Convection in TOGA COARE: Horizontal Scale, Morphology, and Rainfall Production

Abstract

The occurrence frequency and rainfall production of mesoscale convective systems (MCSs) relative to smaller groups of convective clouds over the tropical oceans is not well known. Eighty days of shipboard radar data collected during the recent Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) were used to provide a detailed view of convection in the western Pacific warm pool, a region of global climatological significance. The aim of this study was to document the frequency of occurrence, rainfall production, and depth of convection observed during TOGA COARE within a simple and meaningful framework of convective horizontal organization. Organization was characterized in terms of the horizontal scale and morphology of convective systems. Precipitation events were defined based on whether they attained the length scale of an MCS, and on whether convection was organized into lines.About four-fifths of rainfall during COARE was associated with MCS-scale squall lines. These occurred in a variety of wind regimes but tended to be most common prior to low-level westerly wind maxima. Systems of isolated cells produced 12% of all COARE rainfall and were observed during periods of both very weak and very strong low-level winds. These two modes occurred about equally as often, and together they accounted for about 90% of observed convection. Cloud height populations associated with MCS organization were distinct from sub-MCS-scale cloud systems, with more rainfall from shallower clouds for sub-MCS convection. The distribution of total rainfall by cloud height for COARE was interpreted as a superposition of rainfall-cloud height distributions from each mode of organization. These results raise the possibility that isolated cell periods may represent a distinct, nonnegligible heat source in the large-scale heat budget when compared to the dominant MCS-scale systems.

Convective activities associated with intraseasonal variation over Sumatera, Indonesia, observed with the equatorial atmosphere radar

“…However, a dissimilarity is also observed between our case study and TOGA COARE. During TOGA COARE, mesoscale convective events are observed 2-5 days after the maximum of the westerly wind burst (Rickenbach and Rutledge, 1998). However, in our case study, a well-organized convective event is not observed during the postwesterly wind burst phase of the ISV (Period 3).…”

“…Lau et al, 1989) and observational studies over the western Pacific during TOGA COARE (Rickenbach and Rutledge, 1998;Lin and Johnson, 1996). Moderate vertical shear of horizontal wind (20-30 m s −1 ) plays an important role in the organization of mesoscale convective activities with a horizontal scale of >100 km (Saxen and Rutledge, 2000;Rickenbach and Rutledge, 1998).…”

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

“…Their methods are aimed at assessing the natural variability of the WAM over long periods. However, such generalization may deserve some care since precipitations are known to have patchy features with small spatial scale (Houze and Cheng, 1977;Leary and Houze, 1979;Nicholson, 1995;Chen et al, 1996;Rickenbach and Rutledge, 1998;Lebel et al, 2009). In this study, we show how the local Saloum rainfall is coherent with the regional Sahel rainfall at seasonal to decadal time scales.…”

“…While their work opens stimulating perspectives for the investigation of the seasonality of the WAM in the past, the relationship between local and regional rainfall variability needs to be assessed before extending paleoclimate reconstructions in the Saloum to the WAM in general. Precipitation is indeed known to be a patchy feature with small spatial scale (e.g., Houze and Cheng, 1977;Leary and Houze, 1979;Nicholson, 1995;Chen et al, 1996;Rickenbach and Rutledge, 1998;Lebel et al, 2009).…”

Daily precipitation extremes in isolated and mesoscale precipitation for the southeastern United States

“…In this context, an isolated thunderstorm is an example of IPF organization, while mesoscale convective systems or widespread frontal rain are examples of MPF organization. A similar framework has been used for climatological studies of precipitation systems regionally and globally using surface and space-based radar observations [8][9][10][11][12][13]. Differences in the organization of precipitating systems reflect the influence of the large-scale environment, for example, squall line mesoscale convective systems tend to form in environments of high shear and moderate convective available potential energy [14].…”