Tropical Cyclone SSMI - 85 GHz PCT Tutorial

	NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil

Tropical Cyclone SSMI - 85 GHz PCT Tutorial

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Introduction

Example of PCT image for Bonnie	Bonnie Track Map

The PCT image isolates convection within tropical cyclones and thereby allows an easy-to-understand introduction to the ability of the SSM/Ito detect precipitation. It is sensitive to snowflakes above the freezing level, and thus gives an effective view of conditions aloft in a storm. Although the PCT is a vertical integration, it can be thought of as showing precipitation at roughly 5 to 9 km, or15 to 30 thousand feet. The Polarization Corrected Temperature (PCT) image combines the two polarizations of the 85 GHz channels. It enhances convection and virtually eliminates low-level clouds and ocean characteristics. Tounderstand the usefulness of the PCT, it is necessary to review the properties of 85 GHz in general. Scattering by large precipitation particles, especially by snowflakes above the freezing levels, causes 85GHz H brightness temperatures (Tb) to be low. Thus, convective rain bands tend to have very low Tb, often lower than 200 K. On the other hand,emission from low-level clouds and water vapor raises Tb as high as 280K. Emission effects from the storm atmosphere often completely hide the surface of the ocean from the satellite. However, away from the storm in cloud-free regions, the low-emissvity surface of the ocean can be detected by low 85 GHz H Tb, between 200 and 250 K. Unfortunately, the low Tb in these regions can match the depressed Tb in storm convection. So convection can be mistaken for the sea surface on uncorrected images of 85GHz. The PCT eliminates this ambiguity. See examples below for more clarification. The PCT shows convection in the same Tb as on uncorrectedimages of 85 GHz. Convection-free areas appear at uniform Tb of about 280K on the PCT image. Thus, the PCT image provides a "convective mask",isolating deep convective precipitation bands and eliminating other detail. For an excellent discussion of the PCT and SSM/I (including an example ofa tropical cyclone) see: Spencer, R.W., H.M. Goodman, R.E. Hood, Precipitation retrieval over landand ocean with SSM/I: Identification and characteristics of the scattering signal. J. Atmos. Ocean. Technology, 6, 254-273, 1989.

Example of PCT image for Bonnie

Bonnie Track Map

The PCT image isolates convection within tropical cyclones and thereby allows an easy-to-understand introduction to the ability of the SSM/Ito detect precipitation. It is sensitive to snowflakes above the freezing level, and thus gives an effective view of conditions aloft in a storm. Although the PCT is a vertical integration, it can be thought of as showing precipitation at roughly 5 to 9 km, or15 to 30 thousand feet.

The Polarization Corrected Temperature (PCT) image combines the two polarizations of the 85 GHz channels. It enhances convection and virtually eliminates low-level clouds and ocean characteristics. Tounderstand the usefulness of the PCT, it is necessary to review the properties of 85 GHz in general. Scattering by large precipitation particles, especially by snowflakes above the freezing levels, causes 85GHz H brightness temperatures (Tb) to be low. Thus, convective rain bands tend to have very low Tb, often lower than 200 K. On the other hand,emission from low-level clouds and water vapor raises Tb as high as 280K. Emission effects from the storm atmosphere often completely hide the surface of the ocean from the satellite. However, away from the storm in cloud-free regions, the low-emissvity surface of the ocean can be detected by low 85 GHz H Tb, between 200 and 250 K. Unfortunately, the low Tb in these regions can match the depressed Tb in storm convection. So convection can be mistaken for the sea surface on uncorrected images of 85GHz. The PCT eliminates this ambiguity. See examples below for more clarification. The PCT shows convection in the same Tb as on uncorrectedimages of 85 GHz. Convection-free areas appear at uniform Tb of about 280K on the PCT image. Thus, the PCT image provides a "convective mask",isolating deep convective precipitation bands and eliminating other detail.

For an excellent discussion of the PCT and SSM/I (including an example ofa tropical cyclone) see:

Spencer, R.W., H.M. Goodman, R.E. Hood, Precipitation retrieval over landand ocean with SSM/I: Identification and characteristics of the scattering signal. J. Atmos. Ocean. Technology, 6, 254-273, 1989.

Background

Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat Applications:Tutorial

Advantages

The 85 GHz PCT image shows areas of deep convective precipitation against a uniform background. It isolates the scattering signature produced by large snowflakes above the freezing level. Quite literally, the PCT sees a "snowstorm in a tropical cyclone". Unlike uncorrected images of 85 GHz, itdoes not detect cloud water, water vapor, or the sea surface. There is no ambiguity on the PCT image between cloud-free sea surfaces outside of the storm environment and storm convection: Tb lower than about 255 K will nearly always indicate precipitation.

Limits

The PCT image completely elimininates low cloud bands and low-level water vapor plumes, leaving onlydeep convective bands. Thus, it is not a good tool to observe weak tropical cyclones which are often marked only by low- level clouds. For these weak systems, the PCT image will often show just a uniform blue. For weak systems it is better to select the 85 GHz HW enhancement. Even for strong storms, the PCT image often eliminates useful low-level detail over tropical cyclones. A better choice is the color composite which uses the PCT image but combines it with the 85 GHz horizontal and vertical polarizations. This produces an image that shows deep convection and low cloud bands without ambiguous signatures from the sea surface.

Examples

Bonnie 85 GHz PCT	85 GHz H (W) -- Red color is ambiguous:

The image on the left shows a PCT image of Bonnie as the stormaffects the Carolinas. The PCT shows the convective structure only, and it eliminates any information about low cloud bands or the sea surface.In this image convection appears in green with embedded yellow. Everything else is dark blue. The image on the right shows the 85 GHz H Weak enhancement of the same scene as the PCT image. Notice that the storm convection is red, yellow and black. However, the cloud-free sea surface on either side of Florida is also red and yellow. Without further information, one could confuse the sea surface with convection.The PCT image eliminates this effect caused by the sea surface and thus solves the ambiguity. However, the PCT image also eliminate slow cloud bands. Notice, for example, the low cloud bands (blue) that appear to the east of the storm in the 85 GHz HW image (right). These are eliminated in the PCT image.

Bonnie 85 GHz PCT

85 GHz H (W) -- Red color is ambiguous:

The image on the left shows a PCT image of Bonnie as the stormaffects the Carolinas. The PCT shows the convective structure only, and it eliminates any information about low cloud bands or the sea surface.In this image convection appears in green with embedded yellow. Everything else is dark blue. The image on the right shows the 85 GHz H Weak enhancement of the same scene as the PCT image. Notice that the storm convection is red, yellow and black. However, the cloud-free sea surface on either side of Florida is also red and yellow. Without further information, one could confuse the sea surface with convection.The PCT image eliminates this effect caused by the sea surface and thus solves the ambiguity. However, the PCT image also eliminate slow cloud bands. Notice, for example, the low cloud bands (blue) that appear to the east of the storm in the 85 GHz HW image (right). These are eliminated in the PCT image.

GOES color IR	SSM/I Integrated Water Vapor

The image on the left is the GOES-8 infrared image that corresponds to the SSM/I images above and to the right. The infrared image illustrates that the clouds associated with Bonnie cover a much greater region than the convective precipitation as shown by the PCT image. The water vapor image on the right shows an area of elevated water vapor associated with Bonnie off the Carolinas (red). Notice the regions of lower water vapor surrounding Florida and extending northeastward (lightblue). This low- moisture region corresponds to the region of reds and yellows in the 85 GHz HW enhancement just above. Notice that it isa region of little cloudiness based on the infrared image to the left.

GOES color IR

SSM/I Integrated Water Vapor

The image on the left is the GOES-8 infrared image that corresponds to the SSM/I images above and to the right. The infrared image illustrates that the clouds associated with Bonnie cover a much greater region than the convective precipitation as shown by the PCT image. The water vapor image on the right shows an area of elevated water vapor associated with Bonnie off the Carolinas (red). Notice the regions of lower water vapor surrounding Florida and extending northeastward (lightblue). This low- moisture region corresponds to the region of reds and yellows in the 85 GHz HW enhancement just above. Notice that it isa region of little cloudiness based on the infrared image to the left.

85 GHz PCT	85 GHz Color Composite

The image on the left shows the same PCT image as above. The image on the right is the color composite image for the same scene which uses the PCT image in the red gun and the two polarizations of the 85 GHz channel in thegreen and blue guns. It retains the advantages of the PCT image without the drawbacks. Convective precipitation, contributed by the PCT image, appears in red. Land appears in bright blue. Water vapor over water is light blue; heavy low cloud with possible rain appears in a more solid color, e.g., just off the southern coast of North Carolina. Regions of little moisture over the oceans appear black, e.g., off both coasts of Florida. On the color composite image there is no ambiguity between dry ocean regions and convective precipitation as on image of 85 GHz H (see above). Unlike the PCT, the color composite image shows low-level water vapor and precipitation. Thus, it isthe optimal viewing images for many applications.

85 GHz PCT

85 GHz Color Composite

The image on the left shows the same PCT image as above. The image on the right is the color composite image for the same scene which uses the PCT image in the red gun and the two polarizations of the 85 GHz channel in thegreen and blue guns. It retains the advantages of the PCT image without the drawbacks. Convective precipitation, contributed by the PCT image, appears in red. Land appears in bright blue. Water vapor over water is light blue; heavy low cloud with possible rain appears in a more solid color, e.g., just off the southern coast of North Carolina. Regions of little moisture over the oceans appear black, e.g., off both coasts of Florida. On the color composite image there is no ambiguity between dry ocean regions and convective precipitation as on image of 85 GHz H (see above). Unlike the PCT, the color composite image shows low-level water vapor and precipitation. Thus, it isthe optimal viewing images for many applications.

85 GHz PCT	SSM/I Rainrate

The image on the left is the PCT image as above. The image on the right is the rainrate image for the same scene. Because of the inability of the rainrate algorithm to detect precipitation over coastal waters, a blank strip appears just off North Carolina. Precipitation is undoubtedly falling here based on the PCT image. Notice that the rainrate image, at a 25 km spatial resolution, has a much coarser look than the PCT image at 12 km spatial resolution.

85 GHz PCT

SSM/I Rainrate

The image on the left is the PCT image as above. The image on the right is the rainrate image for the same scene. Because of the inability of the rainrate algorithm to detect precipitation over coastal waters, a blank strip appears just off North Carolina. Precipitation is undoubtedly falling here based on the PCT image. Notice that the rainrate image, at a 25 km spatial resolution, has a much coarser look than the PCT image at 12 km spatial resolution.

Author: 7541tl\@nrlmry.navy.mil
Last Updated: Tue Dec 17 15:27:37 2002
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NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil