Intermittency of Precipitation: Duration, Frequency, Intensity and Amount using Hourly Data

Detailed analyses are made of near-global gridded (about 1°) hourly or 3-hourly precipitation rates from two observational datasets (TRMM 3B42v7 3-hourly, and CMORPH hourly) both of which have been revised and bias corrected, and from the CESM model from January 1998 to December 2013.  Special runs were made of the model to obtain hourly values to explore the intermittency of precipitation: the frequency, intensity, duration and amounts, and compare with observed estimates.  The latter differ somewhat but both are considerably different from the model, which has too much precipitation overall, and it precipitates far too often at low rates and not enough for intense rates, with the divide about 2 mm h^-1.  A focus is on the duration of events, and a new metric is proposed based upon the ratio of the frequency of precipitation at certain rates (0.1 to 1 mm h^-1) for hourly vs 3-hourly vs daily amounts.  A comparison is made for all products of the conditional probability of precipitation (given previous precipitation) for various thresholds.  The model has precipitation durations of just over 1 hour for hourly data vs 1.5 to over 3 hours for observations over much of the globe. For 3-hourly precipitation CMORPH durations are about 25% greater than for TRMM. This signifies the on-again off-again nature of model grid-point precipitation even as perpetual drizzle is widespread.  The precipitation characteristics for the CESM are for a very different planet than Earth.