Real-Time Mesoscale Numerical Simulations in Support of Astronomical Operations at Mauna Kea Observatories

The summit of Mauna Kea on the Big Island of Hawaii is widely recognized as the premier site in the world for ground-based astronomy. Much of the uniqueness of this site is owed to a nearly 14,000' summit with favorable meteorological conditions. Located in the subtropical northeastern Pacific Ocean, the summit typically lies well above the persistent trade wind inversion, beneath which most cloudiness, water vapor, and pollutants are confined. The atmosphere above Mauna Kea most often is clear, clean, dry, and nearly transparent at optical and infrared wavelengths. However, changing flow patterns affect astronomical observing quality and occasionally bring hazardous weather conditions to the summit. A new research and forecasting initiative aims to better understand and anticipate changing conditions to optimize astronomical operations on Mauna Kea.

A dedicated real-time numerical modeling effort utilizing MM5 is a crucial aspect of the forecasting initiative. A 48-hour simulation is run once per day. The outer domain has 27-km horizontal resolution and covers a regional area surrounding the Hawaiian Islands. Intermediate domains cover the island chain (9-km resolution) and the entire Big Island (3-km resolution). The finest horizontal resolution is 1 km in a 55x55-km domain centered on the Mauna Kea summit. This resolution is needed to accurately represent the altitude of the summit and the effects of terrain on atmospheric and astronomical parameters.

Astronomical observing quality can vary significantly due to several atmospheric factors. Turbulence and the resulting changes in the vertical profile of refractive index cause phase distortions to incoming wavefronts, resulting in image motion and blurring. Several astronomical parameters that quantify these effects, generally referred to as "seeing," are explicitly predicted using MM5 temperature and wind profiles. Forecasts compare favorably with actual seeing from the summit, and seeing is shown to improve or degrade within particular synoptic flow patterns. Forecasts of other important parameters, including summit-upward precipitable water and summit temperature, also exhibit significant skill. Advance knowledge of all of these conditions allows telescopes with flexible observing schedules to optimize their strategy each night.

Adverse conditions that occasionally interrupt astronomical operations can occur at any time of year but are most common during the cool season. Strong winds, fog, snowfall, and ice accumulation at the summit are safety hazards that can require closing or evacuation of summit facilities. Cloud cover above the summit can hinder or prevent observing. These conditions generally result from a cold front, cutoff or "Kona" low, or a tropical cloud plume.

Growing archives of satellite data, daily MM5 output, and astronomical observations from the summit afford a unique opportunity to study in detail several aspects of tropical meteorology that affect Mauna Kea. The primary challenges are an incomplete understanding of tropical-extratropical interactions, the effects of mesoscale processes in a region of complex terrain, and data sparsity in a maritime environment. These challenges are being addressed with case studies of systems that present particular forecast difficulty, and with a climatological assessment of meteorological conditions that provide favorable or poor astronomical observing conditions.