Precipitable Water Plots
The precipitable water plots below were created using data from the (1) NCDC Radiosonde Database of North America CD-ROMs, 1948-1997 and (2) FSL on-line radiosonde archive, 1997-2000/2005 (http://raob.fsl.noaa.gov/). The radiosonde identifiers on the map below correspond to active upper-air observing sites. However, the climatology for some of these sites includes two or more stations in order to get a lengthy time series for calculating statistics. For example, the climatology for ABR (Aberdeen, SD) was derived from upper-air observations using ABR and HON (Huron, SD), because from 1953-1995 the upper-air site was at HON and from 1995-present the site has been at ABR. For most cases, differences in station locations do not exceed 60 miles for these “combined” climatologies. The combining of sites is not expected to be problematic since these plots represent a synoptic signal and not a mesoscale signal.
The plots were created as follows: (1) precipitable water values were calculated for all available soundings for the period of record, including 00z, 03z, 12z, 15z, and special observations; (2) questionable soundings were thrown out based on gross quality control measures; (3) the data were put into monthly bins; (4) the maximum, minimum, 75th percentile, 50th percentile (median), and 25th percentile values were obtained for each month; and (5) the standard deviation was also calculated, and twice this value is plotted on the charts. Note that for normal (or Gaussian) distributions, 95% of the values lie within ± two standard deviations of the mean/average value, so when you get to +2SD on the charts, you have a pretty rare event. Since the sample size is very large (i.e., several thousand per month), the Gaussian assumption is reasonable.
The equations that were used for calculating precipitable water (units of cm) are as follows:
a) e = 6.112 * exp [ (17.67 * Td) / (Td + 243.5) ] -- where e is vapor pressure in mb and Td is dewpoint in C [refer to Bolton (1980) for details]
b) density = [ (e / (461.5 * T)) * 10^5] -- where e is from above, 461.5 is the gas constant for water vapor, T is the temperature in K, and the vapor density is in g/m^3
c) pw (layer) = [ mean density over a layer (g/m^3) ] * [thickness of the layer (m) ] * [1 / [1000,000 g/m^3] } -- where the last term is the density of liquid water
d) pw (sfc to 300 mb) = summation of pw (layer) from the sfc to 300 mb for any given sounding -- again, these equations return the pw in cm
If you have any questions regarding these plots, please send them to Matthew.Bunkers@noaa.gov.
