Local time- and space scales of organized tropical deep convection

Abstract

The time- and space scales of tropical deep convection are estimated via analysis of 3-hourly Global Cloud Imagery (GCI) data for 3 yr at 35-70-km resolution. The emphasis is on estimating local time- and space scales rather than traditional zonal wavenumber-frequency spectra. This is accomplished through estimation of local spatial lag autocorrelations, the conditional probability of convection at neighboring points, and the expected duration of convective events. The spatial autocorrelation scale is found to be approximately 130 km. and the mean duration of convective events approximately 5.5 h, in the convectively active areas of the Tropics. There is a tendency for the spatial autocorrelation scales to be shorter over the continents than oceans (95-155 versus 110-170 km). The expected duration of convective events likewise tends to be shorter (4-6 versus 5-7 h). In the far western Pacific, these differences are sharp enough to legitimize the notion of the Indonesian archipelago as an extended maritime continent with a distinctive shape. Consistent with many other studies, the diurnal variation of the convection is also found to be strikingly different over the continents and oceans. The diurnal amplitude over land is comparable to the long-term mean, raising the possibility of significant aliasing across timescales. The simple analysis of this paper should be useful in evaluating and perhaps even improving the representation of convective processes in general circulation models.