Ernst Klinker

The diagnosis of mechanical dissipation in the atmosphere from large-scale balance requirements

Abstract The momentum budget for January 1987 is evaluated with global observations analyzed at the European Centre for Medium-Range Weather Forecasts (ECMWF). The dissipation term is diagnosed from the budget as a balance requirement, that is, as that required to balance the sum of the advection, Coriolis, pressure gradient, and local tendency terms. This is then compared with the parameterized subgrid-scale effects in the ECMWF models momentum equation, with a view of identifying possible errors in those parameterizations. The balance requirement does not support the high parameterized values of orographically induced gravity-wave drag in the lower stratosphere. A deeper analysis also does not suggest a major role for turbulent vertical transports above the boundary layer. On the other hand, our budget does indicate that more effort be spent on a better representation of the potential enstrophy cascade associated with Rossby wave breaking in the upper troposphere. These statements are qualified by the ...

Comparison of Ceilometer, Satellite, and Synoptic Measurements of Boundary-Layer Cloudiness and the ECMWF Diagnostic Cloud Parameterization Scheme during ASTEX

Abstract Cloud fraction is a widely used parameter for estimating the effects of boundary-layer cloud on radiative transfer. During the Atlantic Stratocumulus Transition Experiment (ASTEX) during June 1992, ceilometer and satellite-based measurements of boundary-layer cloud fraction were made in the subtropical North Atlantic, a region typified by a 1–2 km deep marine boundary layer with cumulus clouds rising into a broken stratocumulus layer underneath an inversion. Both the diurnal cycle and day-to-day variations in low-cloud fraction are examined. It is shown that ECMWF low cloudiness analyses do not correlate with the observed variations in cloudiness and substantially underestimate the mean low cloudiness. In these analyses, the parameterization of low cloud fraction is primarily based on the inversion strength. A comparison of ECMWF analyses and ASTEX soundings (most of which were assimilated into the analyses) shows that the thermodynamic structure of the boundary layer and the inversion strength a...

Relation between mean boundary-layer structure and cloudiness at the R/V Valdivia during ASTEX

Abstract The relationship between boundary-layer thermodynamic structure and cloud fields and their diurnal variation are explored using seven days of data from dw R/V Valdivia during the Atlantic Stratocumulus Transition Experiment. Cloudiness is at a maximum before dawn, when the boundary layer (BL) has the thermodynamic structure of a partially mixed, conditionally unstable stratocumulus layer, which is close to mean saturation below the inversion. Cloudiness falls during the daytime, and in the late afternoon the BL has two distinct layers: a warmer, drier cloud layer (characteristic of trade cumulus) above a more well-mixed subcloud layer. The observed mean profiles are consistent with an earlier suggestion that there is a cloud-cover transition once the BL mixing-line slope exceeds half that of the moist adiabat. In contrast, the BL structure in the ECMWF model for the same week has a much drier, warmer, more stable “cloud” layer than the observations.

Reanalysis and reforecast of three major European storms of the twentieth century using the ECMWF forecasting system. Part I: Analyses and deterministic forecasts

In Part I of this study recent versions of the ECWMF Integrated Forecasting System (IFS) are used together with historical observational data to carry out reanalyses and deterministic reforecasts of three major north-west European wind storms of the twentieth century. The storms considered are the Dutch storm of 1 February 1953, the Hamburg storm of 17 February 1962, and the British October storm of 1987 (Great October Storm). Common to all these storms is their severity, which caused large loss of life and widespread damage. Reanalysis of the storms is based on a 3D-Var and 4D-Var assimilation scheme at a horizontal resolution of TL159(≈ 125 km) and TL511(≈ 50 km), respectively. Similarly, two different horizontal resolutions (TL159 and TL511) are used to investigate the deterministic predictability of these storms. The lower-resolution system is exactly that used in the ERA-40 reanalysis project. The high-resolution system is a more recent version of the ECMWF IFS. It is shown that the basic characteristics of the Dutch and Hamburg storms that gave rise to the storm surge are well predicted by the single deterministic forecasts up to about 48 and 84 hours, respectively, in advance. Our capability to predict the Great October Storm is more difficult to assess. On the one hand, even recent versions of the ECMWF IFS underestimate the severity of the storm in the very short-range (12–24-hour forecasts started at 12 UTC 15 October 1987). On the other hand, the high-resolution version of the ECMWF IFS provides excellent deterministic forecasts of the track and intensity of the storm up to 96 hours in advance. However, there are errors in the timing of the storm (12 hours for the 96 hour forecast). From the results presented in this study it is concluded – bearing in mind the limited number of cases considered – that with the current ECMWF forecasting system reliable deterministic predictions of some European wind storms are possible several days in advance. Copyright

Reanalysis and reforecast of three major European storms of the twentieth century using the ECMWF forecasting system. Part II: Ensemble forecasts

In Part II of this study the ECMWF Ensemble Prediction System (EPS) is used to study the probabilistic predictability of three major European storms of the twentieth century. The storms considered are the Dutch storm of 1 February 1953, the Hamburg storm of 17 February 1962, and the British/French storm of October 1987 (Great October storm). Common to all these storms is their severity that caused large loss of life and widespread damage. In Part I of this study it has been found that deterministic predictability of the Dutch and Hamburg storms amount to 48 and 84 hours, respectively. Here, it is shown that the ensemble forecasts supplement the deterministic forecasts. The large number of members in the 48 and 84 hour ensemble forecasts of the Dutch and Hamburg storms, respectively, suggest that at this forecast range and for these storms the sensitivity of the forecasts to analysis and model uncertainties is rather small. From these results, therefore, it is argued that reliable warnings (i.e. low probability for the occurrence of a forecast failure) for the Dutch and Hamburg storms could have been issued 48 and 84 hours, respectively, in advance, had the current ECMWF EPS been available. For the Great October storm it has been found in Part I of this study that short-range and medium-range forecasts of the intensity and track of the storm were very skilful with a high-resolution model of the ECWMF model. The actual timing of the storm, however, was difficult to predict. Here, it is shown that the EPS is capable of predicting large forecast uncertainties associated with the timing of the Great October storm up to 4 days in advance. It is argued that reliable warnings could have been issued at least 96 hours in advance had the ECMWF EPS been available. From the results presented in this study it is concluded that an Ensemble Prediction System is an important component of every early warning system for it allows an a priori quantification of the probability of the occurrence of severe wind storms. Copyright

Aspects of Large Scale Modelling

Large scale modelling plays in important part in several areas, which will be covered by GEWEX activities. One objective, the determination of the hydrological cycle and the energy fluxes from observations, requires an analysis system that can process satellite data and in situ data in an optimum way. As the available observational data is insufficient for a comprehensive description of the state of the atmosphere, the information from current observations has to be complemented by short range forecast fields. In this way the numerical model acts as a tool to interpolate the fields in space and extrapolate them in time. The quality of numerical weather prediction models (NWP-models) has been improved to such an extent, that short range forecast errors in data dense areas may be more accurate than observations. Even in data sparse regions the forecast fields can provide useful information for the quality control of observations.

Applications of satellite radiance data in the medium range forecasting environment at ECMWF

Abstract Some applications of satellite radiance data in the medium range weather forecasting environment are described. In the first, data is used as a proxy for rainfall rates, which may be used for diabatic initialization of the model. The second uses the data to study the behaviour of the parametrized radiation scheme within the forecast model. Finally there is an example of a verification of the integrity of the analyses by a comparison of observed and inferred radiances.