Stress, turbulence, and heat flow measurements over the Gulf of Maine and surrounding land

Abstract

This report presents turbulence, flux, temperature and water vapor data obtained from an airplane flying over the Gulf of Maine and adjacent shores. Measurements of the root-mean-square turbulent deviation velocities, shearing stresses, and heat flows were made at many heights and offshore distances in air masses moving from land to water. Stability effects on the turbulence and fluxes of heat and momentum have been Observed over a wide range of conditions as air flowed over cooler or warmer surfaces. The following conclusions have been drawn from a study of the data: (1) The magnitudes of the root-mean-square turbulent deviation velocities δw and δu 3 increases with height in the lowest 100 meters and then decreases slowly with height up to the inversion where the velocities drop to very low values. (2) Shearing stresses were found to increase with height up to the 100 to 200 in level and then drop off with height. This height variation is in contrast to the generally accepted notion of a decrease of the stress from the surface to the geostrophic level. These observations confirm the findings of Scrase (1930) and others and demand an investigation of the acceleration of the air and the effects of thermal winds. (3) Both the horizontal and vertical components of the turbulent wind are increased by hydrostatic instability and decreased by stability. The horizontal component is affected less than the vertical component by stability differences. (4) The decrease in the turbulent velocities as air passes from land to cooler water is great and rapid while the increase in turbulent velocities as the air passes over warmer water is slight and slow. (5) The observed heat fluxes also first increase and then decrease with height and usually become negative near the top of the mixed ground layer where the potential temperature gradient becomes strongly positive. (6) The stability of the air above about 50 m is a very poor indicator of the temperature difference existing between the underlying surface and the air of the main mixed layer. Diffusion of heat downward from a layer of warm air above the ground layer frequently is the cause of a stable lapse rate regardless of the relative temperature of the surface below. (7) No comparison of the coefficient of turbulent mass exchange for water vapor, heat flow and momentum could be made since the temperature gradient was stable even when heat was flowing upward, and no wind profiles were made over the water.