A Re-examination of Precipitation Activity in the Subtropics and the Mid-latitudes Based on Satellite-derived Data

Abstract

Large-scale precipitation activity in the subtropics and the mid-latitudes, in addition to seasonal variation, were studied using new climatological precipitation datasets: GPCP rain gauge-satellite combined data, and TRMM level-3 data derived from TRMM-PR observations. We found significant large-scale precipitation zones extending between the subtropics and the midlatitudes, referred to as SMPZs (Subtropical Mid-latitude Precipitation Zones) in this study. Most portions of the SMPZs existed over the oceans. Meridional variation of zonal averaged precipitation was quite different between land and the oceans. Mid-latitude peaks at ∼40° in zonal averaged precipitation were observed only over the oceans, because the SMPZs over the ocean formed the 40° peaks. The midlatitude peaks significantly intensified in the fall and winter when the SMPZs zonally extended in the mid-latitudes and kept precipitation as substantial as in summer. In summer, the SMPZs extended diagonally across the subtropics from the active tropical monsoon rainfall areas. Precipitation over the subtropical oceans thus increased in summer. Over land, mid-latitude peaks were not found. Precipitation intensified in summer, both in the subtropics and in the mid-latitudes.
The mid-latitude portions of the SMPZs extended near the storm tracks defined by high baroclinic wave activity. However, many portions of the SMPZs shifted to the lower latitudes from the storm tracks in fall and winter. In summer, baroclinic wave activity was weak for the subtropical portions of the SMPZs, which were characterized by convergence and frontal zones with weak baroclinicity. Vertical structure of precipitation observed by PR suggested that deep (shallow) stratiform and convective precipitation largely contributed to the active rainfall along the SMPZs in summer (fall and winter). A possible relationship between the shallow precipitation and the displacement of the SMPZs from the storm tracks in fall and winter was discussed.