Monthly Weather Review, 132, 422-444.
Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.
Sandra E. Yuter and Christopher S. Bretherton
Department of Atmospheric Sciences, University of Washington, Seattle, WA.
George N. Kiladis
NOAA Aeronomy Laboratory, Boulder, CO.
An overview of the large-scale behavior of the atmosphere during the
Tropical Rainfall Measuring Mission (TRMM) Kwajalein experiment
(KWAJEX) is presented. Sounding and ground radar data
collected during KWAJEX, and several routinely available data sets
including GMS, NOAA OLR, and SSM/I satellite data and ECMWF
operational analyses, are used. One focus is the dynamical
characterization of synoptic-scale systems in the western/central
tropical Pacific during KWAJEX, particularly those which produced the
largest rainfall at Kwajalein. Another is the local
relationships observed on daily time scales among various
thermodynamic variables and areal average rain rate. These
relationships provide evidence regarding the degree and kind of local
thermodynamic control of convection.
Although convection in the Marshall Islands and surrounding regions often appears chaotic when viewed in satellite imagery, the largest rain events at Kwajalein during the experiment were clearly associated with large-scale envelopes of convection, which propagated coherently over several days and thousands of kilometers, had clear signals in the lower-level large-scale wind field, and are classifiable in terms of known wave modes. Spectral filtering identifies mixed Rossby-gravity (MRG) and Kelvin waves prominently in the OLR data. ``Tropical depression-type'' disturbances are also evident. In some cases multiple wave types may be associated with a single event. Three brief case studies involving different wave types are presented.
Daily-mean sounding data averaged over the five sounding sites show evidence of shallow convective adjustment, in that near-surface moist static energy variations correlate closely with lower tropospheric temperature. Evidence of thermodynamic control of deep convection on daily time scales is weaker. Upper-tropospheric temperature is weakly correlated with near-surface moist static energy. There are correlations of relative humidity (RH) with deep convection. Significant area-averaged rainfall only occurs above a threshold lower tropospheric RH near 80\%. Above this threshold there is a weak but significant correlation of further lower tropospheric RH increases with enhanced rain rate. Upper tropospheric RH increases more consistently with rain rate. Lag-correlations suggest that higher lower-tropospheric RH favors subsequent convection while higher upper-tropospheric RH is a result of previous or current convection. Convective available potential energy and surface wind speed have weak negative and positive relationships to rain rate, respectively. A strong relationship between surface wind speed (a proxy for latent heat flux) and rain rate has been recently observed in the eastern Pacific. It is suggested that in the KWAJEX region, this relationship is weaker because there are strong zonal gradients of vertically integrated water vapor. Variations in the mean low-level easterlies on synoptic to intraseasonal timescales affect the humidity of the tropospheric column by horizontal advection; this counteracts and often may overwhelm their impacts on surface fluxes.