Evaluation of the impact of vertical resolution changes on mesoscale COAMPS® forecasts over the Arabian Gulf based on observed and modeled surface ducting events

Fleet Numerical Meteorology and Oceanography Center (FNMOC) is one of only a few elite operational production centers conducting daily mesoscale NWP forecasting throughout the world. FNMOC currently produces over 50 twice daily regional COAMPS® forecasts in support of Navy missions of both an unclassified and classified nature. Due to the large number of forecasts produced, HPC resources are at a premium. This limits both the vertical and horizontal resolutions which can be used for any given area forecast. Currently, area forecasts use 30 vertical levels stretching from the surface to the middle troposphere with horizontal resolutions of its innermost nests ranging from 2 to 27km. FNMOC is in the process of increasing both the vertical and horizontal resolution of its model domains. Preliminary testing has shown that increased horizontal resolution improves forecast skill for synoptic weather metrics; however, increased vertical resolution does not. Those preliminary tests, however, only examined standard FNMOC state-variable statistical metrics (bias, correlation and RMS) for temperature, pressure and moisture conditions representative of the entire domain.

The purpose of this new study is to evaluate the impact of vertical resolution on another key NAVY planetary boundary layer (PBL) parameter, radio frequency (RF) refraction, which was not examined in preliminary studies. RF refraction is sensitive to gradients in both temperature and moisture in the PBL, especially within the lowest ~500 meters. Of particular importance to the Navy are the RF ducts that can form in this region, trapping RF energy and greatly affecting radar propagation. The work presented here shows the impact of increased resolution on temperature, moisture and ducting (in terms of M-Unit deficits) throughout the troposphere using a number of different vertical structures. Observations taken from shipboard radiosonde measurements at high temporal frequencies during October 1999 in the Arabian Gulf are used to verify the mesoscale model results. These results will then be used to develop a strategy at FNMOC for increasing the vertical resolution for the operational forecasts to sufficiently capture ducting events.

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