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structure. Serious attention has also been paid to analysis of data on the content of
water vapor in the stratosphere and mechanisms for the formation of thin cirrus
clouds in the tropics. Randel et al. (2003) undertook studies of the structure and
variability of the temperature
field in the upper troposphere and lower stratosphere
of the tropics (at altitudes about 10
30 km) from the data of radio-occultation
observations for the period from April 1995 till February 1997, using the satellite
system designed for geodetic measurements (GPS). A comparison with a large
number (several hundreds) of synchronous aerological soundings has shown that a
retrieval of the vertical temperature pro
-
les from GPS/MET data provides reliable
enough information.
Analysis of the obtained results suggested that the spatial structure and vari-
ability of the tropopause altitude determined by a
“
cool point
”
(minimum tem-
perature) of the vertical temperature pro
le are governed mainly by wave-like
fl
fluctuations like Kelvin waves. A strong correlation was observed between tem-
perature from GPS/MET data and outgoing longwave radiation, which can serve as
an indirect
indicator of penetrating convection in the tropics. This correlation
con
fluctuation revealed from GPS/MET data and
opens possibilities of quantitative assessments of the response of large-scale tem-
perature
rms a reality of temperature
fl
field in the tropics to time-varying conditions of convection, revealing
coherent wavelike variations at altitudes between 12 and 18 km.
5.3.3 Snow and Ice Cover
Snow and ice cover estimation is important for climate change studies and suc-
cessful water resource management. Sea ice is important component in the global
climate system. Sea ice regulates of heat, moisture and salinity in the polar oceans.
Sea ice area is considerable varied in both hemispheres. In the Northern Hemi-
sphere, the total extent of sea ice varies from a minimum of about 7.8
10
6
km
2
in
×
10
6
km
2
September to a maximum of about 14.8
×
in March. In the Southern
10
6
km
2
Hemisphere, the extent varies from about 4.0
×
in February to about
10
6
km
2
in September (Parkinson 1994; Hall et al. 1995; Parkinson et al.
1987; Orheim 1999). Since the end of the 1960s, a 10 % decrease of the snow cover
extent has been observed, as well as a 2-week reduction of the annual duration of
lakes and river ice-cover in NH middle and high latitudes, while in the non-polar
regions mountain glaciers are retreating. In 2002, NH snow cover extent constituted
25.4 million km
2
, on the average by 0.2 million km
2
, less than during the preceding
30 years. The annual trend of snow cover changes from 2.7 (August) to 46.9
(January) million km
2
(Waple and Lawrimore 2002). Also, starting from the 1950s,
the extent of NH ice cover in spring and summer has been decreasing by 10
20.0
×
-
15 %.
“
—
”
During the last decades (in the periods
) the Arctic sea ice
cover thickness has probably decreased by about 40 %, with winter decrease being
less substantial. From regular satellite observations (starting from the 1970s) no
marked trend in the extent of ice cover in the Antarctic has been observed.
late summer
early fall
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