.10Southern Africa 40°S-5°N 5°E-55°E 3 0.75 4 -4.25Europe 35°N-75°N 15°W-45°E 34 1.01 35 0.49Baltic Sea catchment 50°N-70°N 5°E-40°E 15 -0.39 15 0.16Central Europe 42°N-55°N 5°E-30°E 15 1.37 16 0.78Mediterranean Sea 30°N-45°N 10°W-40°E 15 -0.66 17 4.07Siberia 50°N-80°N 60°E-180°E 7 -19.86 9 -0.14Central Asian Deserts 30°N-50°N 55°E-110°E 2 -23.93 2 23.40Saudi Arabia 10°N-35°N 30°E-60°E 5 -1.08 6 7.19Southern Asia 0°N-35°N 60°E-150°E 5 2.03 4 0.03Tropical Ind./Pac. Ocean 15°S-15°N 60°E-180°E 8 -2.38 7 -1.29Australia 45°S-10°S 110°E-150°E 7 -2.79 8 -2.17Australia + surroundings 60°S-0°S 90°E-180°E 12 -4.10 12 -2.41

July 2000/01January 2000/01Region Latitude Longitude

Regional station averages of normal. bias (GPS- ERA 40) (IWV bias divided by GPS derived IWV) in %

NStat Bias [%] NStat Bias [%]North America 30°N-90°N 170°W-50°W 25 -2.17 22 1.52Central USA/Canada 35°N-65°N 120°W-70°W 12 -10.44 12 3.14Central America 5°N-30°N 115°W-55°W 4 -5.37 4 -0.51South America 55°S-10°N 85°W-30°W 3 -1.35 3 -3.05Southern Africa 40°S-5°N 5°E-55°E 3 -1.85 4 -8.35Europe 35°N-75°N 15°W-45°E 34 0.90 35 -0.10Baltic Sea catchment 50°N-70°N 5°E-40°E 15 -0.67 15 -0.26Central Europe 42°N-55°N 5°E-30°E 15 2.01 16 0.39Mediterranean Sea 30°N-45°N 10°W-40°E 15 -1.77 17 2.06Siberia 50°N-80°N 60°E-180°E 7 -20.40 9 3.04Central Asian Deserts 30°N-50°N 55°E-110°E 2 -27.72 2 14.48Saudi Arabia 10°N-35°N 30°E-60°E 5 -2.35 6 4.74Southern Asia 0°N-35°N 60°E-150°E 5 1.21 4 3.68Tropical Ind./Pac. Ocean 15°S-15°N 60°E-180°E 8 -0.79 7 1.10Australia 45°S-10°S 110°E-150°E 7 -3.58 8 -5.00Australia + surroundings 60°S-0°S 90°E-180°E 12 -4.28 12 -4.37

Region Latitude LongitudeJanuary 2000/01 July 2000/01

Concluding Remarks

Atmospheric water vapour is a key climate parameter

Indications are that water vapour mixing ratio is conserved in the atmosphere so IWV will follow Clausius-Clapeyrons relation ( near exponential increase by temperature)

Long-term climate monitoring of IWV is essential

GPS observations are here very useful. It is strongly recommended to establish a long-term operational network based on an extended world-wide GPS-network

END

Thanks to

Stefan Hagemann, MPI for Meteorology, Hamburg

Gert Gendt, GeoForschungsZentrum, Potsdam