Tropcal Storm Ernesto moved inland over southern Florida this morning, and was downgraded to a Tropical Depression. The AWIPS image below shows a MODIS vs. GOES comparison of the longwave IR (“IR Window”) and shortwave IR channels around 16 UTC. Note on the 1-km resolution MODIS IR Window channel (upper left panel) the much larger circular-shaped area of colder cloud top temperatures (-75 to -80 C, gray to white enhancement) over Florida, compared to the 4-km resolution GOES-12 IR Window channel (upper right panel). Also of interest is a signature of slightly warmer cloud top temperatures (darker grey enhancement) over that same region on the MODIS shortwave IR (lower left panel) — no such signature was yet evident on the corresponding GOES-12 shortwave IR at that time (lower right panel):

This signature on the shortwave IR channel is due to the dominance of smaller cloud particles within the storm tops of the active convection (similar to what is sometimes observed with “overshooting tops” associated with severe convection) — these smaller, more numerous ice particles exhibit a larger shortwave IR albedo, which is manifest as slightly warmer shortwave IR brightness temperatures (due to enhanced solar reflectance). GOES-10 Super Rapid Scan Operations (SRSO) imagery at 1-minute intervals (IR Window | Shortwave IR | Visible) indicated that this particular burst of convection was beginning to rapidly intensify around 16 UTC; this “small ice particle signature” did eventually become obvious on the GOES-10 shortwave IR imagery, but not until around 16:11 UTC.

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The GOES-10 satellite has been placed into Super Rapid Scan Operations (SRSO) mode while the satellite is being re-positioned to support Southern Hemisphere operations this Fall. While in SRSO, the imagery is available at 1-minute intervals for certain portions of each hour. This frequent imaging schedule lets us view the evolution of cloud features on a much shorter time scale than the normal 15-minute scan interval allows.

On 29 August 2006, some interesting boundary layer roll clouds developed in the Oklahoma panhandle region — this is often a signature of strong warm air advection within the lower troposphere. Organized convection is then seen to develop near the western (upstream) edge of these cloud features.

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GOES-12 “water vapor channel” (6.5 µm) imagery on 28 August 2006 showed a rather strong moisture gradient in the southcentral US, oriented along a cold frontal boundary. In the color enhancement applied to an AWIPS water vapor image (above), dry air is denoted by orange to yellow shades, while increasing moisture is indicated by blue to white shades; thick clouds are enhanced with white to green colors. In particular, note the sharp water vapor gradient between the dry atmosphere at Amarillo, Texas (KAMA) and the moist atmosphere at Fort Worth, Texas (KFWD):

Water vapor imagery shows us the distribution of moisture features within the middle to upper troposphere — but the actual altitude of those dry or moist layers that are being detected can vary quite a bit, depending on the temperature/moisture profile of the atmosphere. To assess the altitude and depth of the layer from which the radiation is originating, we can calculate the water vapor weighting function based upon the temperature and moisture profile for that region. The CIMSS GOES Realtime Weighting Functions site allows you to select a particular rawinsonde location to view the corresponding GOES weighting functions for a few of the imager and sounder spectral bands (including the 6.5 µm imager water vapor and the 7.4 µm/7.0 µm/6.5 µm sounder water vapor bands). Note on the plots below that in the dry air at Amarillo TX the peak altitudes of the various imager and sounder water vapor weighting functions is significantly lower than those in the moist atmosphere just 310 miles (500 km) to the southeast at Fort Worth TX:

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Hurricane Wilma rapidly intensified on 19 October 2005, becoming the most intense tropical cyclone ever observed in the Western Hemisphere. GOES-12 10.7 µm IR images (above; QuickTime animation) revealed cloud top brightness temperatures as cold as -87º C (darker purple enhancement) early in the day; the eye of Wilma also demonstrated a pronounced “trochoidal oscillation” or “looping motion” on the image animation.

GOES-12 visible images (below; QuickTime animation) showed that a “pinhole eye” was associated with Wilma on 19 October — at its peak intensity, the pinhole eye of Wilma was only about 3 miles (5 km) in diameter, the smallest eye ever to be observed in an Atlantic hurricane. Such pinhole eyes are often observed with tropical cyclones that are rapidly intensifying.

A comparison of two GOES-12 visible images (below) shows that Wilma’s pinhole eye on 19 October grew dramatically within 48 hours, becoming much larger on 21 October. The two visible images are displayed using the same magnification and projection, and are at the same time (17:45 UTC) on those two days.

The track of Hurricane Wilma passed over southern Florida on 24 October. MODIS true color images of that region before (16 October) and after (25 October) the passage of Wilma (below) show that the turbidity of the offshore waters increased dramatically due to the strong winds and heavy wave action associated with the tropical cyclone.

 

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