Permafrost

anthropogenic disturbance. Despite the reported accelerated climate change that has occurred in
high latitudes as well as high altitudes, a fundamental problem exists: climate warming has
been shown to have occurred disproportionately during the cold season; however, permafrost
degradation tends to occur at the end of the warm season, via, e.g., a deepened active layer.
An important additional variable that has not been studied extensively in regard to its
effect on permafrost is warm-season precipitation. For instance, it has been found that in spite
of increased summer surface air temperatures, summer soil temperatures actually decreased due
to abundant precipitation. Different hypotheses can be made regarding the relationship between
precipitation and permafrost: increased precipitation can increase soil moisture, which results in
a higher thermal conductivity than for dry soil. Therefore, increased precipitation could cause
higher soil temperatures. Conversely, increased precipitation can result in increased evaporation
from the soil, thereby resulting in lower soil temperatures. Additionally, increased soil moisture
from increased precipitation will increase the specific heat of the soil, requiring more energy to
increase soil temperature.
There is thus a potentially missing link in our understanding of permafrost dynamics:
warm season precipitation. This is partly due to a lack of reliable precipitation observations in
the traditionally data-sparse high latitudes. However, the Tibetan Plateau presents a unique
region to study the impacts of precipitation on permafrost. Because of its high elevation, large
parts of the plateau are underlain by permafrost. Furthermore, because of its subtropical
location, precipitation measurements are available for the last nine years (1997–present) from
the Tropical Rainfall Measurement Mission (TRMM) satellite. While there are several methods
of retrieving rainfall from satellite observations, many of these suffer from various inherent
problems, especially in snow-covered terrain. For example, microwave measurements are
commonly used to retrieve precipitation, however they require specific knowledge of the
background surface emissivity. In a snow-covered region such as the Tibetan Plateau,
characterization of the background scene is extremely difficult. The first spaceborne
precipitation radar (PR) on TRMM, therefore allows a direct measurement of precipitation that
will not suffer from the problems inherent in other precipitation retrieval schemes.
The PR footprint is approximately 4 km at nadir, however as the data are not temporally
continuous and the footprint size increases as the instruments scans away from nadir, monthly
average rain rates are calculated on 0.1° × 0.1° grid for the Tibetan Plateau. The PR data are
used to develop a rainfall climatology for the Tibetan Plateau from 1997–present. A comparison
between the PR data and a surface rain gauge network is performed to validate the satellite
retrievals. The rain gauge data from the Chinese Meteorological Administration provides daily
precipitation totals for approximately 200 stations in and around the Tibetan Plateau. While we
do not expect a one-to-one relationship between the data because of sampling differences, the
comparison allows us to validate that the PR captures the spatial variability of the rainfall on the
Tibetan Plateau. Since rain gauge data have sparse spatial coverage, the PR offers a
high-resolution dataset with a previously unavailable spatial coverage for a permafrost region.
A principal components analysis is also performed on the rainfall climatology to examine the
dominant modes of precipitation variability on the Tibetan Plateau. In addition to developing a
satellite rainfall climatology, the rainfall climatology and dominant patterns of precipitation
variability are compared to the distribution of permafrost on the Tibetan Plateau using the
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