Mon. Wea. Rev., in press.

The mesoscale characteristics of tropical oceanic precipitation during Kelvin and mixed Rossby-gravity wave events

Christopher Holder and Sandra E. Yuter
Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC

Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY

Anantha Aiyyer
Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC


We analyze the mesoscale precipitation structures during Kelvin and mixed Rossby-gravity (MRG) wave troughs near Kwajalein Atoll (8.7 oN 167.7 oE) during the 1999-2003 rainy seasons using three-dimensional radar data and upper-air sounding data. Equatorial waves troughs are associated with anomalously cold cloudiness regions in outgoing longwave radiation fields filtered in the wavenumber-frequency domain. Small, isolated convection is the prevailing precipitation organization at Kwajalein and is the sole organization type over half the time during wave troughs. With rain areas 1.3x larger than rainy season climatology, wave troughs are preferred locations for precipitation.

Mesoscale convective systems (MCSs) occurmore frequently within wave troughs compared to rainy season climatology, but MCS activity widely varies among the trough events. Many MCS stratiform regions contain embedded convection and areas of weak reflectivity, which suggest interruptions in MCS flow fields that may be dynamically significant.

There is an observed limit to convective precipitation area that the atmosphere near Kwajalein can support --- roughly 15 500 km2 (20% of radar domain) convective area within 30 000 km2 total rain area. Stratiform area fractions vary widely for small total rain areas. As total precipitation area increases beyond 10 000 km2, the stratiform area fractions converge towards 1.0.

Kelvin trough mesoscale precipitation structures tend to be slightly more organized than MRG. Total, convective, and stratiform rain areas and MCS rain areas can achieve larger sizes during Kelvin troughs, and convective lines occur three to four times more often than during MRG troughs.