WIND FIELD SIMULATION IN METEOROLOGICAL APPLICATIONS IN CROATIA

This paper describes two numerical models for computing the mesoscale influences of orography, friction and heating on the wind field. The DMI model is a diagnostic, one-level, primitive equation model and ths DM2 is a mass-adjusted, three dimensional wind field model where interpolated three-dimensional mean winds are adjusted in a weighted least-squares sense to satisS the continuity equation within the volume specitied. The models are capable of simulating a broad scale of atmospheric phenomena such as orographic channelling, effects due to changes in atmospheric stability, land and sea breezes, and anabatic and katabatic winds. The main attention is paid to the performed models' applications for different purposes in the Meteorological Service of Croatia. The given application examples illustrate the broad spectrum of applications made possible by the models'flexibility in treating different weather situations and regions. Key word index: Numerical modelling, wind field, meteorological application. Saietak U radu su pokazaneiiroke moguinosti kori5tenja numeridkih modela za simulaciju strujanja u orografski sloZenim uvjetima. U hrvatskoj meteorolo5koj sluibi primijenjivana su dva numeridka modela za simulaciju mezoskalnog utjecaja orografrje, trenja i zagrijavanja na polje vjetra: DMI-dijagnostidki, jednorazinski model s primitivnim jednadZbama gibanja i DM2{rodimenzionalni model s ukljudenim uvjetom kontinuiteta mase unitar podrudja rada modela. Analiza rezultata primjene modela na razliditm podrudjima Hrvatske pokazalaje da dobiveno polje vjetra odraiava mnoge realne karakteristike strujanja. Ukazanoje na neophodnost raspolaganja ovakvim numeridkim modelima, narodito u podrudjima s rijetkom mreZom meteorolo5kih mjerenja koja ne omoguiava dobivanje dovoljno realne slike strujanja u orografski veoma razvijenim podrudjima. Kljutne ryec:i: Numeridko modeliranje, polje vjetra, primjena meteorologije. INTRODUCTION cumstances. The advantages which wind can have in physical planning and housing are numerous: it is a Coasts and mountains can greatly modify synoptic source of power, it ventilates, it evaporates moisture scale weather in the lower troposphere. Topographic and dries surfaces, etc. As for the disadvantages, wind blocking, deflection and channelling, differential heatspreads smoke and odours, it assists snow accumulaing and cooling, and differential friction produce tion and the penetration of water into surfaces, it remesoscale circulations that greatly influence and even quires stronger construction to resist it, and so on. If dominate local weather. architects are aware of the direction of prevailing The mesoscale and local circulation significantly winds, they can design accordingly. Planners may site influence many aspects of human activities such as industrial quarters in a way to prevent the winds physical planning and housing; design; agriculture;air, blowing smok€ and other impurities into residential arland and water transport; energy resources; environeas. mental protection etc. Layout design atso has a marked effect upon air Why worry about wind in planning? It is because motion. Wind in hot, moist climates is an asset and wind can be man's friend or enemy under different cirshould be exploited as natural air conditioning, for it tends to reduce temperatures and excessive humidity. In colder regirnes, the main effect of the wind is to induce convective cooling and thus lower the sensation of temperature. Many of the climatic effects on buildings require both macroand microanalysis. This is particularly true of the role of wind for in many situations local topographic effects give rise to highly specialized conditions. For example, wind channelling through valleys requires special design. All phases of air, sea and land transport are influenced by atmospheric conditions. The effects are felt at all levels of operation, from the construction of roads, railroads, landing strips to ths on route trip and the landing and take-off conditions in air transport' On route, aircraft face many problems, Many of these are concerned with meteorological conditions and include such factors as the selection of an optimum cruise altitude in relation to upper level winds. Wind also affects plants in many ways. One of the most obvious effect is the physical damage that high wind can cause. Wind speed influences the plant carbon dioxide intake, and transpiration rates increase with increasing wind speed. Local winds often play a very important part in modifuing the entire plant cover in regions where they prevail. During the last several years, interest has increased in the transport, diffusion, and environmental impact of air pollutants in regions of complex land forms such as valleys, hills and mountains. Knowledge of the wind field over complex terrain is a prerequisite for the analysis of pollutant dispersion and transport in the atmosphere. One possible way to know the mesoscale wind field is to install a dense wind measurement network. This is very expensive and not possible everywhere. The measured wind data available for the mesoscale and local air flow analyses, especially for past weather situations are almost always inadequate or nonexistent. Because of that, in data-sparse areas numerical models are used for wind field simulations which provide as many data as needed for the analysis. Several models may be used to provide two{r three-dimensional wind fields: a) dynamic models predict the evolution of the planetary boundary layer and, hence, of temporal and spatial variation of wind velocity, but only at a relatively high computational expense; b) interpolation techniques reconstruct the wind field on the basis of available measurements. These simple methods are in most cases inadequate, as the provided wind fields in general violate mass conservation; c) diagnostic wind models are a suitable compromise between computational efficiency and modelling accuracy: an initial interpolation of existing measurements provides an estimate for the wind field which is subsequently adjusted to satisfy given conditions. Hrvatski meteoroloSki dasopis, 28, 1993. In the Meteorological Service of Croatia the diagnostic model has been applied for wind field simulation in different regions of the country and for different purposes. The first developed version of the diagnostic model was based on the one-level primitive equation model constructed by Danard (1977). Although this model is simple it gives useful informations about the surface wind field. However, the two dimensional wind field turned out not to be sufficient for many applications which required a third dimension, such as an air pollution transport estimate. The incorporation of a third dimension became, therefore, necessary. For that purpose the mass-adjusted three-dimensional wind field model (MATHEW) made by Sherman (1978) was used for construction of second version of the DM2 model. The second version of the diagnostic model constructed in the Meteorological Service" of Croatia (DM2) uses an initial surface wind field obtained by the DMI model. In the case of a neut'rally stratified atmosphere the DM2 model incorporates the channelling effects caused by river valleys in the model domain according to Wippermann's approach (Wippermann, 1984; Bajii, 1984). The third wind component was obtained by including the continuity equation according to MATHEW. The programming code of Sherman's model was made by Rakovec in the Slovenian Hydrometeorological Institute. The final part of the DM2 model is a calculation of air particle trajectories. In such a way, for the first time, several model approaches were implemented in one model. The model begins with measurements at one point of the model domain and ends with three-dimensional trajectories in the whole domain. The aim of this paper is to give a short description of the DMI and DM2 models and a review of their applications by the Meteorological Service of Croatia.