In order to determine cloud-top heights, knowledge of the cloud top temperature and the temperature structure in the atmosphere is required. Instruments that utilize the infrared (IR) window channel (11.0 um ) only provide estimates of the cloud-top temperatures. On the other hand, data from Rawinsondes, Climatologies and Standard Atmospheres provide the relationships needed to convert such temperatures to heights. However, the major difficulty with the use of these methods arises from the fact that the atmosphere is neither spatially nor temporally constant. Accordingly, errors in excess of 10,000 feet can result. A more accurate means of relating temperature to heights occurs through the use of numerical weather prediction model (NWP) data.
One such method of estimating cloud-top heights currently used at NRL utilizes IR data from geostationary satellites for cloud-top temperatures and standard pressure level air temperatures derived from the Naval Operational Global Atmospheric Prediction System (NOGAPS). The cloud-top temperatures from the IR data are related to model heights by finding the height at which the model derived air temperature matches that of the cloud-top temperature. The resulting heights are real heights above sea level and are then related to the observed Standard Atmosphere altimeter readings used by pilots.
Several examples of the differences between the NRL method and those using Standard Atmospheres highlight the large discrepancies that are possible. The final products produced by NRL methods demonstrate their applicability to tropical weather and their usefulness during Global Hawk flights, Ocean Convection Nowcast Demonstration and Operation Enduring Freedom.