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

Observations indicate that deep convective activity in the tropics is significantly modulated by synoptic waves. In this study, we focus on convectively coupled equatorial Kelvin and mixed-Rossby gravity (MRG) waves, and we examine the mesoscale characteristics of precipitation associated with them. In a recent study, Swann et al. (2006) used a wavenumber-frequency filtering technique (e.g. Wheeler and Kiladis, 1999) to identify the passage of Kelvin and MRG waves near Kwajalein Atoll, Republic of the Marshall Islands in the western tropical Pacific. Their study suggested more stratiform precipitation area and possibly greater mesoscale organization for Kelvin-related storms compared to MRG storms. The current study continues their work with a slightly altered methodology that only considers the convectively active phases of waves during 1999-2003. The present method and that of Swann et al. (2006), which weight OLR and radar differently in identifying wave events, are compared to identify those results which are most robust .We use Kwajalein operational three-dimensional S-band radar data and twice-daily sounding data to analyze the mesoscale environment during wave events.

Small, isolated precipitation areas dominate precipitation activity during both wave types and overwhelm other net statistics. In contrast to the mid-latitudes where there is often a clear synoptic connection to the mesoscale environment, there are few significant differences in the mesoscale precipitation structures of the synoptically different Kelvin and MRG waves. However, when the subset of rain areas on the scale of mesoscale convective systems (MCSs) is considered, some statistically significant results do emerge.

Kelvin events more often contain large rain coverage and large individual contiguous rain areas compared to MRG. The three largest storms observed at Kwajalein were Kelvin events. The frequency of occurrence of stratiform rain area peaks near 0.7 for both wave types, but the frequency is significantly larger for Kelvin waves. Kelvin data also have a higher maximum convective area per total area, and two Kelvin wave events reach and sustain far greater convective rain areas than any MRG event. These results are very similar to the Swann et al. (2006) dataset. Contoured frequency-by-altitude diagrams (CFADs) of radar reflectivity indicate that the near-surface modal reflectivity within both convective precipitation and overall precipitation is two to three dB greater for the ensemble of Kelvin storms compared to MRG storms. Convective lines spanning at least 100 km and lasting at least one hour occur at a 6% higher frequency during Kelvin storms than MRG storms.

These results for MCSs within Kelvin events compared to MRG events – higher frequencies of large rain areas and convective lines, larger rain areas, larger convective and stratiform rain areas, and greater reflectivities – imply that the storms associated with Kelvin waves more frequently contain organized mesoscale circulations than storms within MRG waves. The degree of organization may be related to more efficient preconditioning of the upstream convective environment in Kelvin waves since the Kelvin wave propagates in the same direction as the convective envelope in contrast to the MRG wave which propagates against the convective envelope (Straub and Kiladis, 2003a; Haertel and Johnson, 2000). These findings have implications on satellite retrieval algorithm parameterizations, operational forecasting, and the numerical model representation of the tropical atmosphere.