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This paper presents an approach to postprocess ensemble forecasts for the discrete and bounded weather variable of total cloud cover. Two methods for discrete statistical postprocessing of ensemble predictions are tested: the first approach is based on multinomial logistic regression and the second involves a proportional odds logistic regression model. Applying them to total cloud cover raw ensemble forecasts from the European Centre for Medium-Range Weather Forecasts improves forecast skill significantly. Based on stationwise postprocessing of raw ensemble total cloud cover forecasts for a global set of 3330 stations over the period from 2007 to early 2014, the more parsimonious proportional odds logistic regression model proved to slightly outperform the multinomial logistic regression model. [ABSTRACT FROM AUTHOR]

This paper presents the Integrated Nowcasting through Comprehensive Analysis (INCA) system, which has been developed for use in mountainous terrain. Analysis and nowcasting fields include temperature, humidity, wind, precipitation amount, precipitation type, cloudiness, and global radiation. The analysis part of the system combines surface station data with remote sensing data in such a way that the observations at the station locations are reproduced, whereas the remote sensing data provide the spatial structure for the interpolation. The nowcasting part employs classical correlation-based motion vectors derived from previous consecutive analyses. In the case of precipitation the nowcast includes an intensity-dependent elevation effect. After 2--6 h of forecast time the nowcast is merged into an NWP forecast provided by a limited-area model, using a predefined temporal weighting function. Cross validation of the analysis and verification of the nowcast are performed. Analysis quality is high for temperature, but comparatively low for wind and precipitation, because of the limited representativeness of station data in mountainous terrain, which can be only partially compensated by the analysis algorithm. Significant added value of the system compared to the NWP forecast is found in the first few hours of the nowcast. At longer lead times the effects of the latest observations becomes small, but in the case of temperature the downscaling of the NWP forecast within the INCA system continues to provide some improvement compared to the direct NWP output. [ABSTRACT FROM AUTHOR]

In this paper the skill of seasonal prediction of river discharge and how this skill varies between the branches of European rivers across Europe is assessed. A prediction system of seasonal (winter and summer) discharge is evaluated using 1) predictions of the average North Atlantic Oscillation (NAO) index for the coming winter based on May SST anomalies of the North Atlantic; 2) a global-scale hydrological model; and 3) 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) data. The skill of seasonal discharge predictions is investigated with a numerical experiment. Also Europe-wide patterns of predictive skill are related to the use of NAO-based seasonal weather prediction, the hydrological properties of the river basin, and a correct assessment of initial hydrological states. These patterns, which are also corroborated by observations, show that in many parts of Europe the skill of predicting winter discharge can, in theory, be quite large. However, this achieved skill mainly comes from knowing the correct initial conditions of the hydrological system (i.e., groundwater, surface water, soil water storage of the basin) rather than from the use of NAO-based seasonal weather prediction. These factors are equally important for predicting subsequent summer discharge. [ABSTRACT FROM AUTHOR]

At the end of December 1999, two extremely severe storms only one day apart affected western Europe and caused considerable damage. A variable derived from satellite observations, the so-called temperature of the lower stratosphere (TLS), is used in this study for detecting and tracking the upper-level components of these storms. TLS is computed from a regression over five Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS, aboard NOAA satellites) channels, with coefficients calculated from a climatological dataset [thermodynamical initial-guess retrieval (TIGR)], and provides information on the temperature near the tropopause. The objective of this paper is to assess the ability of TLS, in situations such as these two exceptional storms, to track and depict upper-tropospheric precursors of surface lows. After a brief synoptic description of the meteorological situation, TLS fields as well as the Action de Recherche Petite Échelle Grand Échelle (ARPEGE) model fields (mean sea level pressure, temperature, wind velocity, and geopotential height of the dynamical tropopause) are discussed concurrently for the period 23-27 December. Although the upper-level thermal fields are consistent overall, differences appear, especially during the incipient stage of the second storm. The forecast, which was poor in the operational context, is modified when a configuration close to the TLS one is adopted. Qualitative comparisons of TLS with Microwave Sounding Unit (MSU) channel-3 limb-corrected brightness temperatures and with the water vapor imagery are also shown. One advantage of TLS over these two other fields is the earlier detection of the upper-level precursor of the second storm. Because TLS computation is easy and fast, the suitability of TLS as a possible forecasting aid over midoceanic regions is promoted. [ABSTRACT FROM AUTHOR]

ABSTRACT The European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA) results for July 1987 are compared with stratocumulus observations for the same period made during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE). The FIRE I experiment, conducted off the coast of California, provided a comprehensive observational set of data on marine stratocumulus during July 1987. The observations reported in this paper and used as a basis for comparison with the ERA results include the mean diurnal variation of cloud cover, liquid water path, cloud-base and cloud-top height, incoming short-wave and long-wave radiation at the surface, and monthly mean vertical profiles of (potential) temperature and specific humidity. It is concluded that the ERA thermal structure in the boundary layer is quite realistic and the ERA data reveal some diurnal variability. However, in ERA the cloud thickness is overestimated and cloud cover and liquid water path are strongly underestimated as compared with the FIRE I observations. As a result the ERA downwelling short-wave radiation at the surface is much larger than observed in FIRE I. It is argued that the large difference between the ERA downwelling shortwave radiation at the surface and that observed in FIRE I can lead to large biases in the sea surface temperature in a coupled ocean-atmosphere model. [ABSTRACT FROM AUTHOR]

Results are presented from the project Clear-sky Longwave from ERA (CLERA), in which simulations were performed of clear-sky longwave fluxes and heating rates for the period 1979-93, based on data from the European Centre for Medium-Range Weather Forecasts Re-Analysis project (ERA). This paper presents selected results from CLERA and compares the clear-sky outgoing longwave radiation (OLR) with data from the Earth Radiation Budget Experiment (ERBE). Over much of the globe, especially over the oceans, the clear-sky OLR from CLERA is within the expected uncertainty in the ERBE data of 65 W m 22 . Elsewhere, there are larger differences and to study these the ERA data are compared with independent sources of information: surface synoptic observations of screen-level temperatures and retrievals of the total column moisture and upper-tro-pospheric humidity from satellite data. Over land, the largest clear-sky OLR differences occur at high latitudes in winter and these can be explained by the fact that the ERA surface temperatures are too low in these regions. However, for many other regions over land there was no obvious explanation for the clear-sky OLR differences. Over the oceans, the clear-sky OLR differences in the Tropics are consistent with known systematic biases in ERBE, the most important consequence of which is that the ERBE clear-sky OLR is too high in convective regions such as the ITCZ. [ABSTRACT FROM AUTHOR]