HR: 08:50h
AN: AE41A-04    [PDF]
TI: Intraseasonal Forcing of Lightning and Convective Activity in the Southern Amazon as a Function of Cross Equatorial Flow
AU: * Petersen, W A
EM: walt.petersen@msfc.nasa.gov
AF: Earth Systems Sciences Center/National Space Science and Technology Center, University of Alabama Huntsville, Huntsville, AL 35899 United States
AU: Fu, R
EM: fu@eas.gtech.edu
AF: Georgia Tech. University, Department of Earth and Atmospheric Sciences, Atlanta, GA 30332 United States
AU: Blakeslee, R
EM: rich.blakeslee@msfc.nasa.gov
AF: NASA Marshall Space Flight Center, 320 Sparkman Dr., Huntsville, AL 35805 United States
AU: Chen, M
EM: mchen@eas.gatech.edu
AF: Georgia Tech. University, Department of Earth and Atmospheric Sciences, Atlanta, GA 30332 United States
AB: Recently, Wang and Fu (2002) developed a monsoon-index (V-index; VI), based on changes in cross-equatorial 925 hPa meridional wind flow in the northwest Amazon. This index appears to be a robust metric of seasonal and intraseasonal changes in precipitation regime (e.g., wet vs. dry) across the Amazon and other parts of South America. While the VI identifies continental-scale variability of the monsoon, it yields no information on structural changes in the convective regime. For example, how does the overall three-dimensional structure of convection change as a function of VI-regime? Similarly, how are transitions in VI-regime manifested in lightning trends? In an effort to answer these questions we have examined four wet seasons (Dec.-Mar., 1998-2001) of TRMM satellite Lightning Imaging Sensor (LIS) and Precipitation Radar (PR) data in addition to two wet seasons (2000-2001) of ground-based Brazilian Lightning Detection Network (BLDN) data over South America. Composited LIS data indicate that the most statistically significant wide-spread response to VI-regime changes occurs over the south-central Amazon (SCAMZ), with other noticeable variations observed over portions of the subtropical Altiplano and Parana River basin. Most notably, over the SCAMZ both LIS and BLDN lightning data suggest for the southerly (northerly) VI-regime: 1) a pronounced widespread increase (decrease) in lightning activity; 2) a marked increase (decrease) in the amplitude of the diurnal cycle of lightning; (3) in association with (1) and (2), a factor of two relative increase (decrease) in the probability of any radar reflectivity pixel exceeding 30 dBZ above the freezing level; (4) an associated 20% increase (decrease) in pixel-mean ice water contents between the 7 and 11 km levels; and (5) an increase (decrease) in the relative frequency of occurrence of large rain rates. Interestingly, while our results suggest the presence of more vertically developed convection, lightning, attendant ice processes, and a larger relative fraction of high rain rates over the SCAMZ during the southerly-VI, the results of Wang and Fu (2002) suggest that greater mean daily rainfall totals occur over the SCAMZ during the northerly phase of the VI. One explanation for the apparently opposite trends in behavior between convective vertical structure/lightning and rainfall between VI-regimes, appears to reside in comparisons of the relative area coverage and/or frequency of precipitation in both regimes. For example, TRMM PR data indicate that the number of significantly raining pixels (reflectivity $>$20 dBZ) per orbit below the 4 km level is 33% larger in the northerly VI-regime than that of the southerly. Hence VI-regime transitions over the SCAMZ do affect changes in lightning frequency and vertical distributions of convective intensity, but these trends may behave in an opposite sense to raining-pixel area coverage and daily mean rainfall. Wang, H., and R. Fu, 2002: Cross-Equatorial flow and seasonal cycle of precipitation over South America. {\it J. Climate}, {\bf 15}, 1591-1608.
DE: 3304 Atmospheric electricity
DE: 3314 Convective processes
DE: 3354 Precipitation (1854)
DE: 3374 Tropical meteorology
SC: Atmospheric and Space Electricity [AE]
MN: 2003 Fall Meeting