Fractional coverage of rainfall over a grid: Analyses of NEXRAD data over the southern Plains

An important parameter in land surface hydrology is the fractional area of a model grid receiving rainfall when rain is observed. Using NEXRAD (Next Generation Weather Radar) hourly rainfall estimates for the southern Plains, we examined the temporal variability in this quantity and tested the important assumptions of the threshold method. The following conclusions are reached. First, seasonal/diurnal variations explain less than 18% of the variance at all spatial scales examined; an efficient method of computing fractional coverage must consider the large event-scale variability. Second, stationarity in conditional distributions of spatial rainfall, a key assumption in the threshold method, is not warranted at the event scale for small grids or at the seasonal/ diurnal scale for large grids. Third, biases are introduced by the high correlation between the parameter and the independent variable of the threshold method. Fourth, the spatial conditional mean rain rate is higher than its temporal counterpart for all cases examined; the ergodicity assumption overpredicts the fractional coverage. Two simple methods were proposed to solve the above problems. Results of an intercomparison showed that they can modestly improve the threshold method. One important parameter in modeling land surface water and energy fluxes is the fractional area of a model grid that is covered by rain when rain is observed within the grid. This fractional coverage (FC) is defined as the ratio of the area receiving rain to the total grid area. This quantity determines the degree of localization in atmospheric forcing and hence the land surface response and has been shown to make fundamen- tal differences in the modeled energy and water fluxes (e.g., Pitman et al., 1990). In the present study we attempt to address a few issues regarding the estimation of FC, with three specific objectives. First, we look at the temporal variations in FC for a given spatial scale. In particular, we look for the timescales (e.g., seasonal, diurnal, or event) at which most of the temporal variations are observed. Second, we evaluate one of the most successful methods of computing FC, the threshold method (e.g., Kedem et al., 1990) and its adaptation by Eltahir and Bras (1993) for hydrologic modeling. In particular, we test, at vari- ous spatial and temporal scales, the underlying statistical as- sumptions that are central to the validity and robustness of the method. Third, we propose two simple procedures to improve the threshold method at spatial scales relevant to hydrologic studies and examine their merits by comparing their perfor- mance against that of the original threshold method. The radar rainfall data used for this study are NEXRAD (the Next Generation Weather Radar) hourly rainfall product. A description of the initial products from NEXRAD is given by Hudlow et al. (1991) and Klazura and Imy (1993). The hourly rainfall is compiled from volume scans at every 6 min and projected onto the Hydrological Rainfall Analysis Project (HRAP) at a spatial resolution of about 4 km. Wherever radar measurements are missing, ground rain gauge data are used.