Mesoscale Convective Vortex in Nebraska

June 24th, 2008 |

GOES-12 10.7µm IR images (Animated GIF)

A large mesoscale convective system (MCS) developed over Nebraska during the pre-dawn hours on 24 June 2008, and AWIPS images of the GOES-12 10.7 µm IR channel (above) showed the extensive coverage of very cold cloud top temperatures (-60 to -70º C, red to black color enhancement) associated with this convective activity. This MCS was responsible for some weak tornadoes and small hail across parts of Nebraska (SPC storm reports), but the main impact was heavy rains that produced flooding — in Nebraska, 3.70 inches was reported at Gibbons, and 2.10 inches fell at Waterloo in a 1-hour period.

As the MCS moved eastward and began to decay during the daytime hours over Iowa and Missouri, the cyclonic circulation of a mesoscale convective vortex (MCV) became apparent over southeastern Nebraska on both the GOES-12 IR imagery above, and also on GOES-12 visible channel imagery from the UW-Madison AOS site (below). This MCV apparently played a role in the development of new convection later in the day over southeastern Nebraska, which produced hail up to 1.0 inch in diameter.

GOES-12 visible images (Animated GIF)

Fire activity in California

June 23rd, 2008 |

GOES-11 visible images (Animated GIF)

Numerous wildfires were started by lightning activity across Northern California on 21 June 2008, and GOES-11 visible images from 22 June (above) showed a large number of smoke plumes drifting across that region. A MODIS true color image from the SSEC MODIS Today site (below) showed that areas of thick smoke remained across much of northern California on 23 June, with some of the smoke still trapped in the valleys of the North Coast Range. The thick smoke drifting eastward on 22 June reduced surface visibility to 2 miles at Redding (station identifier KRDD), and was causing major air quality problems over a good deal of that area.

MODIS true color image

An AWIPS image comparison of the 1-km resolution MODIS 3.7 µm and the 4-km resolution GOES-11 3.9 µm shortwave IR channels from around 06 UTC on 23 June (11 PM local time on 22 June) demonstrated the improved ability to detect many of the smaller fires using higher spatial resolution data (below) . In addition, the actual locations of the larger fires were more correctly depicted on the 1-km MODIS image; the comparatively large 4-km GOES IR field of view (and to a lesser extent, the large geostationary satellite viewing angle) tends to diminish the accuracy of such small-scale image details. Improved fire monitoring will eventually be possible using the Advanced Baseline Imager (ABI) instrument on the upcoming GOES-R satellite (scheduled to be launched in 2014), which will offer similar IR imagery at a 2-km spatial resolution (at 5-minute intervals on a routine basis).

MODIS + GOES-11 shortwave IR images (Animated GIF)

UPDATE: A MODIS true color image from 24 June (below, displayed using Google Earth) showed that the thick smoke had increased in areal coverage across much of northern California. MODIS Aerosol Optical Depth values as high as 1.0 were seen across much of the Sacramento Valley region of northern California; surface visibilities on that day at Redding and Red Bluff were as low as 1 mile due to smoke.

MODIS true color image (Google Earth)

Convective cloud top properties: temperature and ice particle size

June 19th, 2008 |

MODIS IR images (Animated GIF)

Numerous severe thunderstorms developed along the foothills of the Rockies and the High Plains during the afternoon hours on 19 June 2008. AWIPS images of the MODIS 11.0 µm “IR window” channel, Cloud Top Temperature product, and 3.7 µm “shortwave IR” channel (above) showed some of these areas of convection around 20:00 UTC (2 PM local time). Shortly after the time of these MODIS images, hail up to 2.75 inches in diameter was reported near Pueblo, Colorado (SPC storm reports). About 90 minutes after the time of the MODIS images, hail of 4.50 inches in diameter and a wind gust to 81 mph was produced by the storm that was seen developing in the Texas panhandle region. Although these storms exhibited similar cold cloud top temperatures on the IR window image (-60 to -77º C, red to black to gray color enhancement) and the Cloud Top Temperature product (-60 to -69º C, darker blue to light violet color enhancement), note the darker gray appearance of many of the westernmost storms on the 3.7 µm shortwave IR image — this darker signal is due to a dominance of smaller anvil top ice particles (in contrast with the brighter white appearance due to larger anvil top ice particles associated with the storms farther to the east). Anvils composed of smaller ice particles are more reflective of incident solar radiation, which results in significantly warmer shortwave IR brightness temperatures (in this case, +5 to +20º C, compared to -10 to -15º C in the brighter white anvil features composed of larger ice particles). According to Lindsey et al. (2006), regions such as the high plains and mountains (having environments with relatively dry boundary layers, steep lapse rates, and large vertical shear values) tend to favor thunderstorms with enhanced 3.9-μm reflectivity.

With cloud top temperatures in the -60 to -70º C range, no supercooled water droplets could have been present at the cirrus anvil top — this was confirmed by the MODIS “cirrus detection channel” image in tandem with the MODIS Cloud Phase product (below), which indicated ice phase (salmon color enhancement) for all the convective storm cirrus anvil features.

MODIS cirrus channel + cloud phase product (Animated GIF)


Additional reference:

  Cloud-top Structure of Northeast Colorado Thunderstorms May 24, 2005

GOES-11 and GOES-12 in Rapid Scan Operations

June 18th, 2008 |

GOES-11 + GOES-12 visible images (Animated GIF)

Both GOES-11 (GOES-West) and GOES-12 (GOES-East) were placed into Rapid Scan Operations (RSO) during the afternoon and early evening on 18 June 2008, to monitor the development of severe thunderstorms across the central US (SPC storm reports). In RSO mode, images are available at 5-7 minute intervals (instead of the usual 15 minute intervals for standard operations) — however, the RSO image times are not exactly the same for GOES-11 and GOES-12. A side-by-side comparison of GOES-11 and GOES-12 visible channel images centered on Bismarck, North Dakota (above) showed the formation and intensification of thunderstorms that produced hail as large as 2.75 inches in diameter at 22:48 UTC and 4.25 inches in diameter at 00:22 UTC (both times at a location between Bismarck KBIS and Garrison KN60), and a tornado around 00:25 UTC (just west of Bismarck KBIS). Both the GOES-11 and GOES-12 images are displayed in their native satellite projections, so the cloud features (and the area shown) appear a bit different due to the differing satellite viewing angles.