THE INFLUENCE OF LAND-SURFACE MOISTURE RETENTION ON PRECIPITATION STATISTICS

The retention of precipitation water in land surface reservoirs damps higher frequencies of evaporation variability and can thereby influenc e the feedback of evaporation on precipitation. The extent of this inf luence is examined in a series of general circulation model simulation s in which the timescale of surface moisture retention is very careful ly controlled. Shorter timescales lead to increased daily precipitatio n variance and one-day-lagged precipitation autocorrelations but to de creased autocorrelations at longer lags. An explanation for the simula ted precipitation statistics is offered in the form of a heuristic mod el of evaporation feedback that describes precipitation variance and a utocorrelation in terms of three parameters: (i) the timescale of prec ipitation persistence in the absence of feedback; (ii) the surface ret ention timescale; and (iii) a parameter describing the atmosphere's re sponsiveness to variations in evaporation. The heuristic model reprodu ces the statistical trends seen in the GCM diagnostics, and it can be used to explain geographical variations in precipitation statistics ge nerated by a GCM coupled to a full biosphere model.