Orographic cirrus over Colorado and Wyoming

December 18th, 2013 |
GOES-13 10.7 µm IR channel images (click to play animation)

GOES-13 10.7 µm IR channel images (click to play animation)

AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data (above; click image to play animation) showed the development of a large shield of orographic cirrus clouds immediately downwind (to the east of) the high terrain of the Rocky Mountains in eastern Colorado and southeastern Wyoming on 18 December 2013. The development and persistence of such cloud features is important to monitor, due to their potential impact on daytime temperatures: if incoming solar radiation is significantly reduced by a canopy of dense cirrus, surface temperatures may not be as warm as forecast. Early in the cirrus shield development during the overnight hours, GOES-13 IR brightness temperatures were as cold as -66º C (darker red color enhancement) along the western edge of the cirrus shield, suggestive of a high and potentially dense ice cloud feature — but during the early morning hours the cirrus was seen to begin to rapidly dissipate after about 15 UTC.

While not directly related to the orographic cirrus cloud shield per se, the strong westerly winds interacting with the complex terrain of the Rocky Mountains produced some areas of turbulence across Colorado. At 14:05 UTC a pilot reported Moderate turbulence throughout a very deep layer (5,500 feet to 20,000 feet), and at 15:22 UTC a pilot reported occasional Severe turbulence between the altitudes of 13,000 and 16,000 feet.

Night-time comparisons of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel images at 08:12 UTC or 1:12 AM local time (above) and 09:51 UTC or 2:51 AM local time (below) revealed that the cirrus along the western (upwind) edge was quite cold on the IR images (with IR brightness temperatures as cold as -71º C at 08:12 UTC, and -76º C at 09:51 UTC), and also appeared much more optically thick along the western portion on the DNB images. However, on the DNB images the cirrus shield — although still exhibiting fairly cold IR temperatures — appeared to be much more optically thin along the eastern portion. Due to ample illumination by a nearly-full moon, the DNB provided vivid “visible images at night” to compliment the IR images; in the upper right corner of the DNB images, snow on the ground could also be seen across northern Nebraska into far southern South Dakota.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

A 1-km resolution POES AVHRR Cloud Top Height product at 11:39 UTC or 4:39 AM local time (below) indicated that the tops of the cirrus shield were at 12 km (darker green color enhancement).

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

The 12 UTC Denver rawinsonde data (below) showed that the tropopause was around 13 km, with an air temperature of -70º C. Winds at that altitude were from the west at 100 knots (with strong westerly winds at all altitudes throughout the tropopause).

Denver, Colorado rawinsonde data

Denver, Colorado rawinsonde data

Additional details and imagery of this orographic cirrus event can be found on the CIRA RAMMB GOES-R Proving Ground Blog.

Turbulence within a Mesoscale Convective System cirrus outflow region

June 14th, 2012 |
Radar reflectivity mosaic

Radar reflectivity mosaic

A radar reflectivity composite (above) showed a large Mesoscale Convective System (MCS) that was moving across parts of Minnesota and Wisconsin on 14 June 2012, producing heavy rainfall (2.99 inches at Zumbrota in southeastern Minnesota) and some hail (SPC storm reports).

AWIPS images of GOES-13 0.63 µm visible channel data (below; click image to play animation) showed a broad area of anticyclonic cirrus outflow around the southern periphery of the MCS as it was dissipating during the late morning and early afternoon hours. There were a number of pilot reports of moderate turbulence seen within this banded area of cirrus outflow.

GOES-13 0.63 µm visible images with pilot reports of turbulence (click image to play animation)

GOES-13 0.63 µm visible images with pilot reports of turbulence (click image to play animation)

The banding structure within the cirrus outflow region was clearly shown on a 375-meter resolution (re-mapped onto an AWIPS 1 km grid) Suomi NPP VIIRS 11.45 µm IR image at 18:35 UTC  (below). A comparison with the corresponding 4-km resolution GOES-13 10.7 µm IR image demonstrated the advantage of higher spatial resolution for depicting such features.

Suomi NPP VIIRS 11.45 µm IR image + GOES-13 10.7 µm IR image

Suomi NPP VIIRS 11.45 µm IR image + GOES-13 10.7 µm IR image

The pronounced anticyclonic motion of the cirrus outflow (also verfied using MADIS 1-hour cloud-tracked winds) was creating strong wind shear aloft over much of eastern Iowa, southern Wisconsin, and northern Illinois — note how different the satellite wind vector directions were from the NAM 500-100 hPa deep-layer wind flow streamlines over that region (below). This strong wind shear aloft may have been a factor contributing to the numerous pilot reports of moderate turbulence within the area of cirrus outflow.

VIIRS 11.45 µm IR image + MADIS cloud-tracked wind vectors + NAM deep layer mean wind

VIIRS 11.45 µm IR image + MADIS cloud-tracked wind vectors + NAM deep layer mean wind

A similar depiction of the pronounced wind shear aloft was seen a few hours earlier on a 16:40 UTC MODIS 11.0 µm IR image (below).

MODIS 11.0 µm IR image + MADIS cloud-tracked wind vectors + NAM deep layer mean wind

MODIS 11.0 µm IR image + MADIS cloud-tracked wind vectors + NAM deep layer mean wind

 

Patch of thin cirrus and contrails over Arkansas and Tennessee

November 8th, 2010 |
GOES-13 0.63 µm "visible channel" image

GOES-13 0.63 µm "visible channel" image

A quick look at a 1-km resolution GOES-13 0.63 µm visible channel image (above) would suggest that it was a cloud-free day across Arkansas, Tennessee, and the surrounding region on 08 November 2010.

However, a comparison of AWIPS images of the 1-km resolution MODIS 0.65 µm “visible channel” , the MODIS 11.0 µm “IR window” channel, and the 1.3 µm “cirrus detection” channel (below) revealed that there was a patch of thin cirrus clouds and contrails over much of Arkansas and Tennessee. As with the GOES-13 visible channel, there was no indication of any cloud features on the MODIS visible image. On the MODIS IR image, there was a signature of some sort of cloud features over that region, but the IR brightness temperatures were quite warm (mostly above 0º C), which would be much too warm for cirrus clouds. However, the MODIS cirrus detection channel did a very good job at highlighting the patch of very thin clouds. This is due to the fact that the near-IR 1.3 µm channel is very effective for detecting features that are good scatterers of light (such as ice crystals, volcanic ash, airborne dust, etc).

MODIS 0.65 µm "visible", 11.0 µm "IR window", and 1.3 µm "Cirrus detection" images

MODIS 0.65 µm "visible", 11.0 µm "IR window", and 1.3 µm "Cirrus detection" images

Rather surprising, however, was the fact that this patch of cirrus clouds and contrails existed within a region that appeared to be a fairly dry area (indicated by the lighter blue to yellow color enhancement) on the MODIS 6.7 µm “water vapor” image (below). The pronounced dryness of the middle to upper troposphere was quite evident of the Little Rock, Arkansas rawinsonde data.

MODIS 6.7 µm "water vapor channel" image

MODIS 6.7 µm "water vapor channel" image

A false color Red/Green/Blue (RGB) POES AVHRR composite image (below) did show a subtle hint of some brighter cloud features against the green to brown background of the land surface, but again not to the extent of what was seen on the MODIS cirrus detection channel image.

POES AVHRR false color Red/Green/Blue (RGB) image

POES AVHRR false color Red/Green/Blue (RGB) image

If we examine the 1-km resolution POES AVHRR Cloud Top Temperature (CTT) product (below), we do begin to see features that exhibited CTT values of -50º C and colder (yellow color enhancement), which is more representative of what you would expect for cirrus cloud features.

POES AVHRR Cloud Top Temperature product

POES AVHRR Cloud Top Temperature product

In addition, the 1-km resolution POES AVHRR Cloud Top Height (CTH) product (below) showed features with CTH values of 10-11 km (cyan color enhancement), which are also more representative of the altitude where cirrus cloud features would usually  be located. These altitudes also matched the Cloud Top Temperature values around -50º C on the rawinsonde data from Little Rock, Arkansas and Nashville, Tennessee.

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

Given that these cloud features were obviously quite thin, the 1-km resolution POES AVHRR Cloud Optical Depth product (below) showed correspondingly low values.

POES AVHRR Cloud Optical Depth product

POES AVHRR Cloud Optical Depth product

Another way to distinguish ice clouds from water droplet clouds is examine the 1-km resolution POES AVHRR Cloud Particle Effective Radius product (below). Ice crystals are typically much larger (in this case, at least 30-40 micrometers in diameter) than water droplets (usually around 20 micrometers or less in diameter).

POES AVHRR Cloud Particle Effective Radius product

POES AVHRR Cloud Particle Effective Radius product

Because of the very thin nature of these cirrus and contrail features, the 1-km resolution POES AVHRR Cloud Tye product (below) did seem to struggle in assigning the correct type to the features — although many were correctly identified as Cirrus (orange color enhancement).

POES AVHRR Cloud Type product

POES AVHRR Cloud Type product

Cirrus detection from satellite

March 17th, 2010 |

The visible imagery loop from GOES-12, above, shows cirrus clouds around southern Wisconsin at 1315 UTC on 16 March (1st image in loop), and at 2215 UTC on 16 March (last image in loop), but relatively few at 1615 UTC (middle image). In contrast, the infrared imagery loop (here), indicates cirrus clouds increasing throughout the day; Brightness temperatures associated with the cirrus over southern Wisconsin are fairly warm — near 270 K — because energy from lower in the atmosphere was able to pass through the cirrus clouds, adding to the radiance emitted by the cold cirrus clouds.

Ground truth from Madison (the yellow dot in the visible and 11 micron loops) from a Tower Camera looking west at 1322 UTC, 1619 UTC (Note also the excellent example of a contrail shadow in this image) and 2224 UTC, and looking north at 1324 UTC, 1624 UTC and 2223 UTC all show similar amounts of cirrus cloud coverage.

Cirrus is difficult to detect because backscatter of radiation from the ice crystals can be limited. Indeed, the 1322 UTC Tower Cam image looking west (here) appears to show little cirrus because visible radiation from the sun rising in the east is not effectively scattered backwards by the cloud. The same thing is happening in the visible satellite imagery at 1615 UTC: apparent clarity in the visible occurs because backscatter from cirrus of solar radiation is small. The 11-micron image from the same time shows clouds over southern Wisconsin, but with warm brightness temperatures.

Although Cirrus clouds do not backscatter visible solar radiation effectively, they do very effectively backscatter radiation with wavelengths near 1.38 microns. Furthermore, radiation with a wavelength of 1.38 microns that is emitted by the Earth is strongly absorbed by water vapor. Thus, the largest signal is from solar radiation reflected off cirrus clouds. The MODIS instrument, aboard both Terra and Aqua satellites, detects radiation at 1.38 microns, and the image for 1622 UTC is shown below. Cirrus clouds are indicated over southern Wisconsin, and also over lower clouds over eastern Minnesota and Iowa. The ABI instrument, to be aboard GOES-R when it launches, will also detect radiation at 1.38 microns.