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Science Briefing

Using computer models of California weather, three MMM scientists have successfully replicated the process that causes a narrow tongue of low clouds and fog to surge northward along the state's coast after clearing has occurred. Their findings will help improve the models used in actual forecasting and could lead to better warnings for transportation, defense, and recreation in the coastal zone.

Rich Rotunno
[Photo by Carlye Calvin.]
Joe Klemp, Bill Skamarock, and Rich Rotunno are presenting first results of their modeling this month at the American Geophysical Union's fall meeting in San Francisco. The modeling was conducted with support from the Office of Naval Research (ONR) under the Navy's Accelerated Research Initiative. Rich previewed the talk in a seminar for staff last month.

While fog can occur under a variety of circumstances, the northward-surge phenomenon--especially common in the summertime--has vexed California forecasters for years. The process begins when high pressure noses eastward into Washington, Oregon, and Idaho, producing light east or southeast winds on the California coast. As they descend from the Coast Range, the breezes bring dry, sunny conditions to the shoreline while they push the marine layer (the cool, moist air that extends a few hundred meters above the Pacific) just offshore.

Forecasters can predict the large-scale pressure changes that cause the winds to blow offshore, but they cannot yet reliably tell when the marine layer might return and surge northward to bring overcast or fog. The computer models used by present-day forecasters trace the atmosphere at points separated by around 30 to 90 kilometers. That resolution is too coarse to fully outline the surges, which can be less than 100 km wide.

A three-dimensional computer model at NCAR produced this simulation of a California coastal surge. The marine layer is depicted by the darker shading, with air temperatures below 294 degrees Kelvin (about 21 degrees Celsius or 70 degrees Fahrenheit). The arrows depict air flow wrapping around a weak low pressure center offshore and surging northward near the coast. (Illustration courtesy Bill Skamarock, Rich Rotunno, and Joe Klemp.)
Klemp and colleagues used a finer-scale research model in their attempt to depict the surges. They found that when the clearing-fog boundary is displaced a few tens of kilometers offshore, a weak area of low pressure--too small to be detected by coastal weather stations--may form just offshore. As the low circulates counterclockwise, it wraps moist marine air around its south side and toward the coast. When the marine air reaches the higher terrain along the shoreline, it is forced upward. Clouds and fog may form, along with a small high-pressure center that pushes the air northward. The result is a narrow tongue of cloud pinched between the coastline and the offshore clear zone. The surge can traverse hundreds of kilometers of coastline over a day or two.

The surge's movement up the coast can be characterized as a Kelvin wave, a particular kind of atmospheric feature in which winds blow in the direction of movement of a pressure disturbance. Research computer models tend to handle Kelvin waves skillfully, so this bodes well for surge prediction efforts. The modelers are now using a simplified version of the coast's topography, so their next step is to add sharper resolution to incorporate the bays, inlets, and peninsulas that dot the California coast.

"This phenomenon is a tough test for a model," says Rich. "Delicate imbalances (in pressure) seem to set it off, and it's too fine-scale to show up in most models. Still, it can produce enough fog to envelop boats and airports."

A Canadian university has joined the fold as the 62nd member institution of UCAR. The application of York University was formally approved by the assembled member representatives at the 1996 UCAR members' meeting, held in October at the Mesa Lab.

York is located in North York, Ontario, just north of Toronto. Founded in 1959, it serves over 40,000 students. York offers master's- and doctoral-level programs through its departments of chemistry, earth and atmospheric science, and physics and astronomy, along with its Centre for Research in Earth and Space Science. York faculty have collaborated with each of NCAR's scientific divisions. Their specialties include remote sensing technologies, regional climate modeling, and boundary layer analysis.


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Prepared by Jacque Marshall, jacque@ucar.edu, 303-497-8616