Using MODIS imagery to detect thin stratus/fog over snow cover

February 19th, 2008 |

MODIS visible + snow/ice + IR images (Animated GIF)

AWIPS images of the MODIS visible channel, the 1.6µm near-IR “snow/ice channel”, and the 11.0µm “IR window” channel (above) revealed a large patch of supercooled water droplet stratus cloud (and/or fog) over northeastern Utah on 19 February 2008. This stratus/fog cloud feature was confined to the lower elevations (AWIPS topography image | Johns Hopkins topography image) where several river valleys converge just south of Vernal, Utah (station identifier KVEL); at the time of the images, the surface visibility at Vernal was restricted to 1 mile with fog. The areal extent of this cloud feature was difficult to judge using either the 1-km resolution MODIS visible image (due to the bright white appearance of both the stratus/fog and the surrounding non-cloudy snow cover) or the 1-km resolution MODIS IR window image (due to the light gray enhancement of both the cloud feature and the cold air that had collected within the remainder of the lower elevations) — but on the snow/ice image, the contrast of the brighter gray-to-white enhancement of the supercooled water droplet stratus/fog feature really stood out against the dark appearance of the surrounding snow-covered terrain.

The corresponding 250-meter resolution MODIS true color image from the SSEC MODIS Today site (below, viewed using Google Earth) showed the “translucent” nature of the patch of stratus/fog — note that you can actually see the outlines of some of the converging rivers (the Green River, White River, and Duchesne River) through the optically-thin cloud feature.

MODIS true color image (Google Earth)

Streaks of snow cover in the Northern Plains

February 18th, 2008 |

MODIS true color + false color images (Animated GIF)

A comparison of MODIS true color and false color RGB images (above) reveals multiple narrow streaks of snow cover across parts of the Northern Plains on 18 February 2008. The white snow streaks show up well against the surrounding brown bare ground regions in central South Dakota and Nebraska on the true color image (created using MODIS channels 01/04/03 as red/green/blue); in the corresponding false color image (created using MODIS channels 02/07/07 as red/green/blue), the snow cover exhibits a red enhancement, while supercooled water droplet cloud features appear as varying shades of white.

Snow cover in southern Califorinia and Baja California

February 15th, 2008 |

MODIS true color image (Google Earth)

MODIS true color imagery from the SSEC MODIS Today site (above, viewed using Google Earth) shows extensive snow cover over the higher elevations of southern California (and Baja California in Mexico) on 15 February 2008. According to media reports, this snow closed sections of Interstate 8 (between Alpine and Ocotillo) for more than 12 hours on the previous day; as much as 8.5 inches of snow was reported at Cuyamaca Peak (elevation 4820 feet) in San Diego county.

An AWIPS image of the MODIS Land Surface Temperature (LST) product (below) indicated that LST values were in the 30º-40º F range (green enhancement) in the region of the snow cover, compared to the much warmer bare ground areas to the west and to the east where LST values were generally between 60º and 70º F (orange enhancement).

MODIS Land Surface Temperature product

How fast can 1-4 inches of snow cover melt?

February 14th, 2008 |

GOES-12 visible images (Animated GIF)

According to GOES-12 visible channel imagery (above), less than 12 hours if you’re in the Piedmont region of Virginia and North Carolina, given the presence of dry air and the “warming power” of the increasing sun angle of mid-February! Besides the rapid disappearance of the snow cover on 14 February 2008, there are 2 other interesting features evident in this animation of GOES-12 visible imagery: (1) tiny cloud plumes, likely originating from large industrial sites (I’m guessing either power plants, or mining operations) along the West Virgina / Ohio border region — the cloud plumes can be seen early in the loop, drifting initially eastward, then northeastward, and then northward as the morning boundary layer winds slowly changed direction; and (2) a fairly long smoke plume drifting southeastward from a fire that was burning along the eastern edge of the Great Dismal Swamp (which straddles the Virginia / North Carolina border) — this smoke plume (along with the late morning snow cover) shows up even better on MODIS true color imagery (below, viewed using Google Earth). This particular fire had been burning since 11 February.

MODIS true color image (Google Earth)