U.S. Army Engineer Research and Development Center (ERDC) Cold Regions Research and Engineering Laboratory (CRREL) Scientists Kathy Jones and Kerry Claffey, along with Contractor Sandra Jones, recently ventured to the summit of Mount Washington (6,288 feet elevation) in New Hampshire.
The team worked at the Mount Washington Observatory for a week in late October 2013, measuring the liquid water content of supercooled clouds and the size of the cloud drops, making measurements around the clock when the summit was in the clouds. When temperatures are below freezing, the supercooled cloud drops accrete as rime ice on the windward side of objects. Ice accretion rate and appearance of the ice depend on the air temperature, wind speed and cloud properties.
While working at the observatory, the team encountered wind speeds of up to 80 mph and temperatures as low as 20 degrees Fahrenheit. The mountain is famous for its severe winter weather, and for 76 years held the record for the highest wind gust – 231 mph – directly measured at the Earth's surface.
Small cloud drops tend to follow wind streamlines around objects in their path, while the greater inertia of large drops tends to make them collide with objects. This tendency is characterized by collision efficiency, which varies with wind speed, drop diameter and diameter of the object. The researchers were exploiting the variation in the collision efficiencies of cloud drops with cylinders of different diameters to determine cloud properties from the mass of ice accreted on the six cylinders of a rotating multicylinder.
Over the past 20 years, CRREL research teams have conducted many short field experiments at the observatory to make these intensive measurements. In the October field experiment, the team observed relatively large cloud water contents and drop sizes compared to typical conditions later in the winter.
The cloud data from these field campaigns will be compared to cloud microphysics schemes and modeled cloud properties from the Weather Research and Forecasting (WRF) model. The WRF model is increasingly being used to determine the risk of in-cloud icing for aircraft and wind turbines.