Application Of Automatic Weather Station Data In Severe Convection Short-term Forecasting And Nowcasting

With the outstanding feature of suddenly happening, it is hard to forecast severe convection weather accurately and timely just using large-scale conventional observation data in summer. High space resolution detection data has to be used as an suppliment. This paper selects severe convection cases in summer (June to August) of two national automatic station in Fangshan District(in southwest of Beijing) in recent five years(2007-2011) to classify these cases into four categories:Northeast Cold Vortex-type, Mongolia Low Vortex-type, Westerly Trough-type and Atypical situtation-type according to weather systems and their moving paths. Then uses automatic weather station data of Beijing to analyse variation features of meteorological element fields in different types of cases before severe convection happening. At last finds out the correlationship between the variation features and severe convection occurence in order to provide scientific basis for Severe Convection short-term forecasting and nowcasting. The results show that:1. Large-scale weather background analysis is necessary for short-term forecasting and nowcasting, but it has great limits too. It reflects in the following aspects:(1) Different disastrous weather has different circulation background, so we can basically estimate the type of disastrous weather if circulation situation has been determined.(2) Severe convection potential can be estimated through physical diagnosis.(3) Due to the low spatial and temporal resolution of large-scale conventional observation data, in terms of some severe convection weather, when large-scale analysis shows the circulation situation is not the typical background, it is impossible to forecast their occurence, because meso-micro scale weather systems can not be recognized. For other severe convection weather, the potential is only the general judgement about the tendency and area of occurrence and development of weather system even if it can be estimated. It is difficult to predict the accurate time, place and intensity of the suddenly happening, small scale and short-time severe convection.(4) Severe convection weather short-time forecasting and nowcasting has to be divided into two steps:firstly, analyse the large-scale weather background and make preliminary estimation of occurrence and development tendency and disastrous weather type; secondly, use high spatial and temporal resolution data as supplement to predict the time and place of severe convection.2. The variation of automatic weather station meteorological element fields display some regulations an hour before the occurence of severe convection:(1) Meteorological element shows different variation in the four type severe convection cases. Among them, Northeast Cold Vortex-type has the largest range of variation, the range of the others is a little weaker.(2) In general, hour-to-hour temperature-change, three hour pressure-change and wind(especially hour-to-hour temperature-change) fields show clear variation, which should be paid more attention to. The total surface temperature and air temperature display relatively weak variation, which even can not be found in a few Atypical situtation-type cases. However, these two elements still have high indicative significance. Dew-point temperature and relative humidity have the smallest range of variation, and the variation of relative humidity is particularly weak. Basically, besides in small number of strong synoptic systems their variation regulation can be captured, it is hard to find out clear variation of these two elements most of the time, so they are less indicative physical quantity.(3) The variation of automatic weather station meteorological element fields generally shows up the formation of dense contour in the upstream area and its moving towards downstream region with the emerging of max value center one hour before the occurence of severe convection. In terms of hour-to-hour temperature-change with most significant variation, temperature drop range of it surpasses5degrees centigrade per hour before disastrous weather happens. It shows that the invasion of the dry cold air plays an very important role in triggering severe convection. The variation of wind field differs from the other meteorological element fields, which manifestes the convergence or shear of wind direction. The severer of weather, the more remarkble of the variation. Besides, the establishment of easterly and southerly air current which helps the water vapor transport should be noticed. In a few severe convection cases with less significant wind field variation, easterly and southerly air current often has already existed for a long time before the occurence of convection weather. In addition to this, the total surface temperature field has its own feature of the decrease of temperature after the convection happens, which can be regarded as an indicator.3. Application examines results shows that the variation of automatic weather station meteorological element fields can make up for the deficiency of large-scale weather analysis to a large extent so as to provide great help for severe convection short-time forecasting and nowcasting. Therefore, when variation regulation of meteorological element fields is captured, severe convection short-time forecasting and nowcasting can be made one hour before their occurence, so that decision-making section and the public have relatively sufficient time to deal with it. Nonetheless, in some local severe convection process, the variation is relatively weaker and it shows features in a very short time ahead of the occurence of disastrous weather, so this type of severe convection short-time forecasting and nowcasting still have large difficulty.4. In addition, forecaster should pay attention to the following points in severe convection short-time forecasting and nowcasting:(1) thunderstorm gale and hail weather process have severer variation of meteorological element fields. These two types of disastrous weather are more likely to happen when the range of variation is very large.(2) Make comprehensive analysis on different meteorological element fields, if most of them have evident variation regulation, the probability of the occurence of severe convection is much greater.(3) The forecasting and nowcasting of severe convection which happens in the nighttime is quiet difficult. Many meteorological elements change weakly at night because of diurnal variation, in this case the variation of meteorological element fields can hardly indicate the occurence of severe convetion.