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Ensemble data assimilation

Meteorological data assimilation provides the initial conditions for the numerical weather forecasting system forecasts. These initial conditions give an accurate an assessment as possible of the current atmospheric conditions. In order to generate these initial conditions, a large amount of measurement data are combined optimally with calculated model conditions with the help of data assimilation to arrive at inherently consistent overall atmospheric and ground conditions. The MeteoSwiss ensemble forecasting systems COSMO-1E and COSMO-2E are based on a multitude of forecasts with slightly differing boundary and initial conditions. To generate these initial conditions, MeteoSwiss deploys KENDA (kilometre data assimilation), the ensemble data assimilation system developed by the COSMO consortium.

Data assimilation is the term given to the process which provides the initial atmospheric conditions, also known as the “analysis”, which is used as the starting point for a model forecast. For this purpose, previous model condition forecasts and real-time observations of temperature, pressure, humidity, wind and precipitation that are distributed irregularly throughout the forecasting region, are combined in such a way as to arrive at a best estimate of the current atmospheric conditions on the model grid.

In the case of ensemble systems such as COSMO-1E and COSMO-2E, a host of slightly differing initial conditions is required. This analysis ensemble is calculated at MeteoSwiss with the help of the KENDA (kilometer-scale ensemble data assimilation) system. For high-resolution, convective-scale ensemble forecasting, the analysis ensemble on the COSMO-1E grid with 1.1-km grid spacing and 80 layers is updated with new measurement data in an assimilation cycle on an hourly basis. The method employed - the Local Ensemble Transform Kalman Filter (LETKF), evaluates observation data and model conditions together with the relevant uncertainties to arrive at the best possible estimate of the current atmospheric conditions. LETKF also provides a multitude of slightly differing analyses that represent the uncertainty of the analysis. Using this analysis ensemble as a basis, the COSMO-1E forecast is generated once every three hours (00:00, 03:00, ... 21:00 UTC). To obtain the initial atmospheric conditions for the COSMO-2E forecasts, the analysis ensemble is interpolated every six hours (00:00, 06:00, 12:00 and 18:00 UTC) by upscaling from the higher-resolution COSMO-1E grid to the COSMO-2E grid.

Measurement data in the data assimilation process

In the assimilation cycle, MeteoSwiss uses all available observations at ground level as well as, and especially, in the atmosphere, in order to be able to describe the atmospheric conditions as accurately and as physically consistently as possible. The KENDA System currently uses measurements from the stations in the ground-level monitoring network, from radiosoundings, aircraft reconnaissance (AMDAR and MODE-S), wind profiler data and notifications from shipping vessels and buoys. Weather radar data, which are particularly valuable for the first few hours of a forecast, are also assimilated with the help of so-called latent heat nudging (LHN).

MeteoSwiss is continually working on integrating new monitoring systems into the data assimilation cycle. At the moment, for instance, the use of wind lidars and temperature radiometers is being investigated as part of the EMER-Met project.

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