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Mauna Kea Weather Center

by Louise Good

A sample weather forecast by MKWC

This forecast graphical summary shows the forecast items used most frequently to predict the quality of observations on a given night: odds of fog/precipitation, cloud cover (CC) and height, summit temperature, precipitable water (PW), seeing, and wind speed and direction for the six-day forecast period. Credit: MKWC.

Ground-based astronomy depends heavily on the weather. In that sense, astronomers may have more in common with farmers than with other physical scientists. When bad weather prevents long-planned observations, an astronomer may have to wait a year to again have telescope time when that object is visible. And even when the weather is not bad enough to prevent observing, such factors as air turbulence and the amount of moisture in the air can affect the quality of the observations.

Thus it is not surprising that in July 1998 IfA and the UH Manoa Department of Meteorology established the Mauna Kea Weather Center (MKWC), a research and forecast facility funded by the astronomical observatories on Mauna Kea to provide forecasts relevant to astronomical observations.

Douglas Simons
Douglas Simons
Bob McLaren
Bob McLaren
Steven Businger
Steven Businger

The MKWC resulted from a serendipitous meeting. In November 1996, Douglas Simons, then associate director for development at Gemini Observatory, and IfA Associate Director Bob McLaren, a member of the Gemini Board of Directors, met with meteorologists in the Honolulu office of the National Weather Service to see if NWS could issue custom forecasts for Mauna Kea. There, they happened to meet Thomas Schroeder, then the chair of the UH Manoa Department of Meteorology, and Steven Businger, also a UH meteorology professor. Schroeder suggested that Businger was the right person for the job, since custom forecasts fall outside the purview of NWS.

The MKWC staff consists of Businger, the principal investigator for the project; Research Meteorologist Tiziana Cherubini, the lead weather modeler and a forecaster, and affiliate research faculty in the Department of Meteorology; and Forecast Meteorologist Ryan Lyman, who serves as the lead forecaster, MKWC website administrator, administrator of the cluster of computers needed for modeling and forecasting, and a general jack-of-all-trades. Businger and Cherubini have offices in the Department of Meteorology, while Lyman works in the IfA Hilo office, and occasionally, on the summit of Mauna Kea, where he has installed weather stations that measure weather variables, including winds, temperature, precipitation, and pressure. Lyman issues twice-daily forecasts Monday through Friday, with occasional updates on weekends. These forecasts are available on the Web. MKWC also issues warnings of tropical cyclones, snows, and strong winds to give the observatories time to close, to secure their domes, and to evacuate the summit, if necessary. By maintaining a data archive that is used for research and development, and comparing its forecasts against the actual weather, MKWC is able to improve its forecasting model.

The ability to predict the seeing on a given night is one of the factors that has contributed to the effectiveness of queued observing, which is now in use  at several of the telescopes on Mauna Kea. In classically scheduled observing, in which each project is assigned one or more specific nights, observers are at the mercy of the weather. However, for queue-scheduled telescopes, which include at least part of the observing on the Gemini North Telescope, the Canada-France-Hawaii Telescope, and the NASA Infrared Telescope Facility, highly ranked programs have a strong likelihood of being completed because they are at the beginning of the queue. A list of objects is submitted for each project, and astronomers who work for that observatory observe them in order of priority. The data they collect is then sent to the project astronomers. With increasingly accurate weather forecasts, the observers are able to consider the predicted weather as one factor when they plan each night’s observing, though they may need to change their plans during the night if the predictions prove to be wrong. If observing conditions are close to ideal, they can make observations that require such conditions. Otherwise, they may make observations that don’t require excellent seeing. This optimizes the use of these telescopes.

MKWC forecasters are working to increase the accuracy of their conventional and seeing forecasts and to be able to issue longer-term seeing forecasts. They also hope to expand their services to Chile, where the best observatories in the Southern Hemisphere are located.

Businger and Cherubini have edited a textbook, Seeing Clearly: The Impact of Atmospheric Turbulence on the Propagation of Extraterrestrial Radiation, which they hope will interest more students in studying the effect of turbulence on astronomical observations.