A case study analysis of the Cloud-Top Height Product (CTOP) during the landfall of Hurricane Frances

This paper presents a case study to evaluate how well the Cloud-Top Height product (CTOP), created by the Federal Aviation Administration Aviation Weather Research Program's (FAA/AWRP) Oceanic Weather Product Development Team (OW PDT), corresponds with other cloud top observational platforms. The CTOP is based on a combination of GOES IR emittance values with temperature and pressure profiles from the Global Forecast System (GFS) numerical weather prediction model. The product diagnoses clouds above 15,000 feet and is intended to diagnose clouds that may be a hazard to aviation.

One of the challenges for the AWRP's Quality Assessment Product Development Team (QA PDT) in evaluating the CTOP is finding high quality data sets that are independent of those used in the creation of the product. It is important to note that none of the data sources compared and contrasted to the CTOP are a true measure of the cloud top height, but all are inferred values. The datasets used in this study include a GOES sounder-based cloud-top pressure product provided by NESDIS (NCTP), echo tops (ET) derived from NEXRAD radar data, and cloud top heights estimated from rawinsonde (RCTH) observations.

For this case study, observations and diagnostics from 5 September 2004 were examined; this day was characterized by interesting and significant weather along the Florida Peninsula due to Hurricane Frances making landfall. All datasets (including the land based radar and rawinsondes) were also available along the coast line. Qualitative results show that the CTOP and NCTP products were similar. However, with a nominal resolution of 14 km in the region examined, the NCTP product did not capture the fine-scale details as well as the CTOP, which has 4 km resolution. The resolution also hindered the ET dataset, which displayed the maximum echo top in each 4x4 km box. For the rawinsonde data, balloons are launched twice a day at 00 UTC and 12 UTC, so the valid times can be difficult to match up with the CTOP. In general, the trends in both the ET and RCTH data provided analogous information regarding the minimum and maximum cloud top areas when compared to the CTOP.

Overall, the comparisons of each dataset with the CTOP indicate good agreement much of the time for this case study. The higher resolution in the CTOP gives more detail with values that are consistent with the other types of observations.