Short period forecasting of catchment-scale precipitation. Part I: the role of Numerical Weather Prediction

摘要

A deterministic forecast ofsurface precipitation involves solving a time-dependent moisture balanceequation satisfying conservation of total water substance. A realistic solutionneeds to take into account feedback between atmospheric dynamics and thediabatic sources of heat energy associated with phase changes, as well ascomplex microphysical processes controlling the conversion between cloud water(or ice) and precipitation. Such processes are taken into account eitherexplicitly or via physical parameterisation schemes in many operationalnumerical weather prediction models; these can therefore generate precipitationforecasts which are fully consistent with the predicted evolution of theatmospheric state as measured by observations of temperature, wind, pressure andhumidity.This paper reviews briefly the atmospheric moisture balance equation and how it maybe solved in practice. Solutions are obtained using the Meteorological OfficeMesoscale version of its operational Unified Numerical Weather Prediction (NWP)model; they verify predicted precipitation rates against catchment-scale valuesbased on observations collected during an Intensive Observation Period (IOP) ofHYREX. Results highlight some limitations of an operational NWP forecast inproviding adequate time and space resolution, and its sensitivity to initialconditions. The large-scale model forecast can, nevertheless, provide importantinformation about the moist dynamical environment which could be incorporatedusefully into a higher resolution, ‘storm-resolving’ prediction scheme. style="line-height: 20px;">Keywords: Precipitation forecasting; moisture budget; numerical weather prediction