Evaluation of convective precipitation using spaceborne radar observation
Evaluation of convective precipitation using space-borne radar observation Kengo Matsubayashi Japan Meteorological Agency The Met Office www. metoffice. gov. uk © Crown Copyright 2018, Met Office
Outline 1. 2. 3. 4. Motivation Observation and evaluation method Evaluation result Summary
Motivation • In many convection schemes, water substances are empirically modelled. • Evaluation of water substances in convection schemes is important because of its latent heat release, evaporation and water loading directly influence the global circulation. • In this study, as a proxy of direct evaluation of vertical profile of convective precipitation in convection schemes, radar reflectivity is evaluated using satellite-based radar observation.
OBSERVATION AND EVALUATION METHOD
Two space-borne radars Cloud. Sat CPR TRMM PR 5 km*5 km Resolution, Range 1. 4 km * 1. 7 km 1. 4 km swath width Vertical 500 m Frequency 94. 0 GHz 13. 8 GHz Minimum detectable echoes -27. 5 d. BZ 18 d. BZ 2 B-GEOPROF P 1_R 05 2 APR Version 06 A (4. 3 km*4. 3 km until 2001 Aug) 245 km swath width Vertical 250 m In this study, 2 kinds of space-born radar observation, Cloud. Sat CPR and TRMM PR, are used. These 2 radars have different characteristics, so using two radars helps understanding what causes discrepancies between models and observation.
Two space-borne radars Cloud. Sat CPR TRMM PR 5 km*5 km Resolution, Range 1. 4 km * 1. 7 km 1. 4 km swath width Vertical 500 m Frequency 94. 0 GHz 13. 8 GHz Minimum detectable echoes -27. 5 d. BZ 18 d. BZ (4. 3 km*4. 3 km until 2001 Aug) 245 km swath width Vertical 250 m While Cloud. Sat CPR has higher horizontal resolution, TRMM PR has wide swath and can observe the horizontal distribution of convection.
Two space-borne radars Cloud. Sat CPR TRMM PR 5 km*5 km Resolution, Range 1. 4 km * 1. 7 km 1. 4 km swath width Vertical 500 m Frequency 94. 0 GHz 13. 8 GHz Minimum detectable echoes -27. 5 d. BZ 18 d. BZ TRMM PR Cloud. Sat CPR (4. 3 km*4. 3 km until 2001 Aug) 245 km swath width Vertical 250 m CPR strongly attenuates by large particles (>1 mm) in heavy precipitation, whereas PR can observe heavy precipitation.
Two space-borne radars Cloud. Sat CPR TRMM PR 5 km*5 km Resolution, Range 1. 4 km * 1. 7 km 1. 4 km swath width Vertical 500 m Frequency 94. 0 GHz 13. 8 GHz Minimum detectable echoes -27. 5 d. BZ 18 d. BZ (4. 3 km*4. 3 km until 2001 Aug) 245 km swath width Vertical 250 m CPR has high sensitivity and can detect very weak echoes by cloud with small particles (e. g. cirrus), on the other hand PR cannot detect weak echoes.
Simulation of radar reflectivity • Unified Model AMIP run • ~208 km*139 km@Equator • COSP(Bodas-Salcedo et al. 2011) 1. Split the model grids into sub-columns 2. Assign large-scale condensation considering cloud overlapping in a consistent manner to radiation scheme. 3. Estimate convective water substances from precipitation flux using particleconvective size distribution(PSD) How do we assign precipitation to subcolumns? and terminal velocity, and assign to subcolumns. We have diagnose convective precipitation fraction(CPF) 4. Calculate verticalto profile of radar reflectivity based on PSDs in each sub-columns. which is the area fraction convective rain precipitate in a model grid.
Convective precipitation fraction(CPF) estimation Using TRMM PR observation, the relation between CPF and grid -box mean convective precipitation is investigated. If CPF has correlation to grid-box mean convective precipitation, CPF can be estimated using grid-box mean convective precipitation which convective scheme predicts. The relation derived from TRMM PR looks to have correlation, but has large spread. So, we handled CPF stochastically depending on convective precipitation amount. We produced PDF of CPF using TRMM PR observation in advance, and simulated the probability of radar reflectivity occurrence using the PDF.
EVALUATION RESULT
Radar detection frequency (N 40 -S 40) Cloud. Sat CPR(>-27. 5 d. BZ) TRMM PR(>18 d. BZ) Obs Mdl Simulated radar detection well corresponds to observation, but there are several differences. In the tropics(N 20 -S 20), simulated CPR and simulated PR over 5 km has too infrequent detection compared to observation.
Reflectivity-Altitude joint histogram(N 20 -S 20) Cloud. Sat CPR Obs Mdl TRMM PR There are two large differences between the observations and simulated fields. 1. Radar reflectivity over 5 km, which is expected to be ice, is too weak in simulated. 2. Radar reflectivity in lower layer is too infrequent compared to observations.
Reflectivity-Altitude joint histogram(N 20 -S 20) Cloud. Sat CPR Obs Mdl TRMM PR 1. Radar reflectivity over 5 km, which is expected to be ice, is too weak in simulated. Wrong ice category classification? In the radar reflectivity simulation, convective ice are assumed to be snow aggregates. If this discrepancy is caused by graupel/hail, which have large diameter, PR radar reflectivity should be stronger and CPR reflectivity should be strongly attenuated.
Reflectivity-Altitude joint histogram(N 20 -S 20) Cloud. Sat CPR Obs TRMM PR 1. Radar reflectivity over 5 km, which is expected to be ice, is too weak in simulated. Wrong particle size distribution? We tried other ice PSDs (e. g. Gunn and Marshall, Sekhon and Srivastava), there was not much difference. Mdl
Reflectivity-Altitude joint histogram(N 20 -S 20) Cloud. Sat CPR Obs Mdl TRMM PR 1. Radar reflectivity over 5 km, which is expected to be ice, is too weak in simulated. Too few ice? The current convection scheme in UM employs a constant threshold to determine the amount of water substance to fall out from updraft. However, actually it should depend on vertical velocity. Much more ice with large diameter may be carried to upper layer.
Reflectivity-Altitude joint histogram(N 20 -S 20) Cloud. Sat CPR Obs Mdl TRMM PR 2. Radar reflectivity in lower layer is too infrequent compared to observations. Weak (and shallow) convection could be too infrequent in the model. The frequency of the convection scheme triggering is too low compared to observations.
Cloud. Sat CPR CFAD Observation Model To see the convective precipitation in detail, extract cloud with cloud base height is lower than 3 km. Congestus can be seen in both observations and simulated data above ~5 km. Although observed congestus reaches ~7 km, congestus predicted in the model is lower than observations and its height is too concentrated. This may be due to the lack of the representation of overshooting in the convection scheme.
Summary As a proxy of direct evaluation of convective precipitation, radar reflectivity evaluation with stochastic convective precipitation fraction has been developed. Radar reflectivity evaluation using 2 radar observation revealed several discrepancies. • • • Too weak reflectivity over 5 km Less frequent convective precipitation in low layer Too low congestus As a next step, we are going to evaluate the new convection scheme called Co. Morph which is being developed at the Met Office.
Questions? For more information please contact www. metoffice. gov. uk www. jma. go. jp/jma/indexe. html kengo. matsubayashi@metoffice. gov. uk www. metoffice. gov. uk © Crown Copyright 2018, Met Office
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