Properties of normalised rain-rate distributions in the tropical Pacific

Passive microwave retrievals of rainfall rate are examined for the tropical Pacific region. Frequency distributions of two years of daily rain rates were compared for coarse grainings (arithmetic averages) of 1, 2, 4, 8, 16 degrees squares for nominal warm sea (warm pool) and relatively cooler sea (cold pool) regions. The histograms displayed a gamma distribution shape with an exponential tail to high rain rates and a power law for lower rain rates (<20 mm d(-1)). The distributions over warmer sea were broader than their counterparts over cooler sea. Coarse graining of the rain rates reduced the breadth of the distributions but the shape was retained. Conditional sampling of 1 degrees rain rates for larger regions with the same mean rain rate shows that the distributions of 1 degrees rain rates within bigger regions of the cold or warm pool with the same mean rain rate are very similar. Additionally, it was found that these 1 degrees rain-rate distributions could be successfully normalised to reveal an underlying dimensionless rain-rate distribution shape. Normalised rain-rate distributions are shown to be insensitive to sensor type, time of day and sensitivity perturbations. This makes the normalised rain-rate distributions good candidates for comparison with model output. Normalised rain-rate distributions from a large-domain cloud-resolving model (CRM) and two operational global models (Met Office UM, ECMWF IFS) were compared with the satellite passive microwave data. The models generally produce more frequent high-intensity rain rates than the satellite passive microwave data suggest. The CRM performs better than the global models in reproducing the normalised distributions obtained from the satellite passive microwave data. This difference is attributed to the CRM's ability to resolve convection. (C) Crown Copyright 2009. Reproduced with the permission of HMSO. Published by John Wiley & Sons, Ltd.