HOP Helicopter Observation Platform @ Duke University

Title: Measurement of ABL Turbulent Fluxes with the Duke HOP in Support of CLASIC

NASA Grant: NNX07AJ60G; PI: R Avissar; Period: 04/01/07-03/31/08

The primary goal of the Cloud and Land Surface Interaction Campaign (CLASIC) that was carried out in Oklahoma in June and August 2007 was to improve the parameterization of cumulus convection and its associated parameterization of land-surface processes. Nine aircraft, including the HOP, participated in this experiment in which an extensive network of ground micrometeorological stations, tethered balloons, radiosondes and lidars was also deployed. Its purpose was to decipher the respective roles of local and regional forcing on the observed cloud structures. Thus, the spatial and temporal observation of turbulent fluxes in the ABL was essential for the overall success of this experiment. These observations were provided by the Duke HOP, which flew a total of about 100 hours for the project.

Four sites were regularly sampled during these periods: (1) the Little Washita River basin; (2) Fort Cobb; (3) the ARM/CART “Central Facility” site; And (4) Okmulgee, which is a forested site located in the eastern part of Oklahoma. A triangular pattern at different altitudes in the ABL (including near the ground surface, near the middle of the ABL, and near the top of the ABL) was flown. Each triangle leg was 10-km long, thus allowing the HOP to complete a triangle in about 15 minutes when flying at 30 m/s, and four heights in about one hour. The figure below illustrates the track (as recorded by the on-board GPS) of the flight conducted on June 19 at the "Central Facility."

The data collected is used to study the balance of heat, moisture, and carbon dioxide at these sites, where clusters of ground observations were also operating. The figures below provide a sample of the turbulence statistics (variances and fluxes) derived from the high-frequency measurements that were performed on August 15 at the "Central Facility." The flight started at 1 p.m. and ended at about 3 p.m. The progression of the data collection is illustrated by different colors (first red, then green and last blue). The horizontal bars indicate one standard deviation, which illustrate the variability between the legs of the triangles.

Two flights corresponding to the passing of the A-Train constellation of satellites were also coordinated with some of the other aircraft to provide a useful sampling of the atmosphere for satellite calibration. More information about this project is available at http://acrf-campaign.arm.gov/clasic/ .

The map below indicates the location of the four triangles flown during CLASIC. In June, the HOP was stationed at Wiley Post Airport (near Oklahoma City) and in August, it was stationed at Ponca City Airport.

The picture below shows the ARM/CART Central Facility. It was taken during one of the HOP flights, on August 14, 2007. Note the land-cover type heterogeneity.

The Table below summarizes all the flights that were conducted during the field campaign in June and August 2007.