Passive Microwave Observations of the Stratiform Regions of Two Tropical Oceanic Mesoscale Convective Systems

Abstract

High-resolution passive microwave observations within the stratiform regions of two different tropical oceanic mesoscale convective systems are investigated in detail. The observations were obtained from the Advanced Microwave Precipitation Radiometer (AMPR) during the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The AMPR records data at 10.7, 19.35, 37.1, and 85.5 GHz. Data collected during one ER-2 leg over the stratiform regions of each system are examined in detail. (Each leg is well coordinated with the low-flying WP-3s.) The passive microwave observations of the two stratiform regions, with similar surface rain rates, are found to differ significantly from one another. The average 85.5-GHz brightness temperature was 30 K colder on 22 February compared with 20 February. This greater ice scattering on 22 February is found to be consistent with the simultaneous radar reflectivity profile, which shows that upper-level reflectivities are greater through a deep layer. This and comparison of the two meteorological situations suggest a more recent injection of ice particles from the adjacent active convection region on 22 February, and demonstrate that the 85.5-GHz temperatures are sensitive to these differences. The 19.35- and 37.1-GHZ brightness temperatures were somewhat higher for a given 10.7-GHz temperature an 20 February. That system has a stronger bright band, and higher radar reflectivities below the melting level, suggesting greater vertically integrated rain water content. The radar reflectivities on 20 February decreased markedly with distance below the melting level, implying that a greater proportion of the rain evaporated. It is suggested that the brightness temperatures of the three lower frequencies were somewhat sensitive to these differences in the vertical distribution of precipitation within the rain layer between the two cases. Indications are that many of the differences between these two stratiform regions on these two days result from differences in their stage of evolution. At the time the detailed analyses were made for the 20 February case, the stratiform region had more time to mature compared with the newly formed stratiform region sampled on 22 February.