The ECMWF reanalysis of the MAP Special Observing Period

Abstract

AbstractA reanalysis of the Mesoscale Alpine Programme (MAP) Special Observing Period (7 September to 15 November 1999) was performed at the European Centre for Medium‐Range Weather Forecasts (ECMWF). Substantial upgrades in the global data assimilation system 4D‐Var (T511/159L60, horizontal resolution approximately 40 km) combined with special MAP observations provide a new reference description for studies on mountain‐related atmospheric phenomena. The MAP observations used in the MAP reanalysis comprise European windprofilers, high‐resolution radiosondes and surface observations. The assimilation of MAP observations results in a slight weakening of the cyclonic flow over central Europe and of the mid‐tropospheric westerly wind over the Alpine region. Also, MAP observations lead to moister conditions in the southern Alpine region, southern France and over the Adriatic Sea. The comparison of forecast daily rainfall from the MAP reanalysis with observed precipitation totals averaged over the Po catchment shows good agreement in timing and rain amount. Investigations of some Intensive Observation Periods (IOPs) illustrate specific aspects of the MAP reanalysis. Different precipitation patterns characteristic of different flow regimes are well captured (IOP2b and IOP8). The assimilation of windprofilers rectifies the divergent wind field, but changes can erroneously feed back on the humidity field (IOP2a) because of the way the humidity analysis is performed (a modified humidity analysis was introduced at ECMWF in 2003). Validation with model‐independent Global Positioning System (GPS) derived integrated water‐vapour contents shows that the humidity field describes well the inter‐diurnal humidity variations at southern Alpine GPS stations. However, the reanalysis appears to be slightly too dry compared with GPS measurements. Copyright © 2004 Royal Meteorological Society