The Automated Surface Observing System (ASOS) is a key component of the modernization program in the National Weather Service (NWS). Among the automated sensors in this sophisticated instrument package is a ceilometer which can delineate clouds up to 12,000 feet. Using radiances measured from the GOES-8/M sounding instrument, a technique for calculating cloud top pressure is used to augment the ASOS cloud observation above 12,000 feet. From these GOES radiance calculations, images are created to show cloud top pressure over the United States. For each cloudy field of view, the CO2 Absorption Method is used to calculate the pressure of the cloud top and the effective cloud amount. Currently, the processing algorithm for GOES-8/M utilizes measurements in the 15 micron CO2 absorption band (channels 2, 3, 4, and 5) for cloud height calculation and two long wave (11-12 micron) window channels (7 and 8) for surface skin temperature determination. With the GOES sounders now operational instruments, data are routinely received over the US, and a new image can be created every sixty minutes.
In order to visually enhance the range of cloud top pressures, the images are color coded according to cloud top pressure. In each image, the gray-shaded areas depict clear regions. The color levels define pressure (in hectoPascals) ranging from orange at the lower heights (higher pressures), to blues and white at the higher heights (lower pressures). There are limitations in the coverage due to the location of the GOES satellites (over the equator and typically at either 90 or 135 degrees West longitude) and the curvature of the earth, rending data beyond 62.5 degrees viewing angle to be of reduced quality. Thus, the "step effect" observed in the extreme northeast and northwest portions of the image is the effective limit of this product. The GOES derived cloud top pressure is a very useful product for aviation, as well as aiding the initialization of numerical models.
From the early days of GOES-8, a historical sample loop of four cloud top pressure images (
0046UTC,
0846UTC,
1246UTC,
and 1946UTC
) is shown for 28 November 1994.
Nowadays, hourly composites of the cloud top pressure images derived from the Sounders on both GOES satellites (-East and -West) are available in near real-time.
