Abstract
Sky cover is a unique parameter because its quantification is subject to the perspective of the observer or characteristics of the observing instrumentation. Forecasting sky cover provided a professional challenge to operational meteorologists seeking to offer a refined forecast beyond numerical weather prediction guidance along the path of totality resulting from a solar eclipse traversing North America on August 21, 2017. A routine analysis with which to monitor subtle trends in sky cover and compare sky cover forecasts is also not widely available. This contribution reviews 1-h gridded forecasts of sky cover from the United States National Weather Service (NWS) on the eclipse day and compares them with hourly satellite and surface sky observations for an area of interest over the southeastern United States. An inconsistency between the real-time mesoscale analysis (RTMA) and the NWS National Digital Forecast Database is revealed during the eclipse totality. A satellite-to-satellite comparison of the adjusted average cloud top emissivity over this same area reveals how resolution and algorithm improvements to next-generation satellite imagers may alter the RTMA of total cloud cover in the latest era of Geostationary Operational Environmental Satellites (GOES), starting with the GOES-16 Advanced Baseline Imager.
Highlights
Ahead of the solar eclipse of August 21, 2017, with a path of totality that crossed the United States from Oregon to South Carolina, interested Americans and international tourists seeking to observe the eclipse needed to find a cloud-free viewing location
In the Federal Meteorological Handbook No 1, sky cover is defined as “the amount of the celestial dome hidden by clouds and/or obscurations.”[1]. The difference between sky cover and cloud fraction is the frame of reference
The presented analyses and forecasts of cloud and sky cover over the southeastern United States on August 21, 2017, highlight the challenges in forecasting sky cover during this unique event and finding a suitable representation of observed sky conditions
Summary
Ahead of the solar eclipse of August 21, 2017, with a path of totality that crossed the United States from Oregon to South Carolina, interested Americans and international tourists seeking to observe the eclipse needed to find a cloud-free viewing location. This increased public interest for sky cover climatologies and forecasts, which typically have more specialized applications. Numerical weather prediction (NWP) models provide total and layer cloud cover fields, which are calculated based on the fractional cloud coverage of a grid cell, and, for some NWP models, are smoothed horizontally to better approximate a celestial dome. As discussed further in Appendix A.1, there are several diagnostic approaches to calculating cloud fraction fields from prognostic variables in NWP models
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