A simple four-dimensional assimilation technique, called Newtonian relaxation, has been applied to the Hamburg climate model (ECHAM), to enable comparison of model output with observations for short periods of time. The prognostic model variables vorticity, divergence, temperature, and surface pressure have been relaxed toward European Center for Medium-Range Weather Forecasts (ECMWF) global meteorological analyses. Several experiments have been carried out, in which the values of the relaxation coefficients have been varied to find out which values are most usable for our purpose.To be able to use the method for validation of model physics or chemistry, good agreement of the model simulated mass and wind field is required. In addition, the model physics should not be disturbed too strongly by the relaxation forcing itself. Both aspects have been investigated. Good agreement with basic observed quantities, like wind, temperature, and pressure is obtained for most simulations in the extratropics. Derived variables, like precipitation and evaporation, have been compared with ECMWF forecasts and observations. Agreement for these variables is smaller than for the basic observed quantities. Nevertheless, considerable improvement is obtained relative to a control run without assimilation. Differences between tropics and extratropics are smaller than for the basic observed quantities. Results also show that precipitation and evaporation are affected by a sort of continuous spin-up which is introduced by the relaxation: the bias (ECMWF-ECHAM) is increasing with increasing relaxation forcing. In agreement with this result we found that with increasing relaxation forcing the vertical exchange of tracers by turbulent boundary layer mixing and, in a lesser extent, by convection, is reduced.
As ozone mixing ratio correlates well with potential vorticity, column integrated ozone (“total ozone”) should correlate with column integrated potential vorticity. Daily maps of these fields are analyzed, and it is shown that this correlation holds well, notably during the winter and spring. During a few short periods however, the correlation breaks down.
Abstract. A three-dimensional off-line tracer transport model coupled to the ECMWF analyses has been used to study the transport of trace gases in the atmosphere. The model gives a reasonable description of their general transport in the atmosphere. The simulation of the transport of aircraft emissions (as NOx) has been studied as well as the transport of passive tracers injected at different altitudes in the North Atlantic flight corridor. A large zonal variation in the NOx concentrations as well as large seasonal and yearly variations was found. The altitude of the flight corridor influences the amount of tracers transported into the troposphere and stratosphere to a great extent.
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SUMMARYSince the disaster in The Netherlands caused by a mesoscale gale on 12 May 1983, an hourly nowcasting and very short-range forecasting system has been developed using all available hourly surface observations of pressure and wind over the North Sea and the surrounding countries.Hourly analyses and prognoses are produced using hourly interpolated background fields and lateral boundaries from the fine mesh United Kingdom prognoses, which are received from Bracknell twice a day based on 00 and 12 GMT up to 36 hours in advance at 3-hour intervals.The computational area consists of a grid of 35x35 points with a grid distance of 50 km covering the North Sea and surroundings. The update and forecast model is a two-parameter model based on filtered equations with the geopotential height fields at 500 and 1OOOmb as input and output. It is called VIMOLA (vertically integrated model on limited area). A new method is used for the analysis at 1OOOmb. The observations of pressure and wind are directly assimilated onto the computational grid, and in this way wind measurements are converted into pressure gradients. All 10-minute-mean winds are taken into account. The analysis system is called APW (assimilation of pressure and wind).Every hour analyses and prognoses are produced up to nine hours in advance. The prognoses are merged with the fine mesh U.K. prognoses, i.e. the final prognoses are a linear combination of VIMOLA prognoses and U.K. prognoses. For one hour in advance no merging takes place and after nine hours the final prognoses are identical to the U.K. prognoses.For The Netherlands and surrounding regions winds computed from the forecasts have been verified against the observations. Winds from the U.K. prognoses have been verified in the same way. The U.K. prognoses show a very good quality and a small error growth during the first 24 hours. However, the hourly incorporation of the observations gives an improvement, especially for the first hours of the forecast period.Because no upper air observations are needed, a very short data cut-off time can be applied (20-45 minutes after the time of observation). In practice, the system is suitable for operational gale warnings and tidal surge forecasts. Due to the hourly character, the system is able to detect temporal variations in the surface wind field such as the diurnal cycle over land and air mass variability. Practical examples are given in this paper, including some results obtained for the storm of 15-16 October 1987, which hit the south-eastern part of England.
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