Mesoscale convective complexes in the Americas

Abstract

Enhanced infrared satellite imagery and conventional surface and sounding data are used to document the existence and climatological characteristics of mesoscale convective complexes (MCCs) over midlatitude South America (south of 20°S) and in the tropical region (20°N to 20°S) between North and South America. The implications of the results, with regard to the structure and dynamics of MCCs, are discussed. It is found that MCCs occur with approximately the same frequency in mid‐latitude South America as they do in mid‐latitude North America. For the most part the characteristics of mid‐latitude South American MCCs are similar to those of MCCs in the United States. The most notable difference between North and South American MCCs is that the South American systems are, on the average, about 60% larger than MCCs in the United States. In addition to the large population of mid‐latitude South American MCCs, an even larger number of low‐latitude (tropical) systems were found. In general, both the mid‐latitude and tropical populations of MCCs are nocturnal and continental; i.e., the great majority of systems occur at night over land. Of the systems that do occur over water, a significant fraction develop into tropical storms. Large populations of MCCs occur in each of the physiographically similar mid‐latitude areas of North and South America where low‐level nocturnal jets frequently develop. Very few MCCs occur over the Amazon River Basin or over the southeastern United States, even though both of these areas exhibit large amounts of deep convective activity. All of the various MCC population centers occur in latitudinal zones of westerlies or easterlies and are concentrated immediately downwind of major mountain ranges. One of the 2 years of data that was investigated was an El Niño year. During the El Niño period the number of mid‐latitude South American systems was more than double the number in the non‐El Niño year. Moreover, several MCCs formed over the anomalously warm water that appeared along the Peruvian coast. Thus on the basis of this very small sample, there may be a direct connection between MCC activity and El Niño.

A Midlevel Vortex Observed in the Taiwan Area Mesoscale Experiment (TAMEX)

“…This was also observed in most of the midlatitude cases (e.g. Menard and Fritsch, 1989;Velasco and Fritsch, 1987;Zhang et al, 1989). The reintensification of the vortex intensity after 2100 UTC, in terms of maximum velocity and vorticity (Table 1), is consistent with the vortex stretching effect accompanied by the intensification of the MCS with some adjustment time lag.…”

Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica

“…This inconsistency of associations within the regime, and their varying provenances, may also be indicative of the hydrometeorological complexity of the area. Poveda & Mesa (1997) have proposed a scheme by which the displacement of the eastern equatorial Pacific inter-tropical convergence zone (ITCZ) during years of a warm Pacific (Pulwarty & Diaz, 1993) decreases the number of storms which penetrate into the Colombian and Brazilian Amazon from the Pacific (Velasco & Frisch, 1987). The resulting reduction in évapotranspiration from the continent diminishes the regional intensity of the ITCZ, thereby diminishing the pressure gradient to the North Atlantic anticyclone, and weakening the northeast trades.…”

Improved long‐term mean annual rainfall fields for Colombia

“…First, the westerly low-level Chocó jet flowing onshore from the Pacific Ocean, tightly linked to the clear-cut diurnal dynamics of mesoscale convective systems over the region (Velasco and Frisch, 1987;Poveda and Mesa, 2000;León et al, 2001;Mapes et al, 2003aMapes et al, , 2003bMejía and Poveda, 2005). Second, the so-called San Andres or Caribbean low-level jet (Amador and Magaña, 1999;Poveda and Mesa, 1999;Wang, 2007;Whyte et al, 2007) and third, a mid-atmosphere jet at 700-600 hPa over Central Colombia .…”