Brightness temperature simulations for the physical and synoptic interpretation of advanced microwave sounding unit moisture channels.

摘要

Radiative transfer simulations are performed in three phases to determine how water vapor and clouds affect passive brightness temperatures ({dollar}rm Tsb{lcub}B{rcub}{dollar} scS) of moisture sounding channels on the Advanced Microwave Sounding Unit (AMSU). Phase 1 employs idealized profiles of water vapor, cloud liquid water, and cloud ice as input to the radiative transfer model to investigate how AMSU frequencies at 23.8, 89, 157, 176, 180 and 182 GHz respond to clear and cloudy non-precipitating atmospheres. Phase 2 represents an effort to verify the microwave radiative transfer approach using observed data from an aircraft-mounted prototype radiometer. Phase 3 of the research employs output from a mesoscale model simulation of the ERICA IOP4 cyclone in an effort to gain insight about the synoptic interpretation of microwave water vapor image signatures. Model soundings are used in the radiative transfer code to generate synthetic 182 GHz imagery, as if a satellite were viewing the model atmosphere.; In Phase 1, {dollar}rm Tsb{lcub}B{rcub}{dollar} scS at 23.8 GHz and 89 GHz are more strongly affected by "altostratus" liquid clouds than by "cirrus" clouds for equivalent water paths. Channels near 157 and 183 GHz are more strongly affected by ice clouds. Phase 2 results indicate that the radiative transfer approach is adequate for simulating observed {dollar}rm Tsb{lcub}B{rcub}{dollar} scS, and that {dollar}rm Tsb{lcub}B{rcub}{dollar} scS at the relatively high microwave frequencies considered here are very sensitive to the ice particle size distribution. Phase 3 results show that 50% of the radiance contribution at 182 GHz for clear atmospheres emanates from the upper 0.35 mm of precipitable water, that is, above 380-400 mb, while 90% comes from the top 1.5 mm. A warm radiometric feature occurs near a tropopause fold. Dry upper and middle tropospheric air over the extremely dry formerly stratospheric air brought down by the tropopause fold allow the simulated 182 GHz channel to sense the strong vertical moisture gradient below 700 mb, at the bottom edge of the upper level frontal zone.