Thursday, 13 February 2003
Modeling Intra-seasonal to Interannual Variability of Precipitation over Central America
A devastating Central American drought occurred during the summer of 2001. This drought ravaged much of northern Central America (C.A.) but was not predicted or anticipated; nor is a cause for it immediately apparent. The catastrophe caused by this drought points out how little is known about precipitation and its variability over this region. Some of the seasonal controls are qualitatively known (e.g., trade winds, orographic effects, seasonal march of the ITCZ) but much less is known about intraseasonal to interannual variability. For example, on intraseasonal time-scales, the midsummer drought (MSD) on the Pacific side of C.A. is well known and documented, but little is known about why it occurs. Almost nothing is known about the interactions between precipitation in C.A. and the rest of the Americas. Because of its location, C.A. is key in linking monsoon variability in both North and South America. C.A. is affected by monsoonal processes, large-scale ocean circulation and related cold tongue-ITCZ (CTIC) features, and quasi-cyclical ENSO events.
Because of the importance of small-scale orography and land use issues, large-scale models and reanalyses by themselves are insufficient to fully understand and model precipitation variability over C.A. Regional modeling capabilities are critical as a means to examine how large-scale forcing gets downscaled by orographic effects, and to explore the impact of land surface state. We are making runs with the Regional Spectral Model (RSM), forced with output from the Global Spectral Model (GSM) and the NASA Seasonal to Interannual Predictability Project (NSIPP) model. We have focused on three key interrelated issues regarding high resolution mesoscale modeling: (i) improving the simulated distribution of precipitation and cloudiness through high resolution simulation of topographical forcing acting on anomalous large scale flows, (ii) evaluating the relative roles of C.A. land surface / east Pacific Warm Pool in controlling convective triggering; and (iii) determining is the ensemble effect of these processes on the diurnal cycle of cloudiness.