Assessment of Met Office Unified Model (UM) quantitative precipitation forecasts during the Indian summer monsoon: Contiguous Rain Area (CRA) approach

The operational medium range rainfall forecasts of the Met Office Unified Model (UM) are evaluated over India using the Contiguous Rainfall Area (CRA) verification technique. In the CRA method, forecast and observed weather systems (defined by a user-specified rain threshold) are objectively matched to estimate location, volume, and pattern errors. In this study, UM rainfall forecasts from nine (2007–2015) Indian monsoon seasons are evaluated against \(0.5^{\circ }\times 0.5^{\circ }\) IMD–NCMRWF gridded observed rainfall over India \((6.5^{\circ }{-}38.5^{\circ }\hbox {N}, 66.5^{\circ }{-}100.5^{\circ }\hbox {E})\). The model forecasts show a wet bias due to excessive number of rainy days particularly of low amounts \(({<}1\,\hbox {mm}\,\hbox {d}^{-1})\). Verification scores consistently suggest good skill the forecasts at threshold of \(10\,\hbox {mm}\,\hbox {d}^{-1}\), while moderate (poor) skill at thresholds of \({<}20\,\hbox {mm}\,\hbox {d}^{-1}\,({<}40\,\hbox {mm}\,\hbox {d}^{-1})\). Spatial verification of rainfall forecasts is carried out for 10, 20, 40 and \(80\,\hbox {mm}\,\hbox {d}^{-1}\) CRA thresholds for four sub-regions namely (i) northwest (NW), (ii) southwest (SW), (iii) eastern (E), and (iv) northeast (NE) sub-region. Over the SW sub-region, the forecasts tend to underestimate rain intensity. In the SW region, the forecast events tended to be displaced to the west and southwest of the observed position on an average by about \(1^{\circ }\) distance. Over eastern India (E) forecasts of light (heavy) rainfall events, like \(10\,\hbox {mm}\,\hbox {d}^{-1}\) (20 and \(40\,\hbox {mm}\,\hbox {d}^{-1}\)) tend to be displaced to the south on an average by about \(1^{\circ }\) (southeast by \(1{-}2^{\circ }\)). In all four regions, the relative contribution to total error due to displacement increases with increasing CRA threshold. These findings can be useful for forecasters and for model developers with regard to the model systematic errors associated with the monsoon rainfall over different parts of India.