A GIS application for weather analysis and forecasting

At NCMRWF, data are received from thousands of weather observing stations over the globe for weather analysis and forecasting. After plotting the observations following a synoptic model, the analyst checks the chart for erroneous and inconsistent values of weather parameters, frequently, it becomes necessary for the analyst the suspected reading with neighboring stations and or previous observations and analyses or observations at other levels in the vertical. The Arc View plotted maps can be utilized for comparing the values of a particular parameter with the neighboring stations, at different levels and between observations and analyses for past days, for properly assessing the accuracy of the observation for inclusion in the analysis or exclusion.

The observations coming from the observing stations, through several processing steps are transferred into weather forecasts. First step in this process is to plot the observations over a base map of the region, global or regional according to the area of interest, after removing possible observational and communication errors.


Figure 2: Wind analysis for 500 hPa on 17th Dec. 2000 at 5.30 A. M. over India and neighborhood areas.


Figure 3: Geoptoential height analysis at 850 hPa level on 17th Dec. 2000 at 5.30 A. M. over India and neighborhood areas.

The plotted maps are analyzed to bring out different weather systems in action in the atmosphere at that particular instant of observation. Fig. 2. shows, such an analyzed chart generated using ArcView GIS at NCMRWF for 5.30 A.M. observation time on 17th December 2000. In the figure, the red arrows represent the wind at 500-hPa level of the atmosphere. The hollow head of the arrow represents the direction towards which the wind is blowing. Fig. 3. presents the geopotential height analysis at 850 hPa on the same day and time.

The forecaster draws contours or isobars of pressure and marks the fronts, lows highs etc. on the chart. The contouring can be done by Arc View plotted map using the necessary analysis tools. In this application, Troughs, ridges, highs and lows can be drawn on the chart in appropriate color and style, and saved as a part of the chart or a separate view. The successive movement of these systems also can be drawn on the same chart by super imposing the successive charts. The trends and rate of movement of the systems can be studied either directly from the charts or using the theme attribute table.


Fig. 4. Wind flow pattern over India and neighborhood at 500 hPa on 17th Dec. 2000 at 5.30 A.M (IST). The green line represents the low pressure troughs. The same trough as manifested at 850 hPa is also marked in the chart.

Through combining analyses at various levels with past analyses, the weather forecaster endeavors to visualize in his mind the weather processes at work, like the large-scale vertical motion, convection, radiative or advective cooling, etc. Fig. 4. is an ArcView generated map of the wind flow pattern at 500 hPa with the low pressure trough marked in green. The same trough as manifested at 850 hPa also is marked in the same chart by superimposing the 850 hPa wind flow pattern over the 500 hPa wind flow pattern. The locations of the trough at various levels of the atmosphere helps the forecaster in understanding the tilt, if any of the trough with elevation, which has high bearing on the expected weather of the trough.

A forecaster studying the sequence of evolution of a weather system in a current chart requires analyzed chart for the previous hour of observation, and indicate the movement of weather systems like centers of low and high pressure, trough and ridges, etc. This is commonly done by marking positions of the centers of the system at the 6-hourly intervals and joining the successive points by a broken line, resulting tracks give ideas on movement of the system. In the Arc View plotted analyzed maps, the time sequence of maps can be super imposed and locations marked. The distance between successive locations is automatically obtained form the tools available. Conventionally, this work is done by the synoptitian using an illuminated tracing or 'light' table. The successive charts are placed one above the other and successive positions of the systems are marked.

In figure 5, the movement of the trough along the 500 hPa level during the period 11th to 19th December 2000 is shown. Superimposing the 500 hPa wind flow pattern successively, for these days in the ArcView GIS created the map. Such maps help the forecaster study the speed and intensity of the trough in the past days for forecasting its behavior in the coming days. In the GIS distance between successive locations of the trough is readily available through the click of the mouse at the locations of interest, for calculation of its speed of progression.

 
Fig. 5. Locations of a low pressure trough in the westerlies at 500 hPa at 5.30 A. M. IST during 11th to 19th Dec. 2000.