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Water vapor channel interpretation

By Scott Bachmeier •

GOES-12 Water Vapor (6.5 µm) image [click to enlarge]

GOES-12 Water Vapor (6.5 µm) image [click to enlarge]

GOES-12 “water vapor channel” (6.5 µm) imagery on 28 August 2006 showed a rather strong moisture gradient in the southcentral US, oriented along a cold frontal boundary. In the color enhancement applied to an AWIPS water vapor image (above), dry air is denoted by orange to yellow shades, while increasing moisture is indicated by blue to white shades; thick clouds are enhanced with white to green colors. In particular, note the sharp water vapor gradient between the dry atmosphere at Amarillo, Texas (KAMA) and the moist atmosphere at Fort Worth, Texas (KFWD):

Water vapor imagery shows us the distribution of moisture features within the middle to upper troposphere — but the actual altitude of those dry or moist layers that are being detected can vary quite a bit, depending on the temperature/moisture profile of the atmosphere. To assess the altitude and depth of the layer from which the radiation is originating, we can calculate the water vapor weighting function based upon the temperature and moisture profile for that region. The CIMSS GOES Realtime Weighting Functions site allows you to select a particular rawinsonde location to view the corresponding GOES weighting functions for a few of the imager and sounder spectral bands (including the 6.5 µm imager water vapor and the 7.4 µm/7.0 µm/6.5 µm sounder water vapor bands). Note on the plots below that in the dry air at Amarillo TX the peak altitudes of the various imager and sounder water vapor weighting functions is significantly lower than those in the moist atmosphere just 310 miles (500 km) to the southeast at Fort Worth TX:

Amarillo, TX water vapor weighting functions [click to enlarge]

Amarillo, Texas water vapor weighting functions [click to enlarge]

Fort Worth TX water vapor weighting functions [click to enlarge]

Fort Worth, Texas water vapor weighting functions [click to enlarge]

Categories: General interpretation