MTSAT IR image

Tropical Systems can occasionally be accompanied by Predecessor Rainfall Events (PREs). This band of heavy rainfall is not associated with the spiral bands of the tropical system, but rather with an interaction with a mid-latitude jet that exists Poleward of the tropical feature. Some notable PRE-producing tropical systems in the United States include Hurricane Ike from 2008 and Hurricane Floyd from 1994. Accumulated rainfall patterns from Ike and from Floyd show very heavy rains far removed from the landfall; in Ike’s case, over northwestern Indiana, and in Floyd’s case over Connecticut and New York. In both cases, PREs have been identified as a likely rain producer.

Typhoon Melor has been approaching the northwestern corner of the tropical Pacific Ocean over the past several days. At the same time, a ribbon of moist air, as denoted by high Precipitable Water values diagnosed from MIMIC, extends southwestward from Japan towards the straight of Luzon (Note also in the Precipitable Water loop the presence of former Typhoon — now Tropical Storm Parma meandering within the straight of Luzon as well.

The 11-micron window channel imagery show a general blossoming of cold cloud tops in and around the ribbon air with high precipitable water as typhoon Melor approaches (Link here). Indeed, the last image in the loop, shown above in this post, bears a resemblance to the enhanced 11-micron GOES-8 image from landfall of Floyd.

Is there evidence of a jet poleward of Melor that would support the development of the PRE? Consider the enhanced water vapor image from MTSAT below. The large gradient in brightness temperature — very warm values in and around Korea and the South China Sea northeastward to the Sea of Japan, and very cold temperatures to the east, suggest the presence of a strong jet. Plots of 300-hPa wind speeds confirm that; note the speeds exceeding 150 knots at Sapporo and at Nakashibetsu on the island of Hokkaido! The position of this jet is such that the left entrance region is supporting upward motion to help support heavy rain in a very moisture-rich atmosphere. (The GFS 00-h analysis at 1200 UTC 6 October shows this jet extending far out into the Pacific Ocean).

MTSAT water vapor image + 300 hPa rawinsonde wind speeds

MTSAT atmospheric motion vectors or “satellite winds”  (calculated by tracking satellite image features on 3 consecutive images) actually showed that wind speeds along the jet stream axis over the western Pacific Ocean were as high as 211 knots at the 218 mb level (below).

MTSAT water vapor image + MTSAT winds

(Added: TRMM measurements of rainfall with Melor are here).

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GOES-12 visible image

Late on October 4th, the weather system in the far northeast Atlantic acquired sufficient tropical characteristics to be classified as a Tropical Storm, and Grace was named. The visible image from GOES-12 above shows the counterclockwise swirl of clouds. GOES-12 is over the Equator at 75 degrees W Longitude, and Tropical Storm Grace at the time of the image above was at 45 degrees North latitude and 16 degrees W Longitude; consequently, the view angle is very oblique. Indeed, the visible image shows a convective spiral band that lies beneath the cirrus shield that covers the system. Note that no overshooting tops penetrate the cirrus overcast over the tropical system. The system sits over sea surface temperatures near 70 degrees Fahrenheit (see the Sea Surface Temperature analysis here, and those temperatures are yielding insufficient CAPEs to produce overshooting tops.

Grace developed underneath a decaying upper-level low. The low was able to draw north modestly high values of precipitable water, as shown in the MIMIC analysis here. Grace is associated with the very small region of enhanced precipitable water that is at 40 N, 20 W at the start of the loop, then moving northeastward towards Ireland.

A comparison of Terra MODIS visible and 11.0 µm IR images (below) showed that Grace exhibited a fairly well-defined banded structure and some semblance of an eye at 11:40 UTC.

MODIS visible and IR images

(Added: Jesse Ferrell at AccuWeather notes that Grace was almost the farthest-east forming tropical system on record! Link).

(Added, 6 October: Grace merged with/was absorbed by a front southwest of Ireland late in the day on the 5th.) AMSU microwave data from early on the day on the 5th clearly show a warm core to the system, one of the hallmarks of a tropical Storm. For example, data from the AMSU-A instrument in NOAA-18 at 0413 UTC on 5 October show a region of warmth at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7); the 89-GHz channel on AMSU-B also shows warmth at the center of the storm. These warm signals were critical in determining that system was tropical in nature. The warmth persisted; AMSU-A data from NOAA-19 at 1406 UTC on the 5th also showed a warm core at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7), as well as in the 89-GHz channel on AMSU-B. (More imagery is available here).

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MODIS visible, 2.1 µm near-IR, and 3.7 µm shortwave IR channels

AWIPS images of the MODIS visible channel, the 2.1 µm near-IR “snow/ice” channel, and the 3.7 µm shortwave IR channel (above) displayed a swath of snow cover on the ground in far northern Quebec, Canada on 04 October 2009. The Environment Canada snow cover analysis at 06 UTC placed a maximum of 21 cm (8 inches) in that area. The area of snow cover appeared bright on the visible image, and darker on the 2.1 µm near-IR snow/ice image (due to the strong absorption of snow at that wavelength) — however, there was a patch of supercooled water cloud over the northern portion of the snow cover, which appeared brighter white on the snow/ice image and darker (warmer) on the shortwave IR image (due to increased solar reflection off the supercooled water droplets).

A comparison of the MODIS visible image and the corresponding MODIS false color Red/Green/Blue (RGB) image constructed using the visible and the near-IR snow/ice channels (below) shows the value of RGB imagery for helping to distinguish between snow cover (which appears darker red on the false color image) and supercooled water droplet clouds (which appear as cyan to white shades on the false color image). Note the semi-transparent nature of this particular cloud deck: surface features (such as rivers, and the edges of the snow cover) can be seen through the thin cloud feature. Farther to the south, glaciated clouds that are composed primarily of ice crystals also appear as varying shades of red on the false color image. The ability to display these types of false-color RGB images will hopefully be available to forecasters using the next generation  of AWIPS II software.

MODIS visible and false color RGB images

The MODIS Land Surface Temperature product (below) indicated that LST values were only in the 30s F (darker green color enhancement) in the region of snow cover, compared to much warmer 40s and 50s F (lighter green to yellow color enhancement) in the surrounding bare ground areas.

MODIS Land Surface Temperature product

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GOES-12 6.5 µm water vapor imagery

AWIPS images of the GOES-12 6.5 µm “water vapor channel” (above) revealed a pair of vortices immediately poleward of a well-defined jet stream  axis that was moving over the southeastern US  on 28 September 2009. It was initially thought that these vortices may have represented either a type of  Kelvin-Helmholtz instability (which can occur when there is sufficient velocity difference across the interface between two fluids) or a type of Rayleigh-Taylor instability (which can occur along an interface of two fluids of different densities) — however, a more likely explanation might be that these vortices were a result of barotropic instability, where the waves grew by extracting kinetic energy from the shear flow from which they were embedded.

If a horizontal circulation  developed due to barotropic instability being forced by the horizontal wind shear, this could result in the formation of the vortex structures seen on the water vapor imagery. The warm/dry spot on the images (exhibiting brightness temperature values as warm as -11º C, darker orange color enhancement) was probably a pocket of warm/dry air that originated from the poleward edge of the moisture gradient — once the vortex formed, the warm/dry air in the center could not escape, and its properties would be preserved.  (Thanks to Jordan Gerth, Justin Sieglaff and Chris Rozoff at CIMSS…and Michael Morgan at UW-AOS for providing valuable inputs and helping to provide an explanation)

Overlays of parameters from the 45-km resolution CRAS model at 12:00 UTC  (below) showed the presence of a 50-60 knot jet axis just south of the primary dry-to-moist gradient on the water vapor image, along with a ribbon of 500 hPa vorticity and a 500 hPa wind shear axis over the region where the water vapor vortices were forming.

CRAS45 maximum wind speed, 500 hPa vorticity, and 500 hPa shear vectors

A comparison of the 1-km resolution MODIS 6.7 µm water vapor image and the 4-km resolution GOES-12 6.5 µm water vapor image (below) show the advantage of improved spatial resolution for displaying the structure and gradients associated with the leading vortex around 18:15 UTC.

1-km MODIS vs 4-km GOES-12 water vapor images

Examining the GOES-12 imager water vapor weighting function profiles at 00:00 UTC for Charleston SC (located in the “dry” portion of the sharp water vapor image gradient) and Jacksonville FL (located in the “moist” portion of the sharp water vapor image gradient) shows that there would be a pronounced downward shift in the altitude of features displayed on the water vapor image in the region of dry air located poleward of the jet stream axis.

GOES-12 water vapor weighting function profile for Charleston SC and Jacksonville FL

A northwest-to-southeast oriented vertical cross section using GFS40 model fields (below) displayed a minor intrusion of potential vorticity (the colored image portion of the cross section) downward into the upper troposphere immediately poleward of the jet stream core (which was located between the 200 and 250 hPa pressure levels). The wind speed shear axis was located at a much lower altitude (between the 400 and 500 hPa pressure levels), closer to the altitude peak of the water vapor channel weighting function in the region of drier air.

GFS40 model cross section

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