These forecasts owe their skill to many types of data, of which that most directly related to precipitation is a set of rain-affected microwave radiances assimilated over sea, in significant numbers from 1992 onwards. Values of precipitation come from a sequence of 12-hour background forecasts. As illustrated below, values are least reliable where surface observations are sparse and the background model forecasts of related variables such as precipitation are biased. Elsewhere, the background forecast model plays a stronger role, enabling values of surface relative humidity to be derived less directly from other types of assimilated observation. Over land, values of the relative humidity of surface air are determined quite directly from observational records for regions where plentiful observations of surface air humidity were made. The three hydrological variables chosen for display differ in the way they are constrained by observations, and in their reliability. The atmospheric observing system underwent several improvements leading up to 1979. ERA-Interim combines information from meteorological observations with background information from a forecast model, using the data assimilation approach developed for numerical weather prediction. The monthly information is from ERA-Interim, a reanalysis for the period from 1979 to the present. The warm colours used to denote above-average temperatures are used to denote regions that are drier than average, while shades of blue denote above-average precipitation, relative humidity or soil moisture. In addition, for soil moisture maps, where some land-points are undefined due to the mismatch between dataset and map-coastline resolution, these land-points are reset using interpolation with the neighbouring sea values and subsequently superimposing the land sea mask. These regions appear in grey on the maps. The qualitative statements in summaries prior to this date thus remain valid.įor all soil moisture calculations a mask is applied for regions designated to have permanent ice cover (principally Antarctica and much of Greenland) or no vegetation, or that otherwise have a climatological mean annual precipitation rate of less than 0.3mm/day. This change only affects the absolute magnitudes of soil moisture values, not the sign of the anomalies. The sub-regions differ appreciably in area, accounting respectively for about 23%, 47%, 14% and 16% of the land area designated as Europe.įrom the November 2017 summary onward, the time-series calculations for soil moisture take into account fractional values in the land sea mask. A dividing latitude of 45 ON and dividing longitude of 15 OE are used to separate this European land area into north-western (NW), north-eastern (NE), south-western (SW) and south-eastern (SE) sub-regions. The averages for Europe are taken over all land areas between 20 OW and 40 OE, and 80 ON and 35 ON. A non-linear scaling is used for some large-domain precipitation plots to enable detail to be shown in both light- and heavy-precipitation regions. Most maps of anomalies use the same colour scale in successive monthly summaries, apart from the uppermost and lowermost bands depicted in each legend, which are adjusted when necessary to cope with extremes. Maps showing precipitation data in the alternative form of percentages of climatological average values are consulted from time to time when preparing the text contained in the summaries, but are not presented in the summaries. The anomalies have units of mm/day for precipitation and % for relative humidity and volumetric soil moisture. The anomaly for a particular variable and month is the difference between the value of the variable for that month and the average value of the variable from 1981 to 2010 for that month of the year. The maps and graphs included in the monthly summaries are based on anomalies relative to the climatological averaging period 1981-2010.
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