Water vapor channel interpretation

August 28th, 2006 |

Today’s GOES-12 6.5 micrometer “water vapor channel” imagery shows a rather strong moisture gradient in the southcentral US, oriented along a cold frontal boundary. In the color enhancement applied to the AWIPS water vapor image below, dry air is denoted by the orange to yellow shades, while increasing moisture is indicated by the blue to white shades (thick clouds are enhanced with the white to green colors). In particular, note the sharp water vapor gradient between the dry atmosphere at Amarillo TX (KAMA) and the moist atmosphere at Fort Worth TX (KFWD):
AWIPS water vapor image
The water vapor channel shows us the distribution of moisture features in the middle to upper troposphere, but the actual altitude of the dry or moist layers that are being detected by the water vapor channel 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 website allows you to select a particular rawinsonde location and then view the GOES weighting functions for a few of the imager and sounder channels (including the imager water vapor channel and the 3 sounder water vapor channels). Note that in the dry air at Amarillo TX (below, left) the altitude of the peaks of the various imager and sounder water vapor channel weighting function plots is significantly lower than those in the moist atmosphere just 310 miles (500 km) to the southeast at Fort Worth TX (below, right):
Amarillo TX weighting functionsFt. Worth TX weighting functions