Undular bore in Florida

March 17th, 2009 |
GOES-13 visible images

GOES-13 visible images

GOES-13 visible imagery (above) showed  an undular bore that was propagating southward across the Florida peninsula  on 17 March 2009. This bore feature appeared to play a role in the initiation of a line of convective clouds that developed over central and southern Florida after about 15 UTC (10am local time). The resulting precipitation was farily light, however, with rainfall amounts of 0.10 inch or less.

A MODIS true color image (below, displayed using Google Earth) revealed that there were at least 10 individual cloud bands (also known as “solitons”) associated with this undular bore. The fact that this particular bore feature appeared to help with convective initiation might seem counter-intuitive, given that the propagation of bores requires a stable layer to propagate — but the convection may have fired along the leading edge of the cold frontal boundary (which was likely out ahead of the undular bore).

MODIS true color image

MODIS true color image (viewed using Google Earth)

The photo below shows the leading edge of the undular bore clouds (taken just south of Melbourne, along the coast, around 9:00 am local time).

Photograph of bore clouds (courtesy of , UW-Madison)

Photograph of bore clouds (courtesy of Dan Hartung (UW-Madison/AOS student)

Stratospheric intrusion vorticies

March 16th, 2009 |
GOES-11 / GOES-12 / GOES-13 waver vapor images

GOES-11 / GOES-12 / GOES-13 waver vapor images

Water vapor imagery from the GOES-11, GOES-12, and GOES-13 satellites (above) showed the spin-up of a well-defined “stratospheric intrusion vortex” over western Kansas on 14 March 2009. The imagery from each of those 3 satellites is displayed in its native projection, which helps to demonstrate the difference that satellite viewing angle has on the depiction of features on the water vapor imagery. Also demonstrated is the fact that imagery from GOES-13 was available through the GOES-11 and GOES-12 “Spring eclipse” periods (when the geostationary satellites are in the Earth’s shadow, and their solar panels cannot generate the power necessary to operate the instruments) — larger batteries on-board the GOES-13 satellite allows operation through the Spring and Fall eclipse periods.

A larger-scale view of AWIPS GOES-12 water vapor imagery (below) shows that there was a string of these vorticies forming during the 14-15 March time period, with a pair of smaller, weaker features seen in Iowa. While these stratospheric intrusion vorticies are seen on water vapor imagery from time to time, they usually do not produce much in the way of “sensible weather” (clouds or precipitation). However, they do seem to be capable of producing areas of mid-tropospheric turbulence, which can be a hazard for aviation. Pilot reports of turbulence (below) did show that there were a few scattered reports of light to moderate turbulence in the general vicinity of the vorticies.

GOES-12 water vapor images + pilot reports of turbulence

GOES-12 water vapor images + pilot reports of turbulence

The GOES-12 sounder Total Column Ozone derived product (below) confirms that these water vapor features were indeed stratospheric intrusion vorticies — the warmer/drier water vapor vortex features  corresponded to ozone values in the 375-400 Dobson Unit range (brighter green to red color enhancement).

GOES sounder Total Column Ozone derived product

GOES sounder Total Column Ozone derived product

The largest and most well-defined vortex (which initially formed over western Kansas on 14 March) was fairly long-lived, and was still evident on water vapor imagery over Illinois 48 hours later on 16 March. Surprisingly, this vortex also spawned some small pockets of convection that even produced a handful of cloud to ground lightning strikes (below).

GOES-12 water vapor images + lightnining strikes

GOES-12 water vapor images + lightnining strikes

A GOES-12 water vapor image with an overlay of NAM40 model fields of 500 hPa height (cyan contours) and PV1.5 pressure (red contours) indicated that there was a Potential Vorticity (PV) anomaly associated with the stratospheric intrusion vortex — the height of the “dynamic tropopause” (taken to be the pressure of the PV1.5 surface) was brought downward to near the 500 hPa pressure level over parts of northern Missouri and central Illinois.

GOES-12 water vapor image (with location of NAM40 cross section line)

GOES-12 water vapor image (with location of NAM40 cross section line)

An AWIPS cross section of the NAM40 model fields (below) allows another way to visualize the lowering of the dynamic tropopause with this PV anomaly (PV is depicted as the multi-colored image feature on the cross section).

Cross section of NAM40 model fields

West-to-east cross section of NAM40 model fields

Drifting ice field in Green Bay

March 11th, 2009 |
GOES-13 visible images

GOES-13 visible images

Strong northwesterly winds (gusting as high as 70 mph at Grand Marais in the Upper Peninsula of Michigan, and 57 mph at Washington Island in northeastern Wisconsin) caused a large portion of the land-fast ice in the far northern portion of Green Bay to break away and begin drifting eastward toward Lake Michigan on 11 March 2009. Once the clouds cleared over that region, GOES-13 visible images (above) showed the large ice feature as it moved slowly eastward.

According to the CIMSS Mesoscale Winds product (below), the speed of the ice field drift was in the 15-25 knot range. These wind vectors were generated by tracking targets on 3 consecutive GOES-12 visible images.

GOES-12 visible image + CIMSS mesoscale winds

GOES-12 visible image + CIMSS mesoscale winds

A false-color Red/Green/Blue (RGB) composite made using AWIPS images of the MODIS visible and the 2.1 µm “Snow/ice” channels (below) confirmed that this was indeed an ice feature — snow and ice are  strong absorbers at the 2.1 µm wavelength, making snow cover (and especially ice features) exhibit a darker red appearance on the false-color imagery. In contrast, the supercooled water droplet clouds appear as cyan to brighter white colored features. The ability to create these types of RGB images should be a new feature available on future releases of AWIPS-2.

MODIS false color image

MODIS false color image (using Visible and Snow/ice channels)

250-meter resolution “true color” and “false color” images from the SSEC MODIS Today site (below) showed better detail of the ice field structure. Also note the long, narrow southwest-to-northeast oriented tornado damage path (from the 07 June 2007 tornado event), located  about 30 miles (48 km) inland to the west of Green Bay.

MODIS 250-m true color and false color images

MODIS 250-m resolution "true color" and "false color" images

Mountain wave signatures on water vapor imagery

March 9th, 2009 |
GOES-11 + GOES-12 water vapor imagery

Composite of GOES-11 (GOES-West) + GOES-12 (GOES-East) water vapor imagery

Strong winds over much of the southwestern US (surface winds gusted to 75 mph at Sierra Rotors Site 7 in California) were responsible for producing large areas of mountain waves across a good deal of that particular region on 09 March 2009. These mountain waves were seen on  AWIPS composite images of the 8-km resolution GOES-11 (GOES-West) 6.7 µm and the 4-km resolution GOES-12 (GOES-East) 6.5 µm water vapor channels (above) — and there were a number of pilot reports of light to moderate turbulence within the 26,000-50,000 feet layer over the southwestern US.

MODIS  + GOES water vapor imagery

MODIS + GOES-11 water vapor imagery

A comparison of the 1-km resolution MODIS 6.7 µm with the 8-km resolution GOES-11 water vapor imagery at around 18:30 UTC (above) showed a marked improvement in the ability to detect the location and areal coverage of the mountain waves.

MODIS + GOES-11/GOES-12 water vapor imagery

MODIS + GOES-11/GOES-12 water vapor imagery

A similar comparison of the the 1-km resolution MODIS 6.7 µm with a composite of the 8-km resolution GOES-11 6.7 µm (GOES-West) plus the 4-km resolution GOES-12 6.5 µm (GOES-East) water vapor images (above) demonstrated the improvement in mountain wave detection capability with the newer 4-km water vapor channel on the newer GOES-12 (and also the GOES-13) satellites — but as expected, neither are as capable as the 1-km MODIS water vapor channel in terms of displaying mountain wave signatures.

GOES-13 water vapor images

GOES-13 water vapor images

GOES-13 shares the same 4-km resolution 6.5 µm water vapor channel as GOES-12, but in this case GOES-13 had the advantage of a more direct satellite view angle (GOES-13 is positioned over the Equator at 105º W longitude). As a result, the GOES-13 water vapor imagery (above) did a very good job of depicting the areal coverage  (as well as the temporal evolution) of the mountain wave signatures. Note how some wave signatures were stationary (anchored to the terrain that caused them to form), while other wave signatures appeared to propagate downwind over time.