The Mesoscale Characteristics of Tropical Oceanic Precipitation during Kelvin and Mixed Rossby-gravity Wave Events

Abstract

We analyze the mesoscale precipitation structures during Kelvin and mixed Rossby-gravity (MRG) wave troughs near Kwajalein Atoll (8.7 °N 167.7 °E) during the 1999-2003 rainy seasons using three-dimensional radar data (radius=157 km) and upper-air sounding data. The large region of anomalously cold cloudiness in the outgoing longwave radiation fields filtered in the wavenumber-frequency domain are suggestive of the presence of the wave trough.

Mesoscale convective systems (MCSs) occur more frequently within Kelvin and MRG wave troughs compared to a multiyear rainy season climatology, but MCS activity widely varies from one trough event to another. Radar volumes during troughs contain only small, isolated rain areas at least half the time, similar to typical Kwajalein conditions and overwhelming many ensemble organizational statistics such as the size, shape, orientation, and reflectivity characteristics of individual contiguous rain areas. This suggests wave trough forcing is variable. Many MCSs contain scattered convective cores and areas of weak reflectivities embedded within the stratiform region, suggestive of perturbations in the MCS air and moisture flow field which may be homogenized away in MCS many schematics and have significant physical implications.

There is an observed limit to convective precipitation area that the atmosphere near Kwajalein can support. This limit is observed in two different datasets near Kwajalein and in the west Pacific warm pool, but the physical reasons for this limit are unclear. Stratiform area fractions vary widely for small total rain areas, and as total precipitation area increases the stratiform area fraction tends to increase and is less variable. This reflects that small total rain areas contain small rain blobs which often have smaller stratiform proportions than larger blobs.

Kelvin trough mesoscale precipitation structures tend to be slightly more organized than MRG. Total, convective, and stratiform rain areas and MCS rain areas are often somewhat larger during Kelvin troughs, and convective lines occur three to four times more often than during MRG troughs. Enhanced organization of mesoscale precipitation structures during Kelvin events may be linked to stronger, deeper, and more sustained convective updraft regions than MRG troughs, and to a potentially more favorable environment for convective initiation due to enhanced wave dynamics in the convective initiation region than with MRG waves.