Model Rain and Clouds over Oceans: Comparison with SSM/I Observations

Frédéric Chevallier ECMWF, Reading, United Kingdom

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Peter Bauer ECMWF, Reading, United Kingdom

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Abstract

A comparison of global model cloud and rain parameterization output with satellite observed radiances was carried out. Hydrometeor profiles from ECMWF operational short-range forecasts were combined with a microwave radiative transfer model to generate observation-equivalent radiances simulating the Special Sensor Microwave Imager (SSM/I) measurements. These were generated for two 15-day periods in January and July 2001 to be compared to SSM/I observations from three DMSP satellites, namely F-13, F-14, and F-15. The simulations were analyzed to isolate the relative contributions of water vapor, cloud water, rain, and snow to the total signal given their frequency of occurrence in the global fields. The 19.35-GHz channel has the great advantage of being less sensitive to cloud geometry and model-generated snow, thus providing a more unique relationship between cloud–rainwater and blackbody equivalent brightness temperatures (TBs). The 37.0-GHz channel showed great skill in separating cloud and (moderate to heavy) rainfall. The uncertainties in cloud geometry and ice microphysics inhibit an interpretation of 85.5-GHz brightness temperatures.

The evaluation was based on 1) the calculation of cloud and rain occurrence applying the same TB threshold screening to both observations and simulation, and 2) the analysis of global TB histograms for clouds and precipitation. From the first part, the model tendency to produce too large cloud and rain systems was identified. While some smaller-scale cloud features are missing, the onset of condensation generally produces larger systems than observed. Since the precipitation scheme is diagnostic, the cloud scheme propagates this problem to the rain coverage. With the results from the second part, the overestimation of extent and intensity was quantified to ≈10–15 K at 19.35 and ≈15–30 K at 37.0 GHz at horizontal polarization.

This was consistent with a direct estimation of retrieved liquid water paths using a variational retrieval scheme and of rainfall rates from a parametric algorithm. The globally averaged liquid water path from the model's first guess was about 75% higher than that from the retrievals, while globally averaged rain rate was 160% higher than retrieved. The major contribution to this overestimation originated from the Tropics, suggesting the convection scheme and/or its inputs as a major source of overestimation.

Corresponding author address: Dr. Peter Bauer, ECMWF, Shinfield Park, Reading, Berkshire RG2 9AX, United Kingdom. Email: peter.bauer@ecmwf.int

Abstract

A comparison of global model cloud and rain parameterization output with satellite observed radiances was carried out. Hydrometeor profiles from ECMWF operational short-range forecasts were combined with a microwave radiative transfer model to generate observation-equivalent radiances simulating the Special Sensor Microwave Imager (SSM/I) measurements. These were generated for two 15-day periods in January and July 2001 to be compared to SSM/I observations from three DMSP satellites, namely F-13, F-14, and F-15. The simulations were analyzed to isolate the relative contributions of water vapor, cloud water, rain, and snow to the total signal given their frequency of occurrence in the global fields. The 19.35-GHz channel has the great advantage of being less sensitive to cloud geometry and model-generated snow, thus providing a more unique relationship between cloud–rainwater and blackbody equivalent brightness temperatures (TBs). The 37.0-GHz channel showed great skill in separating cloud and (moderate to heavy) rainfall. The uncertainties in cloud geometry and ice microphysics inhibit an interpretation of 85.5-GHz brightness temperatures.

The evaluation was based on 1) the calculation of cloud and rain occurrence applying the same TB threshold screening to both observations and simulation, and 2) the analysis of global TB histograms for clouds and precipitation. From the first part, the model tendency to produce too large cloud and rain systems was identified. While some smaller-scale cloud features are missing, the onset of condensation generally produces larger systems than observed. Since the precipitation scheme is diagnostic, the cloud scheme propagates this problem to the rain coverage. With the results from the second part, the overestimation of extent and intensity was quantified to ≈10–15 K at 19.35 and ≈15–30 K at 37.0 GHz at horizontal polarization.

This was consistent with a direct estimation of retrieved liquid water paths using a variational retrieval scheme and of rainfall rates from a parametric algorithm. The globally averaged liquid water path from the model's first guess was about 75% higher than that from the retrievals, while globally averaged rain rate was 160% higher than retrieved. The major contribution to this overestimation originated from the Tropics, suggesting the convection scheme and/or its inputs as a major source of overestimation.

Corresponding author address: Dr. Peter Bauer, ECMWF, Shinfield Park, Reading, Berkshire RG2 9AX, United Kingdom. Email: peter.bauer@ecmwf.int

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