Occluding “Storm Force” cyclone in the Gulf of Alaska

December 21st, 2011
GOES-15 6.5 µm water vapor channel images (click image to play animation)

GOES-15 6.5 µm water vapor channel images (click image to play animation)

McIDAS images of GOES-15 6.5 µm water vapor channel data (above; click image to play animation) revealed a very dramatic signature of the development of an occluding “Storm Force” cyclone over the Gulf of Alaska on 21 December 2011. The characeristic “wrapping spiral” of dry air (yellow to orange color enhancement) around the storm center signals that the mature cyclone is entering the occluded stage of its life cycle. Great detail in the water vapor structure can be seen in the 4-km resolution GOES-15 water vapor imagery that would not have been evident in the previous 8-km resolution GOES-11 water vapor imagery (recent GOES-11 to GOES-15 transition).

AWIPS images of 1-km resolution MODIS 0.64 µm visible channel and 11.0 µm IR channel data (below) showed the occluded cyclone at 21:29 UTC.

MODIS 0.64 µm visible channel + MODIS 11.0 µm IR channel images

MODIS 0.64 µm visible channel + MODIS 11.0 µm IR channel images

AWIPS images of GOES-15 6.5 µm water vapor channel dta with overlays of fixed buoy data and the North Pacific surface analysis (below) showed that the intensity (minimum central pressure) of the storm was underestimated on the 12:00 UTC surface analysis — but after passing over a buoy which registered a pressure value of 976.4 hPa at 15:00 UTC, the storm’s intensity was appropriately adjusted on the subsequent 18:00 UTC surface analysis. Note the large box of Hurricane Force winds that were forecast across the southwestern quadrant of the storm, in the wake of the cyclone’s cold front.

GOES-15 6.5 µm water vapor images + Fixed buoy reports + Surface analysis

GOES-15 6.5 µm water vapor images + Fixed buoy reports + Surface analysis

Winter Storm in the Southern Plains

December 19th, 2011
GOES-13 6.5 µm WV images (click image to play animation)

GOES-13 6.5 µm WV images (click image to play animation)

A potent winter storm moved into the southern Plains on 19 December 2011 as a cut-off circulation off the west coast of the US opened up and moved eastward. GOES water Vapor imagery (looped, above) shows a distinct dry slot. Stationary terrain features in the Cordillera of Northern Mexico are visible, suggesting that the surface radiation emitted at 6.5 micrometers is not absorbed by water vapor within the atmosphere (because of the extreme dryness). Cirrus that develops later in the loop along the southern border of the image does obscure some of the surface features. Note also how a strong moisture signal develops over New Mexico. Rising motion over that state moves water vapor to higher and higher levels in the atmosphere. At the start of the loop, most of the water vapor exists below the mid-tropospheric level where the Imager Sensor is detecting water vapor. Persistent rising motion allows the moist layer to deepen, and the imager starts detecting this higher, colder moisture. The loop in the computed weighting function for Albuquerque at 00 UTC and at 12 UTC on 19 December is here. Note that the amount of water vapor in the atmosphere increases between 00 UTC and 12 UTC as indicated on the linked-to charts. The weighting function describes the relative importance of emitted radiation from different levels in the atmosphere.

Lack of moisture in mid-levels (the peak response is around 500 hPa) at 00 UTC means the water vapor signal is being emitted from farther down in the troposphere, where it is warmer. As moisture deepens, the water vapor signal is emitted from colder regions. The water vapor detector on the imager shows the temperature at the top of the moist layer. It does not reveal the total moisture content in the column.

GOES-13/MODIS 6.5 µm WV images (click image to play animation)

GOES-13/MODIS 6.5 µm WV images (click image to play animation)

MODIS and GOES-13 water vapor imagery (above) from between 1630 and 1700 UTC (that is, just after the loop at the top), show significant brightness temperature differences between sensed water vapor. Values from the MODIS instrument shows water vapor brightness temperatures that are uniformly colder than the GOES-13 values. Why? The Spectral Response Functions below (courtesy of Mat Gunshor, SSEC/CIMSS), for GOES-12 (the imager on GOES-12 is similar to that on GOES-13) and for the MODIS WV Channel suggest a possible reason. The Imager water vapor detection (in blue) spans a larger part of the electromagnetic spectrum, including regions at longer wavelengths. (The MODIS water vapor channel is a single sharply defined peak (shown in red)). As the wavelength increases, the level sensed decreases, so a broader spectrum that includes longer wavelengths will show warmer temperatures because it is detecting more energy from lower in the atmosphere where temperatures are warmer. At Nadir, GOES-13 will be about 1 K warmer than the MODIS Brightness temperature. (Use this website to show different weighting functions.

GOES-12/MODIS Spectral Response Functions for Water Vapor Channel

GOES-12/MODIS Spectral Response Functions for Water Vapor Channel

It is common to relate features in the water vapor imagery to structures in the atmosphere. The figure below shows a 325-K Jet maxima aligned, as expected, with the dry slot in the WV imagery. The dry slot is a region of sinking motion. Warm brightness temperatures develop in the dry slot because water vapor is confined to the lowest levels of the atmosphere, so the emitting surface is warm. A cross-section that is nearly orthogonal to the jet (here) shows an isentropic structure that is characteristic of an intrusion of stratospheric air into the mid-troposphere. It also shows extreme dryness in the middle of the tropopshere.

GOES-13 WV/GFS 325K Winds

GOES-13 WV/GFS 325K Winds

This storm brought needed precipitation to parts of the southern Plains that have been plagued by drought all year.

===== 21 December Update =====

MODIS true color RGB images (viewed using Google Earth)

MODIS true color RGB images (viewed using Google Earth)

Two days after the storm, the clouds had cleared to reveal the large swath of fresh snow cover on 21 December 2011, as seen on a composite of MODIS true color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (above; viewed using Google Earth). Across the Southern Plains, the highest storm total snowfall amounts (in inches) in Texas, Oklahoma, Colorado, and Kansas are highlighted on the image.

A comparison of AWIPS images of the 1-km resolution MODIS 0.65 µm visible channel and the corresponding MODIS false color RGB image created using the visible and “snow/ice” channels 01/07/07 (below) revealed the swath of snow cover (red on the RGB image) on the 16:40 UTC overpass of the Terra satellite. Note the darker red appearance along the far southeastern edge of the snow cover on the false color image — this is a signature of areas where there was a significant accrual of ice due to freezing drizzle. Near Pratt, Kansas (station identifier KPTT) the thickness of the ice accrual was around 0.25 inch. Since ice is a stronger absorber of radiation than snow cover at the 2.1 µm wavelength, this leads to a darker appearance on a single-channel MODIS Band 7 imagery.

MODIS 0.65 µm visible channel + MODIS false color RGB image

MODIS 0.65 µm visible channel + MODIS false color RGB image

Strong winds affect southcentral and eastern Alaska

December 18th, 2011
GOES-15 6.5 µm water vapor channel images (click image to play animation)

GOES-15 6.5 µm water vapor channel images (click image to play animation)

McIDAS images of 4-km resolution GOES-15 6.5 µm water vapor channel data (above; click image to play animation) showed an intense upper level shortwave trough of low pressure moving northeastward across southcentral and eastern Alaska on 18 December 2011. Strong southerly flow associated with this system brought unseasonably warm air into the region, with Anchorage (station identifier PANC) reaching a daily maximum temperature of 45º F (one degree F shy of their record high for the date), and Big Delta (station identifier PABI) tied their daily record high of 37º F. Strong winds were also experienced with this disturbance, with surface winds gusting in excess of 100 mph in southcentral Alaska. It is also interesting to note the development of a small westward-propagating “wave feature” at the end of the water vapor animation near Tanana (station identifier PATA).

Over eastern Alaska the water vapor images also showed a large orographic “banner cloud” that formed downwind of the high terrain of the Alaska Range. A closer look at this banner cloud feature can be seen using an AWIPS image of 1-km resolution MODIS 11.0 µm IR channel data with an overlay of GFS model 500 hPa winds (below). The coldest MODIS IR brightness temperatures along the leading edge of the banner cloud were -65º C, which was just a few degrees colder than the tropopause temperature on the 12:00 UTC Anchorage rawinsonde data. The winds aloft then turned anticyclonically, carrying some of the banner cloud materail eastward into the Yukon Territory of Canada.

MODIS 11.0 µm IR image + GFS 500 hPa winds

MODIS 11.0 µm IR image + GFS 500 hPa winds

The corresponding 1-km resolution MODIS Cloud Type product (below) indicated that much of this banner cloud was of the “opaque ice” category (yellow color enhancement).

MODIS Cloud Type product

MODIS Cloud Type product

The 1-km resolution POES AVHRR Cloud Top Height product (below) indicated that the highest portions of the banner cloud feature were in the 10-11 km range.

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

As an interesting aside, a Boeing 747 flying just off the coast of Alaska encountered severe turbulence at a flight level of 35,000 feet — the captain of the aircraft “said this was the first time he has ever reported severe turbulence” (below).

POES AVHRR 12.0 µm IR image with pilot reports of turbulence

POES AVHRR 12.0 µm IR image with pilot reports of turbulence

Note on the GOES-15 water vapor images shown above that this area was near the leading edge of an advancing dry slot — and 1-km resolution GOES-15 0.63 µm visible channel images (below) depicted a few cloud features resembling banded convective cells along the trailing edge of the cloudiness just ahead of the dry slot. These convective bands (or the strong deformation axis seen developing on the water vapor imagery) may have been responsible for producing high-altitude turbulence across that region.

GOES-15 0.63 µm visible channel images

GOES-15 0.63 µm visible channel images

Tropical Storm Washi (27W) strikes the Philippines

December 17th, 2011
MTSAT-1R 10.8 µm IR images (click image to play animation)

MTSAT-1R 10.8 µm IR images (click image to play animation)

MTSAT-1R 10.8 µm IR images from the CIMSS Tropical Cyclones site (above; click image to play animation) showed a fairly compact cluster of cold convective cloud tops associated with Tropical Storm Washi as it moved westward toward the Philippines during the 15-16 December 2011 period.

A closer view using MIMIC microwave imagery (below) also showed a relatively small area of enhanced brightness temperatures (representing heavy precipitation) crossing Mindanao Island in the southern Philippines on 16 December.

MIMIC microwave imagery

MIMIC microwave imagery

However, AWIPS images of the MIMIC Total Precipitable Water (TPW) product (below; click image to play animation) revealed that Tropical Storm Washi was embedded within a long fetch of very rich tropical moisture, with TPW values in excess of 60 mm or 2.4 inches (darker red color enhancement). This abundance of moisture helped to fuel over 10 hours of heavy rainfall, which resulted in widespread flash flooding and reports of over 900 deaths in the Philippines.

MIMIC Total Precipitable Water product (click image to play animation)

MIMIC Total Precipitable Water product (click image to play animation)