Overshooting Tops in the Tropics

September 1st, 2011 |
GOES-13 Visible (0.63 µm) image and auto-detected Overshooting Tops (click image to toggle)

GOES-13 Visible (0.63 µm) image and auto-detected Overshooting Tops (click image to toggle)

Overshooting tops above a thunderstorm suggest an updraft powerful enough to penetrate through the tropopause into the stratosphere. In mid-latitudes, overshoots are well-correlated with both heavy precipitation and severe weather. In the tropics, overshooting tops are linked to intensification in tropical systems. In the image toggle above of an area of disturbed weather that may become a named tropical system in the Gulf of Mexico over the weekend, visible imagery and infrared data are shown with the automatically-detected overshooting tops (red being a more intense OT, that is, a bigger temperature difference between the tower top and the surrounding anvil cirrus through which the OT penetrates). Overshooting tops over the Continental United States are shown here (computed using data from GOES-13); Data from MSG are used to compute OTs over the tropical Atlantic, as shown here.

GOES-13 IR/WV (10.7 µm - 6.5 µm) used to detect Overshooting Tops

GOES-13 IR/WV (10.7 µm - 6.5 µm) used to detect Overshooting Tops

Difference fields between infrared data at 10.7 µm and the so-called water vapor channel (6.5 µm) can also be used to infer overshooting tops because Hot Towers push water vapor into the stratosphere and this alters the difference between the two channels. The image above, taken from the CIMSS Tropical Weather website, shows maxima in the difference field in the region where strong convection is occurring. (Click here for the toggle between the visible image and a similar WV/IR difference image). The automated detection algorithm seems to miss some towers; the wv/ir difference field identifies too many.

As the system in the Gulf of Mexico intensifies, the signal from the IR/WV difference and the number of autmomated detections should both increase. Intensification for now is inhibited by high shear over the region. The CIMSS Tropical Weather website offers information on this developing Gulf system.

Hurricane Katia

September 1st, 2011 |
GOES-13 10.7 µm IR images

GOES-13 10.7 µm IR images

Tropical Storm 12L intensified on the evening of 31 August to become Hurricane Katia, the second Atlantic basin hurricane of the 2011 season. GOES-13 10.7 µm IR images from the CIMSS Tropical Cyclones site (above) showed the development of deep organized convection both east and west of the storm center.

A DMSP SSMIS 85 GHz microwave image (below) revealed a closed eyewall had formed (although the southern portion of the eyewall was thin and poorly-defined).

DMSP SSMIS 85 GHz microwave image

DMSP SSMIS 85 GHz microwave image

Katia existed in an environment of relatively low 850-200 hPa deep layer wind shear (below), which was one factor that favored further intensification.

GOES-13 IR image + Deep layer wind shear product

GOES-13 IR image + Deep layer wind shear product

An AWIPS image of EUMETSAT MetOp ASCAT scatterometer surface winds (below) indicated that the radius of tropical storm force winds had increased since earlier in the day as Katia intensified.

ASCAT scatterometer surface winds

ASCAT scatterometer surface winds