Surface Data Analysis
Surface Plotting
Surface plotting involves the printing of text and/or symbols on a user defined map. The surface plotting program is sfcwx. The amount of available surface data, if fully plotted, can result in station data overlapping other plotted data. As a result, a filtering mechanism is used to limit the number of stations plotted. The plotting filter can be done using two criteria:
- Station priorities: each station in the city database is prioritized. Priorities run from 1 (highest) to 6 (lowest). The lower the priority, the more stations plot. The default priority is set by the domain size but can be modified using the stat_prior resource.
- Grid filtering: in this mode, only a set number of stations will plot within a grid box as specified by the plot_domain. There is no control over which station plots. Its first come, first plot.
The data are plotted on a user defined base map. This plotting domain is determined either by entering the domain as part of the plot_domain resource or through the Region menu. The menu will have a list of preset regions but if a new region is desired, a plot domain can be specified. The easiest way to do this is by specifying the central latitude and longitude along with a plot size. The plot sizes run from .5 for state or province sized maps to 1 for regional maps, 2.5 for country wide maps and 5 for continental maps. The size of the domain will determine the default station priority to plot. If needed, the latitude and longitude can be replaced with a specific station identifier to center the plot on.
The default background map is a simple low resolution map containing the US plus most of North America (wxp.map). If the plot domain is outside the US, the global maps (cont.map, country.map) will display.
Once the plotting domain has been selected, a variable will be plotted. This can be specified through the variable resource or through a variable menu. a menu of possible surface variables to be plotted is listed. This is a preset list of variables to plot but the whole list is defined by the internal surface variables and those defined in the sfcwx.var file. These include all reported variables saved by the convert process plus derived variables like potential temperature, relative humidity, lifted condensation level (LCL), wind chill and heat index. In addition, there is a composite plot of data:
The composite plot of all data displays temperature, dewpoint, sea level pressure, cloud cover, wind speed and direction and current weather. Valid meteorological symbols that are used to represent weather are:
The wind barb symbols point in the direction from which the wind is blowing and the number of barbs correspond to the wind speed. Each long barb represents ten knots, each half barb represents five knots, and each pennant (looks like a small triangle) represents 50 knots.
Missing data are not plotted. The sfcwx program will also generate flight conditions based on ceiling and visibility information and will plot "VFR", "MVFR" or "IFR". Also, NCEP frontal analyses can be plotted.
Finally, the data type information is printed in the upper left and date/time information is printed in the upper right. A sample surface plot is:
sfcwx -cu=la -re=mw -va=all -de=d
Surface Gridding and Contouring
Surface data are gridded and contoured using the sfccalc program. This program reads surface converted files to fit a specific variable to a grid. These variables represent standard observed and several derived variables. Examples of available derived variables include potential temperature, vapor pressure, relative humidity, wind chill, heat index, convergence and vorticity. As in the surface plotting program, data selection via priority level is necessary to reduce computation time. The grid conforms to the regions used in the surface data plotting program. When using the subregions, more stations are used and, therefore, a more resolute grid is obtained. Using higher priority stations, and, therefore, more reliable data, also reduces grid contamination from bad data elements. Grids can be saved to a file for later use by this program and the contour program. An example of a surface temperature and wind vector field is:
sfccalc -cu=la -re=mw -va=temp -in=3 -pl=cf -de=d
Surface Time Series Plotting
Surface station time series may be plotted with the statlog program. This program can work in either of two modes. The first searches through a series of surface converted data files for a specified station identifier. The second uses surface MOS station data which will be discussed later. The resulting time series information is displayed in a tabular format for easy user viewing. Then, a meteogram will be displayed. The all data meteogram contains temperature, dewpoint plotted in the top graph. This plot scale adjusts to the maximum and minimum temperatures in the data set. The second graph contains cloud level information. Here short lines represent cloud height plotted on a logarithmic height scale. A short line represents scattered (2/8), two dashes are broken (6/8) and a long line for overcast. An X specifies obscured. Cloud ceilings are listed if the ceiling is below 10,000 feet. The third graph is the pressure chart. This will represent sea level pressure or altimeter setting if sea level pressure is not reported at the station. Other plotted information is current weather, snow cover, 3 and 6 hour precipitation totals, visibility in miles, cloud cover, winds and gusts:
statlog -cu=la -nh=-25 -id=KPHL -de=d -va=all
For further information about WXP, email devo@ks.unisys.com
Last updated by Dan Vietor on June 4, 1998