Lee-side cold frontal gravity wave

October 22nd, 2008 |
GOES-12 6.5 µm water vapor images

GOES-12 6.5 µm water vapor images

AWIPS images of the 4-km resolution GOES-12 6.5 µm water vapor channel (above) showed a southward-propagating  lee-side cold frontal gravity wave over New Mexico and Texas on 22 October 2008. This gravity wave was caused by a surface-based cold frontal boundary that was moving southward across the region.

MODIS 6.7 µm water vapor image + fog/stratus product image

MODIS 6.7 µm water vapor image + fog/stratus product image

A comparison of the 1-km resolution MODIS 6.7 µm water vapor channel and the MODIS fog/stratus product (above) indicated that there were narrow cloud bands along the leading edge of the frontal boundary / gravity wave, as well as more extensive patches of fog and/or stratus behind the front in the Texas panhandle.The MODIS Land Surface Temperature (LST) product (below) depicted LST values dropping into the 40s F (green colors) behind the front, with much warmer LST values in the 50s and 60s F (yellow to orange colors) ahead of the front.

MODIS 6.7 µm water vapor channel + Land Surface Temperature product

MODIS 6.7 µm water vapor channel + Land Surface Temperature product

NOAA wind profiler data from Jayton, Texas (below) showed the deepening of the cold northerly flow after the cold front moved through the area — the top of the cold air appeared to be close to the 700 hPa level (around 10,000 feet above ground level).

Jayton, Texas NOAA wind profiler data

Jayton, Texas NOAA wind profiler data

GOES-12 water vapor channel weighting functions calculated for the rawinsonde profiles at Amarillo, Texas (below) demonstrated a significant lowering of the layer being detected by the water vapor channel in the 12 hours between 00 and 12 UTC on 22 October. With the drier air mass in place at 12 UTC, the GOES-12 water vapor channel was able to detect a substantial amount of energy originating from within the 500-700 hPa layer, allowing the signature of the frontal gravity wave to appear on the GOES-12 water vapor imagery. The wave structure was better-defined on the MODIS water vapor image, due to the improved spatial resolution and the more direct satellite viewing angle.

GOES-12 water vapor channel weighting functions for Amarillo TX

GOES-12 water vapor channel weighting functions for Amarillo TX

Mountain waves over Colorado

June 22nd, 2009 |
MODIS 6.7 µm and GOES-12 6.5 µm water vapor images

MODIS 6.7 µm and GOES-12 6.5 µm water vapor images

Moderate southwesterly flow aloft over the Rocky Mountains was aiding in the formation of mountain waves across much of Colorado and parts of the adjacent states on 22 June 2009. AWIPS comparisons of the 1-km resolution MODIS 6.7 µm water vapor image with the corresponding 4-km resolution GOES-12 6.5 µm water vapor image (above) and the 8-km resolution GOES-11 6.7 µm water vapor image (below) demonstrated the value of better spatial resolution for detecting such mesoscale features.

MODIS 6.7 µm and GOES-11 8-km 6.7 µm water vapor images

MODIS 6.7 µm and GOES-11 6.7 µm water vapor images

The appearance of these banded “mountain wave signatures” on water vapor imagery indicates the potential for clear air turbulence in those areas; however, there were no pilot reports of turbulence until 13:02 UTC near Fort Collins (at an altitude of 15,000 feet). An animation of the GOES-12 6.5 µm water vapor imagery (below) also showed the presence of a lee-side cold frontal gravity wave, which could be seen propagating southward across eastern Colorado and western Kansas. In fact, a small packet of waves could be seen along and behind the leading edge of this lee-side cold frontal gravity wave on the MODIS water vapor images above — surface winds behind this front had gusted to 36 knots at McCook, Nebraska (station identifier KMCK) and 20 knots at Goodland, Kansas (station identifier KGLD).

GOES-12 6.5 µm water vapor images

GOES-12 6.5 µm water vapor images

Dusty Cold Front moves south through the Southern Plains

November 11th, 2014 |
GOES-13 0.63 µm Visible imagery (click to play animation)

GOES-13 0.63 µm Visible imagery (click to play animation)

A strong cold front moved southward over the High Plains of the US on Monday 10 November, and the strong winds produced a dust cloud that was apparent in GOES-13 visible imagery, above. The leading edge of the dust cloud in the satellite imagery indicated precisely the leading edge of the cold front. The animation below shows hourly observations plotted on top of the GOES-13 visible imagery. The correspondence between the leading edge of the dust and the wind shift is obvious. Note that multiple stations report Haze (H) after the wind shift occurs.

GOES-13 0.63 µm Visible imagery and surface observations (click to play animation)

GOES-13 0.63 µm Visible imagery and surface observations (click to play animation)

GOES-15 viewed this event as well (Visible animation; Visible animation with observations). The dust in the atmosphere was far more apparent in the GOES-13 imagery, however. This case is an excellent demonstration of how dust effectively forward scatters visible light from the setting sun towards GOES-13 at 75º W, but does not so effectively back scatter towards GOES-15 at 135º W. The toggle below shows visible imagery from GOES-13 and GOES-15, both at 2200 UTC.

GOES-13 0.63 µm Visible imagery and GOES-15 0.62 µm Visible Imagery, both at 2200 UTC 10 November (click to enlarge)

GOES-13 0.63 µm Visible imagery and GOES-15 0.62 µm Visible Imagery, both at 2200 UTC 10 November (click to enlarge)

Both Aqua (MODIS) and Suomi NPP (VIIRS) viewed this haboob in mid-afternoon on 10 November. What can the multispectral views of this feature tell us? Both the Visible and Snow/Ice channels give similar views of the leading edge of the cold front (the biggest difference between the visible and snow/ice channel in this image is that water features are so much darker in the snow/ice channel because water strongly absorbs 2.1 µm radiation; differences in the clouds between the visible and the snow/ice (2.1 µm) channel arise from viewing water-based vs. ice-based clouds). The cirrus channel — 1.37 µm — does not see the surface but it does clearly reveal high clouds. The 3.9-µm image — shortwave infrared — shows very warm temperatures right at the leading edge of the cold front in eastern Colorado. This is a region where the dust is effectively reflecting solar radiation. The longwave infrared imagery (10.7 µm) shows a more uniform cold edge to the cloud. Finally, even the water vapor imagery shows a signal from this cold front (known as a lee-side frontal gravity wave). It is unusual for surface features to have a signal in water vapor imagery; when it does occur, the atmosphere is usually very dry, and that’s the case in this event. Note in the toggle here between GOES water vapor channel weighting functions (computed here) at Amarillo between 0000 UTC — before the cold front — and 1200 UTC — after the cold front — shows how the layer from which 6.5 µm radiation will be detected has dropped in altitude.

Aqua MODIS Visible, Snow/Ice, Cirrus, Shortwave IR, Water Vapor and Longwave IR Imagery at 1917 UTC, 10 November (click to enlarge)

Aqua MODIS Visible, Snow/Ice, Cirrus, Shortwave IR, Water Vapor and Longwave IR Imagery at 1917 UTC, 10 November (click to enlarge)

Suomi NPP viewed the cold front 10 minutes before Aqua, below, and also about 90 minutes later (Favorable orbital geometry allowed sequential orbits to view eastern Colorado). The shortwave IR (3.74 µm) show warmer signatures in some of the dust plumes compared to the longwave IR (11.35 µm), similar to Aqua, a difference that is likely due to solar radiation being reflected by the dust.

Suomi NPP VIIRS data showing Visible, Day Night Band, Snow/Ice, Shortwave IR, and Longwave IR Imagery at 1907 UTC, 10 November (click to enlarge)

Suomi NPP VIIRS data showing Visible, Day Night Band, Snow/Ice, Shortwave IR, and Longwave IR Imagery at 1907 UTC, 10 November (click to enlarge)

Suomi NPP VIIRS data showing Visible, Day Night Band, Snow/Ice, Shortwave IR, and Longwave IR Imagery at 2049 UTC, 10 November (click to enlarge)

Suomi NPP VIIRS data showing Visible, Day Night Band, Snow/Ice, Shortwave IR, and Longwave IR Imagery at 2049 UTC, 10 November (click to enlarge)

Animations of 10.7 µm Brightness Temperature Data from GOES-13 showed the southward plunge of cold air overnight. The progress of this cold front could be monitored from space. Even the water vapor imagery continued to include a signature of the cold front.

GOES-13 Water Vapor (6.7 µm) Infrared Imagery (click to play animation)

GOES-13 Water Vapor (6.7 µm) Infrared Imagery (click to play animation)

The visible imagery at the top of this post ably captured the signature associated with blowing dust. Did the blowing dust continue through the night? Single-channel detection of dust at night is difficult. Historically, dust could be detected with brightness temperature differences between 10.7 µm and 12 µm channels on the GOES Imager, but that capability ended when the 13.3 µm channel replaced the 12 µm channel on the GOES Imager (the GOES-R ABI will contain a 12 µm channel). The VIIRS Day Night Band, below, from Suomi NPP at 0905 UTC on 11 November, does not show a distinct dust signature over south Texas. The leading edge of the front is obvious, however, as it is preceded by a Bore structure with parallel lines of clouds.

Suomi NPP VIIRS Day Night Band (.7 µm) Visible Imagery at 0905 UTC 11 November 2014 (click to enlarge)

Suomi NPP VIIRS Day Night Band (.7 µm) Visible Imagery at 0905 UTC 11 November 2014 (click to enlarge)

Blowing dust in Texas

January 29th, 2008 |

GOES-12 6.5µm water vapor images (Animated GIF)

A very cold arctic air mass was in place across parts of the Canadian Prairies, with many sites in Alberta reporting surface temperatures colder than -40º F (-40º C) on the morning of 29 January 2008; the leading edge of this arctic air was surging rapidly southward across the Great Plains of the US as a strong cold frontal boundary. AWIPS images of the GOES-12 6.5µm “water vapor channel” (above) actually showed a signature of a gravity wave along the leading edge of the cold front as it moved southward across Nebraska and Kansas into Texas and Oklahoma. This lee-side cold frontal gravity wave feature could be detected on the water vapor channel imagery due to the fact that the airmass ahead of the front was quite dry, which shifted the altitude of the GOES-12 water vapor channel weighting function to much lower altitudes compared to what would be seen in a more typical airmass.

Strong pre-frontal and post-frontal winds were responsible for creating a large area of blowing dust across Texas during the afternoon hours, as seen on consecutive Terra (18:15 UTC, or 12:15 pm local time) and Aqua (19:50 UTC, or 1:50 pm local time) MODIS true color images from the SSEC MODIS Today site (below). This blowing dust reduced surface visibility to as low as 2 miles at San Angelo, Texas.

MODIS true color images (Animated GIF)

The windy conditions (several wind gusts in Texas were in excess of 60 mph) and very dry air were also creating an environment favorable for small wildfires; note that a number of fire “hot spots” (black pixels) could be seen on an AWIPS image of the MODIS 3.7µm shortwave IR channel (below).

MODIS 3.7µm IR image

The SSEC MODIS Today true color image from the Aqua satellite displayed using Google Earth (below) showed exactly which counties and highways in Texas were being impacted by the large cloud of blowing dust. In addition, a smoke plume from small wildfire that was burning just southeast of Abilene, Texas could be seen streaming south/southeastward.

MODIS true color image (Google Earth)