An Experimental Study of the Small-Scale Variability of Rainfall

Abstract

mall-scale variability of rainfall has been studied employing six dual rain gauge sites at Wallops Island, Virginia. The rain gauge sites were separated between 0.4 and 5 km, matching the beamwidth of Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) precipitation radars. During a 2-yr observational period, over 7100 rainy samples were received at 5-min integration. A single gauge did not report as high as 67% of the time when at least one of the other gauges had rainfall in one of the seasons. Since rainfall from one of the six rain gauges is sufficient for the rainy footprint from a satellite, this demonstrates the common occurrence of the partial beamfilling. For the periods where all gauges were reporting rainfall, a single gauge had at most 13% difference from the areal average rainfall in one of the seasons. This suggests that at the spatial scale of 5 km, the variability caused by the rain gradient is relatively less important than the variability arising from a partially filled footprint. During the passage of frontal systems and tropical cyclones, the beam was filled by rain most of the time and this resulted in relatively higher correlation distances. The correlation distance had a sharp drop off from 45 km in moderately variable rainfall to 3 km in highly variable rainfall and ranged from 5 to 35 km between the different seasons. This demonstrates its highly variable nature. Considering temporal sampling, the monthly rainfall error was 35% and 73% for 3-hourly and twice-daily observations, respectively.