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Abstract. A numerical model of the atmosphere has been employed to evaluate the assumptions used in the simple two-parameter model that is utilized for many probable maximum precipitation (PMP) calculations. These assumptions are (1) the precipitation is linearly related to the precipitable water; (2) the precipitation efficiency of the storm does not change as the moisture available to the storm increases; and (3) terrain modulates the distribution of the precipitation but does not affect the synoptic-scale dynamics of the storm. A single case study is used to illustrate the techniques employed and to describe the results that were common to four case studies. We show that long-lived, moderate rainfall processes are important contributors to the total precipitation produced by the storm. Increases in the moisture availability result in the heavy rainfall beginning earlier, lasting longer, and being more continuous. As the moisture availability changes, the spatial distribution of the area over which more than 50% of the total rainfall falls as heavy rainfall changes. As the moisture availability is increased, the precipitation efficiency of the storm does not change significantly. Terrain effects are shown to have an effect on the amount of rainfall that occurs over the higher terrain as well as on the distribution of the rainfall due to the "convergence component" of the storm. Despite these deficiencies in the assumptions used to estimate PMP, improvements in the estimation of PMP may soon be possible if increased effort is placed on (amongst other things) the numerical modeling of extreme rainfall events. These improvements are only possible if the results of these efforts are communicated to the hydrological community.

The Global Energy and Water Cycle Experiment has identified the poor representation of clouds in atmospheric general circulation models as one of the major impediments for the use of these models in reliably predicting future climate change. One of the most commonly encountered types of cloud system in midlatitudes is that associated with cyclones. The purpose of this study is to investigate the representation of frontal cloud systems in a hierarchy of models in order to identify their relative weaknesses. The hierarchy of models was classified according to the horizontal resolution: cloud-resolving models (5-km resolution), limited-area models (20-km resolution), coarse-grid single-column models (300 km), and an atmospheric general circulation model (Ͼ100 km). The models were evaluated using both in situ and satellite data. The study shows, as expected, that the higher-resolution models give a more complete description of the front and capture many of the observed nonlinear features of the front. At the low resolution, the simulations are unable to capture the front accurately due to the lack of the nonlinear features seen in the high-resolution simulations. The model intercomparison identified problems in applying single-column models to rapidly advecting baroclinic systems. Mesoscale circulations driven by subgrid-scale dynamical, thermodynamical, and microphysical processes are identified as an important feedback mechanism linking the frontal circulations and the cloud field. Finally it is shown that the same techniques used to validate climatological studies with International Satellite Cloud Climatology Project data are also valid for case studies, thereby providing a methodology to generalize the single case studies to climatological studies.

The ability of General Circulation Models (GCMs) to generate Tropical Cyclones (TCs) over the North Atlantic Main Development Region (MDR; 10-20°N, 20-80°W;Goldenberg and Shapiro, 1996) is examined through a subset of ocean-atmosphere coupled simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set and a high-resolution (0.5°) Sea Surface Temperature (SST)-forced simulation from the Australian Conformal-Cubic Atmospheric Model (CCAM) GCM. The results are compared with National Center for Environmental Prediction (NCEP-2) and European Center for Medium Range Weather Forecasts Re-Analysis (ERA-40) reanalyses over a common period from 1980 to 1998. Important biases in the representation of the TC activity are encountered over the MDR. This study emphasizes the strong link inthe GCMs between African Easterly Waves (AEWs) and TC activity in this region. However, the generation of AEWs is not a sufficient condition alone for the models to produce TCs. Precipitation over the Sahel, especially rainfall over the Fouta Djallon highlands (cf. Figure 1), is playing a role in the generation of TCs over the MDR. The influence of large-scale fields such as SST, vertical wind shear and tropospheric humidity on TC genesis is also examined.The ability of TC genesis indices, such as the Genesis Potential Index (GPI) and the Convective Yearly Genesis Potential (CYGP), to represent TC activity over the MDR in simulations at low to high spatial resolutions is analysed. These indices are found to be a reasonable method for comparing cyclogenesis in different models, even though other factors such as AEW activity should also be considered.

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