气象科技英语-3G.ppt

1、Prof. Shou ShaowenCollege of Atmospheric Sciences,Nanjing Uni. Of Information Science GOES can also transmit processed satellite images in facsimile传真and the NOAA polar orbiters have an automatic picture transmission (APT) system that is utilized at 900 stations worldwide.,2Forecasting Modern foreca

2、sting did not become possible until weather information could be rapidly collected, assembled组装，汇编and processed. The first development came in the middle of the last century with the invention of telegraphy, which permitted immediate analysis of weather data by the drawing of synoptic charts. These

3、were first displayed in Britain at the Great Exhibition of 1851 Sequences of weather change were correlated with barometric pressure patterns both in space and time by such workers as Fitzroy and Abereroleby, but it was not until later that theoretical models of weather systems were devisednotably t

4、he Bjerknes depression model. Forecasts are usually referred to as short-range, medium (or extended) range and long-range. The first two can for present purposes be considered together.,Short-range forecasting Forecasting procedures developed up to the 1950s were based on synoptic principles but, si

5、nce the 1960s, practices have been revolutionized by numerical forecasting models and the adoption of nowcasting techniques. During the first half of the century, short-range forecasts were based on synoptic principles, empirical rules and extrapolation of pressure changes. Since 1955 routine foreca

6、sts have been based on numerical models. These predict the evolution of physical processes in the atmosphere by determinations of the conservation of mass, energy and momentum. The basic principle is that the rise or fall of surface pressure is related to mass convergence or divergence, respectively

7、, in the overlying air column.,Forecast practices in the major national centers are basically similar. The forecasts are essentially derived from twice-daily (00 and 12 GMT) prognoses of atmospheric circulation. Since most techniques are now largely automated, the analyses of synoptic fields are bas

8、ed on the previous 12-hour forecast maps as a first guess. Three different interpolation methods are used to obtain smoothed, grided data on temperature, moisture, wind and geopotential height for the surface at standard pressure levels (850,700,500,400,300,250,200 and 100 mb) over the globe. The NM

9、C currently has two basic prediction models: a special model with (6 or) 12 layers (from the boundary layer into the upper stratosphere), which is integrated for up to 10 days, and a regionally applicable nested grid model with finer horizontal resolution. It should be noted that typically the compu

10、ter time required increases several-fold when the grid spacing is halved减半. The essential forecast products are MSL pressure, temperature and wind velocity for standard pressure levels, 1000500 mb thickness, vertical motion and moisture content in the lower troposphere, and precipitation amounts.,Ac

11、tual weather conditions are now commonly predicted using the Model Output Statistics (MOS) technique developed by the US National Weather Service. Rather than relating weather variables to the predicted pressure/height patterns and taking account of frontal models, for example, a series of regressio

12、n equations are developed for specific locations between the variable of interest and up to 10 predictors calculated by the numerical models. Weather elements so predicted for numerous locations include daily maximum/minimum temperature, 12hour probability of precipitation occurrences and precipitat

13、ion amount, probability of frozen precipitation, thunderstorm occurrence, cloud cover and surface winds. These forecasts are distributed as facsimile maps and tables to weather offices for local use.,Errors in numerical forecasts arise from several sources. One of the most serious is the limited acc

14、uracy of the initial analyses due to data deficiencies. The average over the oceans is sparse and only a quarter of the possible ship reports may be received within 12 hours; even over land more than one-third of the synoptic reports may be delayed beyond 6 hours. However, satellite derived informat

15、ion and aircraft reports can help fill some gaps for the upper air. Another limitation is imposed by the horizontal and vertical resolution of the models and the need to parameterize sub-grid processes such as cumulus convection. The small-scale nature of the turbulent motion of the atmosphere means

16、 that some weather phenomena are basically unpredictable, for example, the specific locations of shower cells in an unstable air mass. Greater precision that the showers and bright periods or scattered showers of the forecast language is impossible with present techniques. The procedure for preparin

17、g a forecast is becoming much less subjective, although in complex weather situations the skill of the experienced forecaster still makes the technique almost as much as art as a science. Detailed regional or local predictions can only be made within the framework of the general forecast situation f

18、or the country and demand thorough knowledge of possible topographic or other local effects by the forecaster.,Nowcasting Severe weather is typically short-lived (2 hr) and, due to its mesoscale character(100 km), it affects local/regional areas necessitating site specific forecasts. Included in thi

19、s category are thunderstorms, gust fronts, tornadoes, high winds especially along coasts, over lakes and mountains, heavy snow and freezing precipitation. The development of radar networks, now instruments and high speed communication links has provided a means of issuing warnings of such phenomena.

20、 Several countries have recently developed integrated satellite and radar systems to provide information on the horizontal and vertical extent of thunderstorms, for example. Such data are supplemented by networks of automatic weather stations (including buoys) that measure wind, temperature and humi

21、dity. In addition, for detailed boundary layer and lower troposphere data, there is now an array of vertical soundersacoustic sounders (measuring wind speed and direction from echoes created by thermal eddies), specialized (Doppler) radar measuring winds in clear air by returns either from insects (

22、35 cm wavelength radar) or from variations in the airs refractive index (10 cm wavelength radar). Nowcasting techniques use highly automated computers and image analysis systems to integrate data from a variety of sources rapidly. Interpretation of the data displays requires skilled personnel and/or extensive software to provide appropriate information. The prompt forecasting of wind shear and downburst hazards at airports is one example of the importance of nowcasting procedures.,Overall, the greatest benefits from improved forecasting can be exp