Typical Patterns of Microwave Signatures and Vertical Profiles of Precipitation in the Midlatitudes from TRMM Data

摘要

Representative patterns from multichannel microwave brightness temperature T_b in the midlatitude oceanic region, observed by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), are studied during precipitation events detected by the TRMM precipitation radar (PR) for three summer and winter seasons using empirical orthogonal function (eof) analysis. The first three patterns are interpreted as rain liquid water, solid particles, and rain type based on the frequency distributions ofvertical profiles of the radar reflectivity factor and the heights of the storm top, cloud top, and freezing level. The first eof (eof1) correlates with the near-surface rain rate. While the eigenvector for the 85.5-GHz channel is less significant for eof1 variability in summer, those in all channels contribute equally to the variability in winter. This difference suggests that summer precipitation is caused by additional solid particles formed in developing precipitation systems. The second eof (eof2)represents the number of solid particles and also corresponds to the near-surface rain rate. This result suggests an increase of solid particles with the development of precipitation systems. eof2 varies largely by echo-top height in summer and by echo-top height and freezing height in winter. The positive component score has double T_b peaks. Dividing the score into two patterns according to these peaks reveals highly developed precipitation systems, such as convective rainbands and frontal systems, and weak precipitation with shallow systems caused by cold outbreaks in the winter case. The negative component score also shows shallow and weak precipitation systems with warm rain. The third eof (eof3) is related to rain type. Vertical profiles show a significant bright band with a small height difference between the echo top and freezing level for negative eof3, while positive eof3 has no bright band with a high echo top relative to the freezing height. The results indicate that stratiform and convective precipitation systems can be characterized by eof3.