Convective Downbursts and Heatbursts in Wisconsin

May 15th, 2013 |
GOES-13 Sounder Derived Lifted Index (click image to play animation)

GOES-13 Sounder Derived Lifted Index (click image to play animation)

Strong convection in the late afternoon/early evening produced wind damage and heat bursts over southern Wisconsin late in the day on May 14, 2013 in a region where severe convection was not considered likely. GOES Sounder data did an excellent job of depicting the instability that developed in the late afternoon. The animation above shows strong destabilization starting shortly after 1800 UTC, and persisting as the convection moved through southern Wisconsin.

GOES-13 Sounder Derived Lifted Index

GOES-13 Sounder Derived Lifted Index

The instability associated with this convective event was very localized, and easily slipped in between the radiosonde stations. This is therefore another example of the benefit of the GOES Sounder DPI products: Not only do they provide hour-by-hour coverage, so that an evolving situation can be monitored, but they can show mesoscale features that are poorly sampled by conventional radiosonde data. The above image shows the 2346 UTC 14 May GOES Sounder DPI LI over the upper midwest; superimposed upon the image are the Lifted Indices computed from radiosondes and the LI computed from the GFS model. The strongest instability is not well sampled by the radiosonde network.

GOES-13 Sounder Derived CAPE

GOES-13 Sounder Derived CAPE

Convective Available Potential Energy (CAPE) can also be used to diagnose the potential for convection. In regions where CAPE values are large, convection can grow explosively. The AWIPS screen capture of CAPE computed from the sounder, above, shows values exceeding 4000 J/kg even after the convection has passed!

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

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

Visible imagery from GOES-13, above, shows the development of the convection as it moves into the area of diagnosed instability. The Microburst Windspeed Potential Index (MWPI) predicts maximum wind gusts that might occur given the thermal profiles associated with developing convection. Attributes that promote downbursts are steep mid-level lapse rates (to enhance convective instability) and abundant dry air (to enhance evaporative cooling). The two animations below (created using McIDAS-V and this bundle) show a maximum in MWPI (with values near 50 — the relationship between MWPI and convective gusts is here) developing over southwest WI as the convection develops. (Data are from the Rapid Refresh model run at 2200 UTC on Tuesday 14 May). The animation of model soundings over Madison (bottom) indicates strong destabilization and mid-level drying, two components that enhance the potential for microbursts. (McIDAS-V animations courtesy of Ken Pryor, NOAA/NESDIS)

Microbust Windspeed Potential Index (MWPI) from 2200 UTC 14 May-0100 UTC 15 May over Wisconsin.  Data from Rapid Refresh Model

Microbust Windspeed Potential Index (MWPI) from 2200 UTC 14 May-0100 UTC 15 May over Wisconsin. Data from Rapid Refresh Model

Rapid Refresh Model Soundings over Madison, WI from 2200 UTC 14 May-0100 UTC 15 May over Wisconsin

Rapid Refresh Model Soundings over Madison, WI from 2200 UTC 14 May-0100 UTC 15 May over Wisconsin

GOES Sounder DPI products are available here. YouTube videos of the convection, obtained from the cameras on the roof of SSEC, are available here (looking east) and here (looking north).

Stray Light in GOES Imager data

May 15th, 2013 |
GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (click image to play animation)

Each year, about every 6 months, the Earth-Sun-Satellite geometry is such that the GOES Imager can look right at the Sun. In the past, there were ‘keep-out zones’ in which the satellites did not image because it was known to be looking at the Sun during those times. The imagery above, from GOES-13, shows visible light in the night-time imagery. (Click here for a similar GOES-15 animation). Stray light values typically peak around 0500 UTC for GOES-East and around 0900 UTC for GOES-West.

In addition, imagery was not possible during the so-called ‘eclipse season’ because the satellites lacked sufficient batteries to power the instruments as they passed through Earth’s shadow. Now, an improved battery system on the current generation GOES-13/14/15 satellites allows for imaging to proceed while the satellite is in the Earth’s shadow.

This new scheduling, however, introduces issues. The GOES Imager is calibrated by periodic looks into deep space, regions from which only very small amounts of radiation (at 3.9, 6.5, 10.7 and 13.3 µm) are being emitted. These ‘space looks’ are on either side of the full-disk GOES Image. During the ‘eclipse season’, that space look can include part of the solar energy, meaning the very small amount of radiation that the satellite is designed to detect is actually potentially significant. Thus, the calibration of the image can be affected. NOAA NESDIS does operationally correct images with ‘stray light’, but this correction does not consider the impact of a corrupted space view. The GOES-13 stray light corrections were implemented in 2012, as discussed here on this blog.

In addition to the calibration images, solar radiation can also be scattered off clouds towards the imager. So, instead of detecting only emitted radiation at night, the GOES Imager is detecting emitted terrestrial radiation in addition to scattered/reflected solar radiation. This solar radiation contaminates the signal, and results in ‘too much’ radiance being detected, resulting in warmer-than-actual inferred blackbody/brightness temperatures.

GOES-13 imagery from infrared channels (click image to enlarge)

GOES-13 imagery from infrared channels (click image to enlarge)

When Stray Light issues occur, the most noticeable effects are in the 3.9 µm channel (Above loop, bottom left) and in products that use the 3.9 µm channel, such as the brightness temperature difference (Above loop, top left). In other words, this calibration issue can affect derived products that use 3.9 µm data at night. The image below shows how the 3.9 µm imagery can change when Stray Light is an issue. Compare the 0415 UTC image, on the left, when Stray Light did not contaminate the space look, with the 0502 UTC image on the right, when Stray Light was an issue.

GOES-13 3.9 µm imagery

GOES-13 3.9 µm imagery

NESDIS is considering methods of mitigating the stray light issues that occasionally occur in the GOES Imager.

Germann Road fire in northern Wisconsin

May 14th, 2013 |
GOES-13 0.63 µm visible channel (top) and 3.9 µm shortwave IR channel (bottom) images (click to play animation)

GOES-13 0.63 µm visible channel (top) and 3.9 µm shortwave IR channel (bottom) images (click to play animation)

McIDAS images of GOES-13 0.63 µm visible channel and 3.9 µm shortwave IR channel data (above; click image to play animation) showed the large smoke plumes and fire “hot spots” (dark black pixels on the shortwave IR imagery) associated with the Germann Road Fire in northwestern Wisconsin and the Green Valley Fire in Minnesota on 14 May 2013. The Germann Road Fire burned 8495 acres, making it the largest wildfire in northern Wisconsin in 33 years. In Minnesota, the Green Valley fire burned 7100 acres.

Items of interest to note on the GOES-13 imagery: (1) the presence of a well-defined lake breeze (lighter gray color enhancement on the IR images) which extended quite a distance inland from the colder waters of Lake Superior (which still exhibited Sea Surface Temperature values in the middle to upper 30s F); (2) the change in wind direction from southwesterly to westerly/northwesterly as a frontal boundary moved eastward across the region; (3) the apparent “flare-up” of the Germann Road Fire as the frontal boundary arrived around 00:45 UTC — the size of the cluster of black “hot spot” pixels increased on the shortwave IR image, concurrent with the rapid growth of an area of pyrocumulus clouds; (4) the eastward motion of the thin lake ice that remained on Mille Lacs in Minnesota (the large lake just south of the Green Valley smoke plume).

2 days after the fire, the burn scar was apparent on an Aqua MODIS false-color Red/Green/Blue (RGB) image (below), viewed using the SSEC Web Map Server. Note the “right turn”on the northern end of the burn scar, caused by a change from southwesterly winds to strong westerly winds in the wake of a frontal passage (which altered the direction of the fire’s progress).

Aqua MODIS false-color image showing wildfire location and burn scar

Aqua MODIS false-color image showing wildfire location and burn scar

 

Formation of an “Otter Eddy” in Monterey Bay, California

May 13th, 2013 |
GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (click image to play animation)

Strong northwesterly winds along the California coast interacted with the complex terrain and orientation of Monterey Bay to promote the formation of a cyclonic coastal eddy (known locally as an “Otter Eddy”) early in the day on 13 May 2013. McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) showed the evolution of the eddy feature, which gradually dissipated by the early afternoon hours. “MRY” denotes the location of Monterey.

Farther to the north, an interesting type of “bow shock wave” formed downwind of Point Reyes (labelled “PR” on the images). Better detail of this feature could be seen in an AWIPS image of Suomi NPP VIIRS 0.64 µm visible channel data (below). At the time of this image, surface winds at the offshore buoy just to the north of Point Reyes were gusting to 33 knots (38 mph).

Suomi NPP VIIRS 0.64 µm visible channel image

Suomi NPP VIIRS 0.64 µm visible channel image