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In order to provide nowcasts of CI using IR satellite indicators, a scoring system is developed that incorporates the interest fields described above as a simple sum of positive indicators for the occurrence of CI. It is important to restate that each IR-based CI interest field used in the scoring is related to the physics of cloud growth and glaciation related to precipitation formation in cumuliform clouds, and thus describes the time-evolution of the CI process from an IR perspective. A summary of the criteria incorporated into this scoring system is presented in the table above, which justifies why eight of the possible 14 IR information pieces from GOES are used; in essence, redundant information exists across several of the IR interest fields. For the CI nowcasting assessments, one point (score) is assigned to each pixel when a CI interest field criterion is met. Satellite pixels that meet at least 7 of 8 CI criteria have been determined to represent rapidly growing, immature (non-precipitating) cumulus in a pre-CI state. The underlying assumption in this nowcasting system is that immature cumulus exhibiting recent signs of rapid development will continue to evolve into precipitating convective storms, provided that the cloud has access to sufficient ABL/elevated moisture source.
A nowcast map of future CI can be produced by combining the eight CI interest fields from the table above. Pixels that meet at least seven of the eight criteria as outlined in the table have been highlighted in red, and provide a forecast of CI over the following 30-45 mins. A red pixel represents a vertically developing, newly glaciated cumulus with a cloud-top TB within the 0 to -20º C range (from Roberts and Rutledge 2003) that meets seven CI criteria. Pixels highlighted in grey represent mature cumulus that are likely precipitating (cumulonimbus) or cirrus clouds, and have been omitted from processing. A comparison of the red pixels to future radar imagery at 2030 and 2100 UTC demonstrates the algorithm's skill.
The CI nowcast product shown above identifies future development of the primary convective line in region 1. Pixels identified in western Missouri also evolved into precipitating convective storms (not shown in radar imagery). Within region 2, the nowcast identifies future CI in north-central Kansas, as well as weaker convective growth in southeast Kansas. For this particular case, the nowcast has demonstrated predictive skill in identifying future CI associated with moving convective storms at 30-45 min lead times. Accuracies of approximately 70% are obtained when pixel-by-pixel comparisons are made between the CI nowcast pixels and radar echoes >= 30 dBZ in subsequent imagery. It needs to be stated that the underlying premise of the CI nowcast product is the assumption that linear trends in cumulus development will continue in the future. As important, the algorithm identifies locations where the mesoscale convergent forcing is supporting organized updrafts of sufficient scale to produce precipitation, and the upscale growth of cumulus clouds.
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