Modeling of passive microwave responses over a cyclone using outputs from mesoscale models and comparison with TRMM/TMI satellite observations
Résumé
Passive microwave observations are sensitive to the whole hydrometeor column, in contrast to infrared and visible observations, which essentially sense cloud tops. Therefore passive microwave observations are a very promising tool to study the internal structure of precipitating clouds. A microwave radiative transfer model (Atmospheric Transmission at Microwaves (ATM)) has been developed to accurately simulate brightness temperature TB fields using output from nonhydrostatic mesoscale atmospheric model, Meso‐NH, simulations. The radiative transfer code takes the detailed description of the hydrometeor properties (as simulated by the Meso‐NH model) into account. The sensitivity of the predicted brightness temperature TB to the hydrometeor properties is carefully analyzed. Depending on the frequency, the passive microwave simulations show different sensitivities to the hydrometeor and surface properties: The low frequencies (10–30 GHz) sense essentially the surface properties and the liquid water column, whereas the higher frequencies (30–90 GHz) are most sensitive to the large icy hydrometeors (graupel and snow). TB simulations are generated for two real convective situations studied with Meso‐NH: Hurricane Bret on 22–23 August 1999 in the Gulf of Mexico and a South Atlantic Convergence Zone case off the Brazilian coast on 6–7 February 2001. The radiative transfer simulations are compared to the corresponding Tropical Rainfall Measuring Mission (TRMM) Microwave Instrument (TMI) observations on board TRMM, at 10.65, 19.35, 22.24, 37, and 85.5 GHz. To our knowledge, no direct comparisons between simulated TB and satellite observations had been conducted before for a systematic evaluation of the mesoscale cloud models. An overall good agreement is obtained for both situations, especially for the second one. At high frequencies the agreement is particularly remarkable, given the high sensitivity of these frequencies to the particle characteristics, especially in the ice phase. This result gives us strong confidence not only in the radiative transfer model but also in the bulk microphysical scheme of Meso‐NH.
