A GOES imager-derived microburst product

Kenneth L. Pryor Center for Satellite Applications and Research (NOAA/NESDIS)
Camp Springs, MD

1. Introduction

A suite of products has been developed and evaluated to assess meteorological hazards to aircraft in flight derived from the current generation of Geostationary Operational Environmental Satellite (GOES). The existing suite of products includes derived images to address major aviation hazards including fog, aircraft icing, and volcanic ash. The products, derived from the GOES imager, utilize algorithms that employ temperature differencing techniques to highlight regions of elevated risk to aircraft. A new multispectral GOES imager product has been developed to assess downburst potential over the western United States. Wakimoto (1985), based on his study of microbursts that occurred during the Joint Airport Weather Studies (JAWS) project, noted favorable environmental conditions over the western United States:
(1) intense solar heating of the surface and a resulting superadiabatic surface layer; (2) a deep, dry-adiabatic convective boundary layer (Sorbjan 1989) that extends upward to near the 500mb level; (3) a well-mixed moisture profile with a large relative humidity gradient between the mid-troposphere and the surface. Peak downdraft speeds associated with microbursts over the western U.S. result from negative buoyancy due to evaporation of precipitation during descent below cloud base.

Transmittance weighting functions specify the relative contribution each atmospheric layer makes to the radiation emitted to space and thereby determine regions of the atmosphere which are sensed from space at a particular wavelength. Temperature and moisture profiling for the purpose of inferring the presence of a favorable environment for microbursts would be accomplished with a group of spectral bands selected to detect radiation emitted from layers of interest in the atmosphere. GOES-11 imager channel characteristics are available online:
http://cimss.ssec.wisc.edu/goes/goesmain.html#imgrinfo . As illustrated in Figure 1, transmittance weighting functions over the northwestern United States in the summer would dictate that band 3 is most sensitive to water vapor present between the 200 and 500 mb levels. Band 5 is more sensitive to moisture in the boundary layer, and thus, when significant moisture is present, brightness temperatures observed by band 5 would be slightly lower than that observed in band 4.

Figure 1. Transmittance weighting functions for GOES-10/11 bands 3 and 5.

Accordingly, a new GOES-West (GOES-11) imager microburst algorithm employs brightness temperature differences (BTD) between band 3 (upper level water vapor, 6.7μm), band 4 (longwave infrared window, 10.7μm), and split window band 5 (12μm). Band 3 is intended to indicate mid to upper-level moisture content and advection while band 5 indicates low-level moisture content. It follows that large BTDs between bands 3 and 5 imply a large relative humidity gradient between the mid- troposphere and the surface, a condition favorable for strong convective downdraft generation due to evaporational cooling of precipitation in the deep sub-cloud layer. In addition, small BTDs between bands 4 and 5 indicate a relatively dry surface layer with solar heating in progress. Thus the GOES imager microburst risk (MBR) product is based on the following algorithm in which the output brightness temperature (B) is proportional to microburst potential:

MBR (B) = {T5 – T3} – {T4 – T5}

(1)

Where the parameter Tn represents the brightness temperature observed in a particular imager band. The relationship between BTDs and microburst risk in the product image is based on the following assumptions: (1) A deep, well-mixed convective boundary layer exists in the region of interest; (2) moisture for convective storm development is based in the mid-troposphere and is advected over the region of interest.

This paper will outline the development of the GOES-West imager microburst product and present case studies that feature example images, outline potential operational use and assess performance of the algorithm.

2. Methodology

The objective of this validation effort was to qualitatively and quantitatively assess the performance of the GOES imager-derived microburst product by employing classical statistical analysis of real-time data. Accordingly, this effort entailed a study of downburst events over the Eastern Snake River Plain (ESRP) of southeastern Idaho during the summer of 2007 that was executed in a manner that emulates historic field projects such as the 1982 Joint Airport Weather Studies (JAWS) (Wakimoto 1985). GOES-11 image data was collected for pre-convective environments associated with eight microburst events that occurred within the Idaho National Laboratory (INL) mesonet do

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