Near-Global Covariability of Hourly Precipitation in Space and Time

A detailed analysis of hourly precipitation from 60 degrees N to 60 degrees S for the covariability is performed at 0.25 degrees resolution using the new CMORPH dataset. For all points, correlations are computed with surrounding points both concurrently and for various leads and lags up to a day. Results are more coherent over the oceans than land; the contours of constant correlation tend to be elliptical, oriented northeast-southwest in the northern extratropics and southeast-northwest in the southern extratropics. An ellipse is fitted to the correlation pattern, and major and minor axis vectors and eccentricity are mapped. Based upon both the isotropic correlations and ellipse, points are allocated to one of 20 clusters, and 16 are documented. Over the main extratropical ocean storm tracks, correlations exceed 0.8 for points 50 km distant and fall to about 0.3 at about 5 degrees radius. In the tropics values drop to 0.65 within 50 km and 0.2 at 5 degrees radius. Over land, values are lower in summer and drop to 0.1 at 5 degrees radius. Decorrelation e-folding distances range from less than 50 km over land to 200 km over extratropical ocean storm tracks. The movement of precipitation is compared with mean atmospheric winds. The lead-lag relationships indicate movement of systems but reveal the relatively short lifetimes of precipitation, of less than 12 h, even taking movement into account. The orientation of the ellipse reflects the structures of rain phenomena (fronts, etc.) rather than movement. These statistics demonstrate that daily averages fail to capture the essential character of precipitation.