CIMSS Satellite Blog

GOES-12 and GOES-13 visible images (Animated GIF)

A comparison of GOES-12 and GOES-13 visible images centered near Rolla (station identifier KVIH) in southern Missouri (above) showed that widespread river valley fog was dissipating during the morning hours on 27 August 2008. Note the improvement in image-to-image navigation on the GOES-13 data (less “wobble” of the images), due to changes in the spacecraft design on the newer GOES-13 satellite.

A comparison of GOES-11 and GOES-13 water vapor channel images (below) demonstrates the improved detection of mesoscale mountain waves over far southwestern Alberta and northwestern Montana on that same day. The spatial resolution of the GOES-13 water vapor channel is 4 km, compared to 8 km on the older GOES-11 satellite.

GOES-11 and GOES-13 water vapor images (Animated GIF)

Mountain waves seen on water vapor imagery have long been recognized as an indicator of potential clear air turbulence. There was one pilot report (PIREP) of light to moderate turbulence at an altitude of 32,000 feet near Great Falls, Montana around 17:40 UTC (below).

AWIPS image of GOES water vapor image + PIREP

AWIPS images of the MODIS + GOES-12 "IR window" channel (Animated GIF)

Areas of low cloudiness (such as stratus and/or fog) at night can be important aviation hazards, but they are sometimes difficult to identify simply by examining standard IR imagery. A comparison of 1-km resolution MODIS 11.0 µm and 4-km resolution GOES-12 10.7 µm “IR window” images (above) does not give the sense that there was widespread fog and stratus clouds in place over much of eastern Colorado during the pre-dawn hours on 20 August 2008. Even with a color enhancement that highlights the warmer range of IR brightness temperatures (below), the edges of the low cloud feature are difficult to pick out unambiguously.

AWIPS images of the MODIS + GOES-12 "IR window" channel (Animated GIF)

There are a variety of satellite products available in AWIPS that can be used to better identify and characterize areas of low cloudiness and fog. In this case, the 1-km resolution MODIS fog/stratus product (below) showed very good details about the edges and structure of the area of low clouds and fog, with the corresponding  4-km resolution MODIS Cloud Top Temperature and Cloud Phase products  indicating that the cloud tops over eastern Colorado were composed of water droplets (blue enhancement) with cloud top temperatures around +5º C (red enhancement). However, note that there was another separate area of supercooled cloud tops farther to the east (located over western Kansas), where the Cloud Top Temperatures were below freezing (yellow enhancement) while the Cloud Phase product still indicated water droplet clouds (blue enhancement) — regions such as this might pose a higher risk for aircraft icing.

AWIPS images of the MODIS fog/stratus + cloud top temperature + cloud phase products (Animated GIF)

Using the GOES-12 satellite, one can examine the 4-km resolution fog/stratus product (below), and also utilize the 4-km resolution Low Cloud Base product along with the 10-km resolution sounder-derived Effective Cloud Amount and Cloud Top Height products to gain additional information about the cloud base height, cloud coverage, and cloud top height.

AWIPS images of the GOES-12 fog/stratus + low cloud base + effective cloud amount + cloud top height products (Animated GIF)

Fog that formed in the Gulf of Maine on Tuesday July 8th developed when relatively moist air moved from the continent over the cooler ocean waters and was cooled from beneath by conduction to the dewpoint. In other words, Advection Fog.

An obvious question arises from the loop: Why does the fog persist over the ocean east of Nantucket and along the coast of Maine even as it clears out in the central part of the Gulf of Maine? The answer is shown in the color-shaded image of SST (available from the Applied Physics Lab at Johns Hopkins University) below. Warmer waters over the central Gulf of Maine will promote the faster erosion of ocean fog because of enhanced vertical mixing because of lower stability. Ocean fog that moves over land — as over southwestern Nova Scotia — also dissipates as heating over land promotes vertical mixing. Dryer air aloft mixed towards the surface reduces the boundary layer relative humidity and the fog evaporates. The dryer air just above the surface is readily apparent in Skew-T plots from Yarmouth, Nova Scotia and from Gray, Maine.