Satellite-Monitored Snow Cover in the Climate System - Satellite-based Applications on Climate Change - page 177

Barnett et al. 1988 ; Dash et al. 2005 ; Dong and Valdes 1998 ; Vernekar et al. 1995 )
have been conducted to support this snow-monsoon inverse relationship.
The direct impact of snow cover, known as the snow albedo effect, will dramatically
change the land surface energy budget and then influences air temperature, density,
pressure, etc. Walsh et al. ( 1982 ) demonstrated that the presence of snow cover is
associated with near-surface cooling of 5-10 K in the lower troposphere. The indirect
impact, also known as the snow hydrological effect, is a result of anomalous soil
moisture from snowmelt that will later impact the atmosphere through land-climate
interactions. However, very few studies have investigated this issue, partially due to the
complicated snowmelt and runoff processes and the unavailability of accurate snow
water content datasets.
Besides the direct and indirect snow effects, positive and negative snow-atmosphere
feedbacks will further amplify or ameliorate anomalies. The most important positive
feedback is the snow albedo feedback. Snow has the highest albedo in nature. This
causes the land surface to reflect more of the incoming solar radiation. With warmer
temperatures, the area of snow-cover decreases and land surfaces absorb an increasing
fraction of solar radiation. This increase of total absorbed solar radiation contributes to
continued and accelerated melting and warming. On the other hand, a colder climate
will keep more snow cover and sustain lower air temperatures (Wiscombe and Warren
1980 ). Another important but less known negative (self-regulating) feedback is the
snowfall-stability feedback, first suggested by Walland and Simmonds ( 1996 ). With the
sudden increase in snow cover after a snow storm, the air temperature in the lower
troposphere decreases and static stability of the atmosphere increases; this reduces the
probability of subsequent snowfall; reducing snowfall further results in decreasing snow
cover by snow sublimation and blowing snow event; decreasing snow cover at the land
surface increases the sensible heating to the atmosphere and decreases the static
stability, increasing the probability of snowstorms. This negative feedback keeps the
snow cover relatively stable over high latitudes in the winter.
12.2 Satellite Snow Monitoring
Accurate observations or monitoring of the snow cover across the globe has great
potential applications to weather, climate, and hydrology. Systematic measurements
of snow depth at meteorological observation stations have been collected for over a
century. At these stations, only the presence or absence of snow along with snow
depth is measured on a daily basis by a snow stick or stake. Due to the sparseness of
measuring stations, it is difficult to adequately capture the spatial variability of snow
cover on a global scale. Furthermore, most of these observations are limited to snow
depth, which is not suitable for snow modeling due to rapid gravity compaction.
In other words, density can vary greatly, making it difficult to estimate the mass of
water in the snow pack. Until the development of automatic stations like SNOTEL
(snow telemetry) in recent decades, accurate real-time measurement of snow water
equivalent (SWE) was not available. However, the point measurement at stations can