Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Relationships Between Precipitable Water and Geographical Latitude in the
Baltic Region
Erko Jakobson 1 , Hanno Ohvril 1 , Oleg Okulov 2 , Nels Laulainen 3
1 Institute of Environmental Physics, University of Tartu, ohvril@ut.ee
2 Tiirikoja Lake Station, Estonian Meteorological and Hydrological Institute
3 Pacific Northwest National Laboratory, Richland, USA
1. Introduction
Water vapor, one of the most variable atmospheric
substances, is an important link in the hydrological cycle
and is the most important greenhouse gas. Beside this, it also
has a significant influence on the accuracy of satellite
monitoring information (satellite images) and of the GPS
applications.
Quantitatively, the total amount of water vapor in the zenith
direction, W (also called precipitable water vapor,
column(ar) water vapor, integrated water vapor, water
equivalent, or simply, precipitable water), is expressed as
mass per unit area. This unit, mass per unit area, is in
practice usually considered as the thickness of the layer of
liquid water that would be formed if all the vapor in the
zenith direction were condensed at the surface of a unit area:
1 mm of the layer corresponds to 1 kg m –2 , and 1 cm to
1 g cm –2 . In this research we quantify relationships between
precipitable water and the degree of geographical latitude in
the Baltic area.
2. Databases
Night-time radiosonde reports (00 UTC) from 17 aerologic
stations for 1989–2002 in the Baltic area (Table 1, Fig. 1)
were used to obtain values of W. The northernmost of
stations, Sodankylä (67.36°N), is located just beyond the
polar circle (66.55°), while the southermost, Wroclaw
(51.78°N), is situated 15.5 degrees southward.
Table 1. List of stations.
Nr Station Code Lat Long Elev
1 Sodankylä 2836 67.36 26.65 179 m
2 Lulea 2185 65.55 22.13 34 m
3 Sundsvall 2365 62.53 17.45 6 m
4 Jyvaskylä 2935 62.4 25.68 145 m
5 Jokioinen 2963 60.81 23.5 103 m
6 Voejkovo (St Ptrb) 26063 59.95 30.7 78 m
7 Tallinn 26038 59.38 24.58 34 m
8 Göteborg 2527 57.66 12.5 164 m
9 Riga 26422 56.96 24.05 26 m
10 Koebenhavn 6181 55.76 12.53 42 m
11 Leba 12120 54.75 17.53 6 m
12 Visby 2591 57.65 18.35 47 m
13 Schleswig 10035 54.53 9.55 48 m
14 Legionowo 12374 52.4 20.96 96 m
15 Greifswald 10184 54.1 13.4 6 m
16 Lindenberg 10393 52.21 14.11 115 m
17 Wroclaw 12425 51.78 16.88 122 m
Usually radiosondes are launched twice daily, at 00 UTC
and 12 UTC. We omitted the 12 UTC observations for
several reasons.
Figure 1. Map of considered stations.
First, and this is the main reason, the set of 12 UTC
soundings is less complete. For example, in Tallinn, the 12
UTC observations were stopped after 11.01.2001. Second,
the diurnal course of precipitable water is weak.
According to an investigation made in Podsdam (52.38 N,
13.06 E, 81 m ASL) by Güldner and Spänkuch (1999)
during 1–14 August 1997, when W was rather constant at
about 25 mm, the interdiurnal variation (difference
between the extremal values) was about 2.2 mm or less
than 10% of the daily mean of W.
Precipitable water measured at 11 UTC corresponded to
the daily mean. Note that 12 UTC = 12:52:16 Local Mean
Time in Potsdam, or the difference between local time and
UTC in Potsdam is about + 1 h. From 13 to 21 UTC there
was a broad maximum exceeding the daily mean by 0.6–
1.2 mm. At 24 (or 00) UTC the daily mean was reached
again. The lowest values of W, about 1 mm less from the
daily mean, one met from 03 to 07 UTC. In the Potsdam
experiment the 12 UTC values of W exceeded the 00 UTC
ones by 0.3 mm.
The Potsdam’s result does not coincide with the parameterization
of precipitable water according to radiosoundings
in the Tallinn Aerological Station (59.48 N,
24.60 E, 37 m ASL) during 12 years, 1990–2001. Okulov
et al. (2002) found that results for 00 UTC (= 01:38:24
Local Mean Time in Tallinn) systematically exceeded 12
UTC soundings:
W(12 UTC) = 1.58 e0 + 0.68 (mm), (1)
W(00 UTC) = 1.67 e0 + 0.49 (mm), (2)
where e0 is surface water vapor pressure in mbars. For a
typical summer value, e0 = 15 mbar, these equations give: