GOES-16 Split Window Difference Precursor to Convective Initiation

There are two GOES-16 products related to convective activity.  One is related to convective initiation; that is, this product will identify new cumulus that will further develop into mature thunderstorms.  The second product identifies which mature thunderstorms have a high probability of producing severe weather.  However, both GOES-16 products require active cumulus development.  We seek to fill a void by providing a product that aids in the identification of future convective initiation in a clear sky scene.  The purpose of this blog entry has 3 parts: 1) training, 2) two case studies, and 3) both color and gray-scale enhancement tables, applied to GOES-16 split window difference product.

A common goal of the both VISIT and SHyMet programs is to provide training to WFO, CWSU, and National Center forecasters.  One way to provide training information to these users is through the use of readily accessible information that appears on this training blog.  Previous blog training has focused on a variety of weather events; for example, winter weather, severe convection, tropical cyclones, aviation applications, and fire weather.  In this blog entry two case studies that focus on precursors to convective initiation are discussed.

Although there has been somewhat of a lack of severe convection during spring of 2018, two cases have been identified to highlight the use of the split window difference product in the identification of precursors to convective initiation.  They occurred on 15 and 29 May 2018  both in the Texas panhandle.  On 15 May 2018 a slowly moving outflow from previous convection interacted with a low-level convergence boundary.  In contrast, 29 May 2018 focuses on a dryline / cold front interaction event.  As a reminder to the reader, the split window difference product aids in the identification of where cumulus may first develop within clear sky conditions.  We urge the reader to exercise caution in that the use of this product is inappropriate for the identification of severe thunderstorms.  One significant challenge is the development of a satellite enhancement table to highlight features of interest.

To begin with, the 15 May 2018 case will be used to illustrate modification of satellite enhancement tables.  Even though a default color table exists for the split window difference product in AWIPS, we will demonstrate the usefulness of modifying satellite enhancement tables.  An example image is taken at 1607 UTC, see image below:

A black oval is used to denote the clear sky precursor signature in both images on the left side.  Within the gray-scale image, two arrows are used to denote the western and eastern edges of the signature.  An oval was not used due to variations in visual perception, i.e., the precursor may not be apparent to some people if a black oval is used.  Note in particular the lack of clouds within the oval in the visible image located in the top right panel.

To illustrate the utility of the 3 types of enhancements, the following GOES-16 animation is provided:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/29may18/swd/swd_4panel&loop_speed_ms=60

Top Left panel: Split Window Difference (10.3 – 12.3 micron) product in the range of -10 to +10 Celsius with the CIRA SLIDER enhancement (http://rammb-slider.cira.colostate.edu/?sat=goes-16&sec=conus&x=5000&y=5000&z=0&im=12&ts=1&st=0&et=0&speed=130&motion=loop&map=1&lat=0&p%5B0%5D=35&opacity%5B0%5D=1&hidden%5B0%5D=0&pause=0&slider=-1&hide_controls=0&mouse_draw=0&s=rammb-slider) overlaid with METARs.

Top right: Visible (0.64 micron)

Bottom left:  Split Window Difference (10.3 – 12.3 micron) product with the default AWIPS color table, in the range of -15 to +15 Celsius.

Bottom right: Split Window Difference (10.3 – 12.3 micron) product with the gray-scale linear enhancement on AWIPS in the range from -10 to 10 Celsius.

The point of the animation is to provide the reader an opportunity to view different enhancements at once.  Of importance are the variations in both the ranges and colors / gray-scale used for each split difference produce panel.  To some, the top left may be preferred while others may prefer the bottom right and yet some will show preference to the bottom left.  One key aspect to keep in mind is that the precursor signature corresponds to clear skies in the visible imagery prior to approximately 1700 UTC.

For completeness, a brief physical explanation of the split window difference signature now follows.  Water vapor in the boundary layer is an absorbing gas to energy at 10.3 and 12.3 microns that is emitted from the earth’s surface.  Water vapor absorbs more energy at 12.3 compared to 10.3 microns; therefore, when the temperature decreases with height the brightness temperature at 12.3 microns is less than the brightness temperature at 10.3 microns, hence the difference is positive.  Along convergence zones, water vapor is transported upwards, making the moist air relatively deeper compared to its surroundings and thus amplifying the channel difference.

Another example of a gray-scale enhancement highlights the usefulness of modifying the range of the split window difference product.  For clarity, the gray-scale loop on the bottom right in the previous animation is repeated along with a modified range (from 1.7 to 10 Celsius):

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/29may18/swd/swd_2panel&loop_speed_ms=60

This example illustrates the use of changing the scale from -10 to 10 Celsius (top) to 1.7 to 10 Celsius (bottom) while preserving the same enhancement table (linear).  One consequence of decreasing the range of 1.7 to 10 C is to increase the contrast.  The motivation is to provide the reader with another example of how the data can be displayed.  That is, some may prefer the top loop while others may prefer the bottom.

Finally, a larger version of the upper left loop of the 4 panel animation above is shown here as another way to view the data:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/29may18/swd/swd_metar&loop_speed_ms=60

The 29 May 2018 case focuses on convective initiation in the northeast Texas panhandle as shown in this animation:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/29may18/swd/swd_29may_4panel&loop_speed_ms=60

Top Left panel: Split Window Difference (10.3 – 12.3 micron) product in the range of -10 to +10 Celsius with the CIRA SLIDER enhancement (http://rammb-slider.cira.colostate.edu/?sat=goes-16&sec=conus&x=5000&y=5000&z=0&im=12&ts=1&st=0&et=0&speed=130&motion=loop&map=1&lat=0&p%5B0%5D=35&opacity%5B0%5D=1&hidden%5B0%5D=0&pause=0&slider=-1&hide_controls=0&mouse_draw=0&s=rammb-slider) overlaid with METARs.

Top right: Visible (0.64 micron)

Bottom left:  Same as top left, except the range has been changed to -5.1 to +10 Celsius.

Bottom right: Split Window Difference (10.3 – 12.3 micron) product with the gray-scale linear enhancement on AWIPS, except the range has been modified to 0.9 to 10 Celsius.

In general, upper level cirrus clouds are seen stream from west to east from eastern New Mexico over the Texas panhandle into southwest Oklahoma.  The moisture gradient associated with the dryline is shown with red colors on the moist side and green/yellow/blue colors on the dry side (note the eastward movement of the moisture gradient just south of the cirrus in the southern Texas panhandle).  Followed by convective initiation in the northeast Texas panhandle around 2000 UTC.

The technique for modifying the range on the bottom 2 panels in AWIPS is as follows:

Hold down the right mouse button on the split window difference product, choose colormap.

A GUI will appear titled “Set Color Table Range”, scroll the Min bar until the contrast on the imagery is to your preference.

The above technique applies in general in modifying the enhancement and/or data range.

Further information:

Lindsey et al. (2014): Use of the GOES-R Split Window Difference to Diagnose Deepening Low-Level Water Vapor

Lindsey et al. (2018): Using the GOES-16 Split Window Difference to Detect a Boundary Prior to Cloud Formation

FDTD GOES-16 Applications webinar on the Split Window Difference

 

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