A radar-based rainfall climatology of Great Britain and Ireland

W ether – M ay 215, ol. 0, o. 5 adjustment for the variation in the vertical profile of reflectivity, to produce an estimate of the reflectivity at the Earth’s surface (Kitchen et al., 1994; Smyth and Illingworth, 1998). This radar reflectivity is converted into precipitation rate (R) by a constant Z–R power law relationship of Z = 200R1.6 (Marshall and Palmer, 1948; Marshall et al., 1955). This retrieval does not take into account variations in the microphysical species of the precipitation (e.g. graupel, snow, or hail) as it assumes that all precipitation is liquid. However, use of a microphysical species-based Z–R relationship may only reduce the error in the precipitation rate retrieval by 10% (Koistinen et al., 2004). The retrieved precipitation rate is then corrected for orographic enhancement (Georgiou et al., 2011) and adjusted to match local gauge observations on a radar-by-radar basis (Kitchen et al., 1994; Lewis and Harrison, 2007; Harrison et al., 2012). These single-radar rain-rate retrievals are then composited to a uniform grid, weighting by both distance to the radar and pixel quality (Harrison et al., 2009; 2012). An overview of the process of precipitation retrieval from radar reflectivities over Great is known about how the radar precipitation composite performs over multi-annual scales. Some climatologies from radar-derived precipitation data have been created for the Himalayan region (Houze et al., 2007), USA (Carbone and Tuttle, 2008), The Netherlands (Overeem et al., 2009) and France (Tabary et al., 2012). Radar-based climatologies using space-based radar have also been performed (e.g. Nesbitt and And ers, 2009). In this study, we examine the accuracy of a long-term precipitation climatology derived from the Great Britain and Ireland radar precipitation composite. To this end, we use 8 years (2006–2013) of radar composite data to compute annual-average precipitation totals and compare these to annual-average precipitation totals from gauge observations over the whole of Great Britain and Ireland. A secondary outcome of this work is to provide insights into any long-term biases of the radar measurements themselves.