Geoscience Reference
In-Depth Information
Figure 4.6.
The annual mean and summer (June-August) tropopause pressure (hPa)
based on the lapse rate definition derived from ECMWF data for 1200 UTC, 1979-1993
(adapted from Hoinka,
1998
, by permission of AMS).
4.3
The Arctic Tropopause
4.3.1
Deinition
The tropopause is the transition between the troposphere and stratosphere (see
Figure 4.1
). The tropopause can be defined on both thermal and dynamic cri-
teria. The thermal definition is based on the lapse rate. The standard World
Meteorological Organization definition of the tropopause is the lowest level above
500 hPa where the vertical temperature gradient decreases to less than or equal
to 2°C km
−1
(provided that the average gradient in the upper 2 km layer is <2°C
km
−1
). The existence of a nearly constant lapse rate in the troposphere and an iso-
thermal lower stratosphere generally allows the tropopause to be distinguished
in the Arctic even during the polar night (Highwood, Hoskins, and Berrisford,
2000
). The dynamical criterion is based on an arbitrary threshold value of PV. K.
Hoinka (
1998
) finds that the spatial pattern of tropopause heights based on the
lapse rate criterion and on a value of 3.5 PV units (PVU = 1.0 x 10-
6
m
2
s
−1
K kg
−1
)
is generally similar.
4.3.2
Mean Characteristics
The lower boundary of the thermal tropopause at 85-90°N, based on soundings
from the NP program for 1954-1991, has a minimum altitude of about 8.4 km in
April and a maximum of 9.4 km in August (Nagurny,
1998
). The mean thickness
of the tropopause layer is only about 600 m over the Kara-Laptev seas in January,
whereas in summer, it exceeds 1.6 km thickness in a belt from northern Norway to
northern Novaya Zemlya and eastward to Wrangel Island (Makhover,
1983
).
