Rapidly-occluding cyclone off the mid-Atlantic coast

January 22nd, 2010 |
GOES-12 water vapor images + surface pressure and frontal analysis

GOES-12 water vapor images + surface pressure and frontal analysis

A cyclone off the mid-Atlantic coast was quickly transitioning to the occluded stage on the morning of 22 January 2010 (above). As the cyclone (in its mature stage) was just beginning to move out over the offshore waters of the Atlantic Ocean, winds gusted to 51 knots at buoy 41025 (Diamond Shoals NC) around 07 UTC and 37 knots at buoy 44014 (64 miles east of Virginia Beach VA) around 11 UTC.

The evolution of a classic “dry swirl” signature was seen on AWIPS images of the 4-km resolution GOES-12 6.5 µm water vapor channel (below) — this dry swirl water vapor signature is a tell-tale sign that a cyclone has reached occlusion (and is often seen with systems over the open oceans).

GOES-12 6.5 µm water vapor images

GOES-12 6.5 µm water vapor images

The appearance of an subtle elongated filament structure on the water vapor imagery (exiting the Georgia / South Carolina coast after about 10 UTC, to the southwest of the occluding cyclone) suggested the presence of a northeastward-propagating jet streak. A plot of 1-hourly MADIS satellite-derived water vapor atmospheric motion vectors (below) revealed one target with a velocity of 151 knots in the general vicinity of the thin filament signature — this wind speed was significantly higher than the 100-110 knots that was initialized by the GFS40 model over that particular area.

GOES-12 water vapor image + satellite winds + GFS 250 hPa isotachs

GOES-12 water vapor image + satellite winds + GFS 250 hPa isotachs

McIDAS images of the 1-km resolution GOES-12 visible channel data (below) showed finer details in the low-level cloud structure, as well as the formation of convective bursts near the circulation center toward the end of the animation.

GOES-12 visible images + buoy and ship wind reports

GOES-12 visible images + buoy and ship wind reports

A comparison of 1-km resolution NOAA-17 AVHRR visible channel and 10.8 µm IR channel images at 14:10 UTC (below) showed the cloud structures around the time that the dry swirl signature was becoming more well-defined on the GOES-12 water vapor imagery.

NOAA-17 AVHRR visible and 10.8 µm IR images

NOAA-17 AVHRR visible and 10.8 µm IR images

The AVHRR Cloud Top Temperature (CTT) product (below) indicated that CTT values were as cold as -70º C (black color enhancement) prior to the cyclone transitioning to the occluded stage.

AVHRR Cloud Top Temperature product

AVHRR Cloud Top Temperature product

Open-cell vs. closed-cell convection over the Pacific Ocean

January 21st, 2010 |

GOES-11 visible images

GOES-11 visible images [click to enlarge]

GOES-11 (GOES-West) visible channel images (above) displayed an unusually large area of open-cell cumulus clouds across the North Pacific Ocean on 20 January 2010. This type of open-cell mesoscale convective cloud pattern is a signature of strong instability (via boundary layer cold air advection over relatively warmer waters) in an environment of cyclonic flow. These cloud patterns tend to be fairly shallow, as indicated by their relatively warm appearance on IR imagery (below) — and the presence of open cell convection usually indicates that winds within the marine boundary layer are greater than about 25 knots.

AWIPS composite IR image + GFS360 surface winds

AWIPS composite IR image + GFS360 surface winds [click to enlarge]

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GOES-11 visible images

GOES-11 visible images [click to enlarge]

On the following day (21 January 2010), the cold front marking the leading edge of the cold air advection had moved south of 20º N latitude — and GOES-11 visible images (above) showed the formation of a large area of closed-cell stratocumulus clouds to the northeast of the Hawaiian Islands. This type of closed-cell convection often forms in regions of anticyclonic flow, as confirmed by the GFS360 surface wind field (below) — and stronger subsidence causes the cumulus cloud features to flatten out and form stratocumulus clouds beneath the subsidence inversion. Also note the “barrier effect” of the Hawaiian Islands on the marine boundary layer stratocumulus, as well as the formation of lee cloud lines downwind of the islands.

AWIPS composite IR image + GFS360 suface winds

AWIPS composite IR image + GFS360 surface winds [click to enlarge]

As a deep cyclone approached the California coast on 21 January, a number of all-time minimum pressure records were set across the state. A MODIS false color Red/Green/Blue (RGB) image (below) shows the extensive cloudiness associated with this storm; on this RBG image supercooled clouds appear as white features, while glaciated clouds take on more of a lighter pink color. Snow cover shows up as darker pink areas (such as those over northern Nevada and southern Oregon/Idaho).

MODIS Red/Green/Blue (RGB) false color image

MODIS Red/Green/Blue (RGB) false color image [click to enlarge]

The first US tornado of 2010: in southern California, of all places

January 19th, 2010 |
GOES-11 10.7 µm IR images

GOES-11 10.7 µm IR images

McIDAS images of the 4-km resolution GOES-11 10.7 µm IR channel data (above) showed the development of convection that moved onshore across southern California during the afternoon hours on 19 January 2010. Of particular interest is the strong convective cluster that developed just offshore over the Santa Catalina and San Clemente Island area — the GOES-11 IR brightness temperatures of this feature cooled to -53º C (darker orange color enhancement) as it was moving inland around 20:46 UTC (12:46 pm local time).

A closer view using AWIPS images of the GOES-11 IR channel with overlays of surface METAR reports and cloud-to-ground lightning strikes (below) revealed that this convective cell with the coldest cloud tops exhibited a number of lightning strikes (as many as 31 for the 15-minute period ending at 21:30 UTC). These storms were developing in the vicinity of a surface wave that had formed along a strong cold frontal boundary — and according to SPC Storm Reports (overlaid on a MODIS 11.0 µm IR image) there were two reports of a tornado in the Huntington Beach area and a number of high wind gusts (including 93 mph at Newport Beach, and 92 mph at Huntington Beach) as the severe thunderstorm moved onshore. For radar images and additional information, see the WeatherMatrix blog.

GOES-11 10.7 µm IR images + Surface METAR reports + Lightning strikes

GOES-11 10.7 µm IR images + Surface METAR reports + Lightning strikes

POES AVHRR Cloud Top Temperature product

POES AVHRR Cloud Top Temperature product

The 1-km resolution POES AVHRR Cloud Top Temperature (CTT) product (above) indicated that the coldest CTT value with this storm was -56º C, with the Cloud Top Height product (below) showing the maximum cloud tops at a height of 14 km.

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

A MODIS 6.7 µm water vapor image with an overlay of CRAS model Level of Maximum Wind isotachs (below) showed that southern California was located beneath the left exit region of a strong upper level jet stream, which likely contributed to strong deep layer wind shear and ageostrophic forcing to aid in the development of the severe thunderstorms.

MODIS 6.7 µm water vapor mage + CRAS model Level of Maximum Wind isotachs

MODIS 6.7 µm water vapor mage + CRAS model Level of Maximum Wind isotachs

A sequence of water vapor images from 15 January to 19 January (below) showed that this particular storm was one of a series of strong disturbances that was moving across the Pacific Ocean along a very strong zonal (west to east) jet stream.

MTSAT + GOES-11 composite water vapor images (15 - 19 January)

MTSAT + GOES-11 composite water vapor images (15 - 19 January)

On 18 January, The GFS360 model Level of Maximum Wind isotach field indicated that winds just exceeded 200 knots within the core of the jet stream; however, GOES-11 satellite-derived water vapor winds (below) suggested that speeds were as high as 239 knots along the jet stream axis.

GOES-11 water vapor image + GOES-11 water vapor winds

GOES-11 water vapor image + GOES-11 water vapor winds

Fog across the western Great Lakes region

January 16th, 2010 |
POES AVHRR fog/stratus product + METAR surface reports

POES AVHRR fog/stratus product + METAR surface reports

An AWIPS image of the 1-km resolution POES AVHRR 11.0-3.7 µm “fog/stratus product” (above) revealed a very interesting banded structure in the fog and stratus features (yellow to red color enhancement) that covered a large part of the western Great Lakes region at 07:49 UTC on 16 January 2010. With stagnant high pressure in place and light winds over much of the area, there was little boundary layer mixing to mitigate the formation and persistence of the low clouds that were trapped under a very strong low-level temperature inversion (Rawinsonde data: Davenport IA | Lincoln IL | Wilmington OH; NAM12 Line A-A’ west-to-east cross section).

An animation of the 4-km resolution GOES-12 11.0-3.9 µm fog/stratus product (below) suggested that there might be a thin veil of high-altitude cirrus clouds (which show up as darker black features) drifting over the banded portion of the fog and stratus — the presence of overlying high clouds could have been contributing to the unusual appearance of those low cloud features on the fog/stratus product imagery.

GOES-12 fog/stratus product + surface pressure/fronts

GOES-12 fog/stratus product + surface pressure/fronts

To further explore whether or not there were indeed cirrus clouds overhead that might be masking the low-altitude fog/stratus signal and creating the false appearance of banding as seen on the fog/stratus product, let’s examine a few additional satellite images. The POES AVHRR 10.8 µm IR image (below) did show some thin filaments that exhibited slightly colder IR brightness temperatures, but most were warmer than -20º C — much warmer than would be expected of typical cirrus clouds. However, with very thin cirrus features, a significant amount of thermal energy reaches the satellite IR detectors from the warmer surfaces below; this then leads to IR brightness temperatures values that are much warmer than those of the actual cirrus clouds themselves.

POES AVHRR 10.8 µm IR image

POES AVHRR 10.8 µm IR image

Multi-spectral POES AVHRR derived products such as Cloud Type, Cloud Top Temperature, and Cloud Top Height (below) did a better job at identifying more of the thin features as Cirrus (orange color enhancement on the Cloud Type product) — with cloud top temperatures of -30º C to -40º C and cloud top heights of 7-8 km — but some of the thinnest cirrus filaments were still flagged as supercooled water droplet clouds (cyan color enhancement on the Cloud Type product).

POES AVHRR Cloud Type, Cloud Top Temperature, and Cloud Top Height products

POES AVHRR Cloud Type, Cloud Top Temperature, and Cloud Top Height products

In this case, perhaps the best satellite product to confirm the presence of thin cirrus filaments aloft was the 1-km resolution MODIS 6.7 µm water vapor channel (below). Since the weighting function of such a water vapor channel normally peaks at higher altitudes in the middle troposphere, it is generally immune to the effects of warm surface radiation that can sometimes plague proper cirrus cloud classification using conventional IR imagery alone.

MODIS 6.7 µm water vapor image

MODIS 6.7 µm water vapor image

As it turns out, a broad region of high-altitude “transverse banding” had formed from the southern Great Lakes to the mid-Atlantic region — this banding was generally located along the axis of a strong (120 to 130 knot) anticyclonically-curved jet stream axis, as confirmed by an overlay of CRAS model Level of Maximum Wind isotachs (below).

MODIS 6.7 µm water vapor image + CRAS45 Level of Maximum Wind isotachs

MODIS 6.7 µm water vapor image + CRAS45 Level of Maximum Wind isotachs

This transverse banding is a satellite signature that indicates a potential for high-altitude turbulence — and there were a few pilot reports of moderate turbulence over the area during the 02-11 UTC time period, shown overlaid on 4-km resolution GOES-12 6.5 µm water vapor images (below).

GOES-12 6.5 µm water vapor images + pilot reports of turbulence

GOES-12 6.5 µm water vapor images + pilot reports of turbulence