Monitoring Change in the Arctic - Satellite-based Applications on Climate Change - page 130

Fig. 9.4 Wind vectors over the Arctic on 7 June 2009, derived fromModerate Resolution Imaging
Spectroradiometer (MODIS) data. Colors indicate wind heights, from blue (low, near surface) to
red (high, 10 km)

Wind vector heights are assigned by any one of three methods. The infrared
windowmethod assumes that the mean of the lowest (coldest) brightness temperature
values in the target sample is the temperature at the cloud top. This temperature is
compared to a numerical forecast of the vertical temperature profile to determine the
cloud height. The method is reasonably accurate for opaque clouds but inaccurate for
semitransparent clouds. The CO 2 slicing method works well for both opaque and
semitransparent clouds. Cloudy and clear radiance differences in one or more carbon
dioxide bands (e.g., 13.3, 13.6, 13.9, or 14.2 m on MODIS) and infrared window

bands are ratioed and compared to the theoretical ratio of the same quantities,
calculated for a range of cloud pressures. The cloud pressure that gives the best
match between the observed and theoretical ratios is chosen (Menzel et al. 1983 ; Frey
et al. 1999 ). The H 2 O-intercept method of height determination can be used as an
additional metric or in the absence of a CO 2 band. This method examines the linear
relationship between clusters of clear and cloudy pixel values in water vapor-infrared
window brightness temperature space, predicated on the fact that radiances from a
single cloud deck for two spectral bands vary linearly with cloud fraction within a
pixel (Schmetz et al. 1993 ). The height of clear-sky water vapor wind vectors is
determined by comparing the water vapor brightness temperature to a collocated
model temperature profile, analogous to the IR window method for cloud features.
However, the brightness temperature of the feature being tracked corresponds more
to a layer than a level, as will the retrieved wind vector height (Rao et al. 2002 ).
An example of polar winds derived from MODIS over the course of a day is
shown in Fig. 9.4 . Wind vectors are color-coded to represent their relative heights.

μ