The Mesoscale Characteristics of Tropical Oceanic Precipitation during Kelvin and Mixed Rossby–Gravity Wave Events

Abstract

Precipitation structures within Kelvin and mixed Rossby?gravity (MRG) wave troughs near Kwajalein Atoll during the 1999?2003 rainy seasons are analyzed using three-dimensional ground-based radar data and upper-air sounding data. Consistent with previous work, wave troughs are preferred locations for precipitation and typically yield 1.3 times more rain area compared to the overall rainy season climatology. Although the contiguous areas of cold cloudiness associated with tropical wave troughs are large and long lived, the underlying precipitation structure is most frequently small, isolated convection from mixed-phase clouds. This mismatch in instantaneous cold cloudiness area versus radar-observed precipitation area indicates differences in the rate and nature of evolution between the mesoscale anvil cloud and the underlying precipitating portion of the cloud. Mesoscale convective systems (MCSs) were identified during portions of 32 of the 39 wave trough events examined. Convective cells are frequently embedded within stratiform regions. Reflectivity holes or pores in contiguous radar echo have been frequently observed in other regions but are quantified for the first time in this study. Based on characteristics such as total size of precipitating area and occurrence of convective lines, MCSs within Kelvin troughs are slightly more organized than those occurring within MRG troughs. Similar to the west Pacific warm pool region, there is a well-defined separation between observed and unobserved stratiform area fraction and convective precipitation area, each as a function of total precipitation area. At precipitation area sizes near 40% of the radar domain, the maximum observed convective area changes from increasing to decreasing with increasing precipitation area. The maximum observed convective precipitation area occupied ?20% of the radar domain. These characteristics suggest that the atmosphere in the west Pacific can sustain a limited area of updrafts capable of supporting precipitation growth by collision/coalescence and riming.