Fourth Study Conference on BALTEX Scala Cinema Gudhjem

W(12 UTC) = 24.4 mm, W(00 UTC) = 25.5 mm, the
difference, ∆ W = – 1.1 mm, or – 4.5% from the daily mean
of W, is considerably less than standard deviation of
precipitable water in Tallinn (3.3 mm).
W (mm)
W (mm)
W (mm)
W (mm)
45
40
35
30
25
20
15
10
5
0
45
40
35
30
25
20
15
10
5
0
45
40
35
30
25
20
15
10
5
0
45
40
35
30
25
20
15
10
5
0
Spring: R 2 = 0.93
50 52 54 56 58 60 62 64 66 68
Summer: R 2 = 0.67
50 52 54 56 58 60 62 64 66 68
Autumn: R 2 = 0.95
50 52 54 56 58 60 62 64 66 68
Winter: R 2 = 0.86
50 52 54 56 58 60 62 64 66 68
Latitude degree, φ
Figure 2. Precipitable water as a function of geographical
latitude. Bold straight line on each graph – seasonal average
for 1989–2002. Thin continuous lines just above and below
the seasonal mean – extremal monthly mean values. Dotted
lines – the highest and lowest instant values of precipitable
water during a season.
Apparently, the diurnal cycle of precipitable water depends
on geographical location and is disguised by the variability
resulting from air mass changes.
Sounding profiles, in low vertical resolution WMO TEMP
format, are accessible for public use at the Web site of the
University of Wyoming (http://www.uwyo.edu). We have
considered only soundings with at least 14 levels, usually
15–30 levels were represented. For each sounding
precipitable water is already calculated and presented on the
Web site. Although the calculation scheme is not presented,
- 54 -
it appeared that for each level z, using observed relative
humidity RH, and temperature T(z), water vapor pressure
e(z) = R H·E(T) was calculated. Then, assimilating water
vapor as a perfect gas, its density was found and integrated
along the vertical profile. It should be mentioned, that
following the WMO recommendations, saturation water
vapor pressures E(T), for temperatures less than 0°C, were
also calculated with respect to water (WMO, 1988).
3. Precipitable water versus the latitude degree
In order to study tendency of increase of precipitable
water W in southern direction, we averaged the 00 UTC
values of W over four seasons:
1) spring (M – March, A – April, M – May),
2) summer (J – June, J – July, A – August),
3) autumn (S – Sept., O – October, N – November),
4) winter (D – December, J – January, F – February).
For all seasons a linear formula
W (season) = a·ϕ + b (3)
expressed latitudinal dependence of seasonal means of W
on the latitude degree ϕ (Table 2). Scatter of seasonal
means of precipitable water from the linear law is greatest
during summer and winter (R 2 = 0.67 and 0.86
respectively) when atmosphere is horizontally less mixed.
Table 2. Coefficients a, b and correlation R 2 .
Season a b R 2
Spring – 0.382 33.5 0.934
Summer – 0.345 41.3 0.675
Autumn – 0.406 37.7 0.945
Winter –0.287 25.2 0.862
Figure 2, besides a visual confirmation of applicability of
linear formula (3) for seasonal means of precipitable
water, contains information on extremal (maximal and
minimal) instant values of W, and monthly mean extremal
values during each season. For example, instant values of
precipitable water in the Baltic region never exceeded 45
mm, and monthly means 33 mm. During the winter season
the lowest values of W are in the range 1–2 mm.
Acknowledgements
This investigation was supported by national grants
No. 4140, No. 5475 and No. 5857 of the Estonian Science
Foundation. University of Wyoming is highly appreciated
for making accessible the sounding profiles.
References
WMO, Technical regulations, vol. 1, General
meteorological standards and recommended practices,
Appendix B, p.1-Ap-B-3, 1988.
Güldner J., Spänkuch D., Results of year-round remotely
sensed integrated water vapor by ground-based
microwave radiometry, J. Appl. Met., 38, 981–989,
1999.
Okulov O., Ohvril H., Kivi R., Atmospheric precipitable
water in Estonia, 1990–2002, Boreal Environment
Research, 7, 291–300, 2002.