Temperature deviation from Equilibrium Temperature:

Convective Overshoot from Satellite Imagery


Robert Rabin1, Phillip Bothwell2 and Steven Weiss2

1NOAA/NSSL and UW-Madison/CIMSS
NOAA/NWS/SPC2


BACKGROUND

Satellite imagery from the window channel (11 microns), commonly refered to as infrared (IR) imagery, is often used to monitor the temperature of anvil and penetrating convective clouds as an indication of storm intensity.  As summarized by Rabin (2004), there has been a long history of research on this topic.  Several studies have related the rate of cloud top expansion and cooling to updraft strength, and to some extent, storm severity.  One particular study stressed the need to consider the cloud top temperature in relation to the tropopause temperature as an indication updraft strength and possible hail production (Reynolds, 1980).  In one example, an analysis of a storm which moved over a dense surface observing network indicated that hail size was related to the temperature difference between the cloud top and presumed equilibrium parcel temperature.  Peak hail size occurred when the temperature difference was most negative, indicating the greatest overshoot of the updraft above its equilibrium level. 

Ideally, it would be preferable to infer updraft strength from the height of the penetrating cloud tops above the equilibrium level rather than the temperture deviation.  Unfortunately, estimation of cloud top heights is quite uncertain. Derivation of height from cloud top temperature generally assumes that cloud tops are in equilibrium with the air temperature along an observed or modeled temperature profile.  This is not always valid for overshooting convective tops since the cloud top temperature may be several degress colder than the large-scale environmental temperature at the same height, depending on the lapse rate.  Other methods of measuring cloud heights include stereoscopic analysis from two or more satellites with sychronized scanning over common locations. Such techniques have yet to be automated with sufficient accuracy. 

Given the difficulty in estimating convective cloud top heights, use of temperature deviations from equilibrium temperature is explored here.  The equilibrium temperature is obtained from parcel analysis using an objective analysis performed at the SPC using three-dimensional temperature fields from hourly RUC analyses and the most recent surface observations.  Equilibrium temperature is obtained from the "best CAPE" or the parcel yielding the most thermodynamic instability originating from any initial level in the first 300 mb.  Gridded equilibrium  temperatures are converted to McIDAS image areas and subtracted from window channel temperature from GOES-12, obtained from the Cooperative Instutute of Meteorological Satellite Studies (CIMSS), University of Wisconsin-Madison.  Resultant imagery is color inhanced to indicate the amount of temperature deviation. 

Also explored here is the use of brightness temperature differences, BTD, between the water vapor (6.7 microns, GOES channel 3) and window IR  (11 microns, GOES channel 4).  Observations have shown that BTD > 0 can occur when water vapor exists above cloud tops in a stably stratified lower stratosphere.  Water vapor may reach the lower stratosphere from overshooting tops.; hence BTD > 0 has been used a measure for intensity of overshooting convection.   

Java based applications used for interactive animations were developed by Tom Whittaker of the Space Science and Engineering Center  SSEC, University of Wisconsin-Madison.  The animations can take a while to load, depending on network speed, computer systems, etc. Also, there can be a problem viewing these on certain machines (Macs).

Click here for 4km imagery

REAL TIME DATA
 
   

Table 1. Most recent images

 
Equilibrium Level Temperature
Cloud Top Temperature
Overshoot Temperature
BT(6.7)-BT(11)


Click here to animate these images (useful to visualize differences)



ARCHIVED DATA (Last 24 hours)

Table 2.

Time Period (UTC)
Overshoot temperature
Cloud Top and Overshoot temperature
Cloud Top temperature
Equilibrium Temperature
BT(6.7)-BT(11)
0000-0245
X
X
X
X
X
0300-0545
X
X
X
X
X
0600-0845
X
X
X
X
X
0900-1145
X
X
X
X
X
1200-1445
X
X
X
X
X
1500-1745
X
X
X
X
X
1800-2045
X
X
X
X
X
2100-2345
X
X
X
X
X


ARCHIVED CASES
 
29-30 October 2004

10-11 November 2004

17 November 2004

NOTE: Starting with 21 November 2004, data from previous days may be viewed from the following address:

http://overshoot.nssl.noaa.gov/ddmmmyy (8km imagery)

or since 13 April 2005:

http://overshoot.nssl.noaa.gov/ddmmmyy/4km.html (4km imagery)



where dd=day, mmm=First three letters of the month (first letter capitalized), yy=year.

For example,  http://overshoot.nssl.noaa.gov/21Nov04

or http://overshoot.nssl.noaa.gov/13Apr05/4km.html



References:

Rabin, R.M., 2004: Nowcasting thunderstorm intensity from satellite, a review. The 2004 EUMETSAT Meteorological Satellite Conference, Prague, czech Republic, 31 May- 4 June, 180-188.

Reynolds, D.W., 1980: Observations of damaging hailstorms from geosynchronous satellite digital data. Mon. Wea. Rev., 108, 337-348.

        A more complete manuscript including an analysis of several convective cases is available by clicking here .
 


Disclaimer. The products from GOES or other satellites shown here are experimental. These have been generated within a research environment and are not intended to be considered operational. Timeliness, availability, and accuracy are sought but not guaranteed.

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Last update was 12 September 2004. Feedback.


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