CIMSS Satellite Blog

McIDAS-V images of CRAS model Precipitable Water pre-forecast spin-up

MODIS instruments (see here, as well) on board NASA’s Terra and Aqua satellites offer high resolution multi-banded views of the Earth’s atmosphere. Information from the channels can be used to derive total precipitable water in regions where clouds do not exist (as explained here). In the present case, MODIS TPW is compared to colocated TPW values in a CRAS model run that is centered on the direct broadcast MODIS ground station site at SSEC. Where the values differ, mixing ratios are adjusted so that the model value more closely matches the satellite-observed TPWs (Lapse rates are preserved in the adjustment). Satellite-observed TPWs are available only in clear fields of view; cloud initializations, however, are adding information where clouds are observed.

The case above (imagery produced using McIDAS-V) shows the 12-hour pre-forecast spin-up for the model with an initial time of 12:00 UTC on 25 August 2008. Six different MODIS orbits that were received at the SSEC direct broadcast ground station between 00:00 UTC and 12:00 UTC directly affect the initial model fields that are derived from GFS output. Note how the addition of MODIS data moistens the atmosphere in and around the remains of Tropical Storm Fay over the south central US, and also moistens the atmosphere over the Pacific Ocean west of California.

This method is used to introduce satellite information downloaded locally into a model run; more accurate initial fields are helpful in producing a more accurate forecast. In the present case, once the more accurate initial fields are generated, the model then steps forward in time (with GFS fields used to constrain the boundaries).

AWIPS images of MODIS and GOES shortwave IR channels (Animated GIF)

Lightning was the cause of a wildfire in southeastern Montana (about 45 miles east-southeast of Miles City) during the afternoon hours on 22 August 2008, which burned a total of about 2600 acres. AWIPS images of the MODIS 3.7 µm and GOES 3.9 µm shortwave IR channels (above) showed a significantly warmer “fire hot spot” on the MODIS 18:20 UTC image (51.0º C, orange pixels) compared to the 18:25 GOES image (36.5º C, dark black pixels). The fire hot spot on GOES imagery did not approach the temperatures seen on the MODIS image until about 20:15 UTC.

This wildfire could be viewed using GOES-11 (GOES-West), GOES-12 (GOES-East), and GOES-13 (undergoing a period of operational testing as “GOES-Central”) — you can see that there was a slight amount of image “wobble”  on the GOES-11 and GOES-12 shortwave IR and visible images, while the corresponding GOES-13 image navigation was very steady (below). Changes to the GOES-13 spacecraft have resulted in improved image navigation compared to the previous GOES satellites.

Also note that the smoke plume on the visible images was much more apparent on the GOES-12 and GOES-13 images — this is due to the fact that the forward scattering angle was more favorable from the viewing perspectives of GOES-12 (positioned at 35º W longitude) and GOES-13 (positioned at 105º W longitude) during the late afternoon hours, which helped to highlight the smoke plume as it drifted southeastward.

GOES-11 + GOES-12 + GOES-13 shortwave IR and visible images (Animated GIF)

A plot of the GOES-11, GOES-12, and GOES-13 3.9 µm shortwave IR brightness temperature values (below) showed a fair amount of variability between the maximum “fire hot spot” temperatures sensed by the different satellites. Although there was general agreement in the trend of increasing temperatures as the wildfire continued to burn, at times the measured IR temperatures differed by as much as 20º K (for example, at 19:45 UTC).  The different satellite viewing angle was a contributing factor to some of these differences — at times the fire was partially obscured by cloudiness that was moving through the region during that period.

GOES-11 / GOES-12 / GOES-13 3.9 µm IR brightness temperatures

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)