The GOES Hybrid Microburst Index

A favorable environment for downbursts associated with deep convective storms systems that occur over the central and eastern continental United States includes strong static instability with large amounts of convective available potential energy (CAPE) and the presence of a mid-tropospheric layer of dry (low theta-e) air. A Geostationary Operational Environmental Satellite (GOES) sounder-derived wet microburst severity index (WMSI) was developed and implemented to assess the potential magnitude of convective downbursts, incorporating CAPE as well as the vertical theta-e difference (TeD) between the surface and mid-troposphere to infer the presence of a mid-level dry air layer. However, previous research has identified that over the central United States, especially in the Great Plains region, an environment between that favorable for wet and dry microbursts may exist during the convective season, resulting in the generation of "hybrid" type microbursts. Hybrid microbursts have been found to originate from deep convective storms that generate heavy precipitation, with sub-cloud evaporation of precipitation a significant factor in downdraft acceleration. Accordingly, a new GOES sounder derived product is under development that is designed to assess the potential for convective downbursts that develop in an intermediate environment between a “wet” type, associated with heavy precipitation, and a “dry” type associated with convection in which very little to no precipitation is observed at the surface. The GOES Hybrid Microburst Index (HMI) algorithm is designed to infer the presence of a convective boundary layer by incorporating the sub-cloud temperature lapse rate (between the 670 and 850 millibar (mb) levels) as well as the dew point depression difference between the typical level of a convective cloud base (670 mb) and the sub-cloud layer (850 mb). Used in conjunction with the GOES WMSI product, the HMI product is intended to indicate the potential magnitude of convective downbursts associated with intermediate type thermodynamic environments over the Great Plains of the United States. In addition, it has been found that the GOES HMI product can be an effective indicator of the presence of the dryline, which can serve as an initiating mechanism for deep convective storm activity. Preliminary validation, using surface observation data from the Oklahoma Mesonet for the period of 1 June to 31 August 2005, has shown favorable results for the coordinated use of the GOES HMI and WMSI. A statistically significant correlation of 0.69 has been found between GOES WMSI values and the magnitude of convective wind gusts for HMI values greater than 16 (considered to be a significant risk). This result highlights the importance of both sub-cloud evaporational cooling as well as static instability in the generation of convective downbursts in an environment typical of the Southern Plains region of the United States. This paper will outline the development of the HMI algorithm and provide examples of the new HMI product, in which index values at each sounding retrieval location are plotted on GOES water vapor imagery. Validation data for the 2005 convective season will be presented. Case studies will then be presented that demonstrate the performance of the coordinated use of the WMSI and HMI during convective events over the Southern Plains. Finally, a modification to convective wind gust prediction with the coordinated use of the WMSI and HMI will be presented.