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CBLAST program to improve forecasting

Jan 1, 2003
From Ocean Navigator #127 January/February 2003

From Ocean Navigator #127 January/February 2003

A new weather-prediction program focused on modeling air turbulence above the sea surface will likely lead to better weather forecasts for mariners. The so-called Coupled Boundary Layers and Air-Sea Transfer program, recently initiated off Martha’s Vineyard’s south shore, will examine “the turbulent processes that are really responsible for exchange of the sun’s energy at the ocean’s surface,” said Dr. Jim Edson, Woods Hole Oceanographic Institution’s lead meteorologist on the project. The program is funded by the Office of Naval Research and supervised by scientists at the WHOI in Woods Hole, Mass.

According to Edson, the data collected will be used to refine the numerical models used to forecast storms, hurricanes and other weather phenomena. “The model is only as good as the data it’s initialized with. You’ve got to start it off with reality,” he said. The turbulence at the ocean-atmosphere interface has never been added to the equation. It is important because the air and water exchange of heat drives weather processes. By altering the temperature of lower atmospheric layers, the ocean creates imbalance. The imbalance causes vertical motion as upper and lower atmospheric layers exchange heat in search of equilibrium.

“If you have vertical motion, you need to fill in what’s going up with something from the sides,” Edson said. That something is wind.

The greater the imbalance, the more vertical motion and the more severe the weather system. A high-pressure system has a downward vertical velocity, while a low-pressure system has an aggressive upward vertical velocity.

But it does not end there. Just as it creates an imbalance in temperatures, the ocean creates an imbalance in momentum. Friction causes the movement of air at the ocean’s surface to be slower than higher levels of air. In an attempt to reach equilibrium, faster, higher air drives down into the lower air, causing what mariners know as wind gusts. The wind gusts drive waves and currents.

Forecast models are currently incapable of analyzing the turbulence at the ocean-air interface, which causes the heat and momentum imbalances in the atmosphere. But scientists will be able to parameterize that turbulence through the CBLAST program, Edson said.

In late October, the WHOI completed the installation of an Air-Sea Interaction Tower located two miles south of Edgartown Great Pond in 50 feet of water. Shaped like a camera tripod, ASIT extends 76 feet into the air from the ocean floor. The end result is that oceanographers will be conducting underwater tests as meteorologists conduct atmospheric tests. As of yet, the condition of the ocean is an uncharted variable in weather forecasting.

“This really limits the validity of a five-day marine forecast, because over five days the ocean has changed considerably,” Edson added. “What’s really cool about the tower is that, probably for the first time, the oceanographers are going to try and do the same thing under the ocean as meteorologists are doing above the ocean.”

Invariably, what is going on beneath the ocean’s surface has an effect on the rate of energy exchange between the ocean and the atmosphere. In turn, this has an effect on ocean circulation. On a larger scale, CBLAST will enable physicists to improve the numerical models used in weather forecasting. The data collected through ASIT and other weather observatories — i.e., buoys, ships and aircraft — will be added to the models to produce a weather forecast. Then the model forecasts will be tested for accuracy by examining what is actually happening in the section of the ocean being monitored. Inaccuracies will be examined in light of new data supplied through the CBLAST program.

ASIT has been permitted for five years and without maintenance has a shelf life of 10 years. CBLAST has been funded for two years, but Edson said that the WHOI expects additional funding for many more years of research.

ASIT is linked to WHOI’s Martha’s Vineyard Coastal Observatory (MVCO). Running for more than a year now, MVCO consists of an onshore lab and meteorological mast, and an offshore underwater node.

Jonathan Burke