The interactions between Earth’s surface and atmosphere are crucial to understanding their impact on surface layer optical turbulence, specifically the temperature structure function (CT2) and refractive index structure function (Cn2). The Energy Balance Bowen Ratio (EBBR) – the ratio of sensible heat flux to latent heat flux – has shown promising capabilities to calculate sensible heat flux, a key component for computing CT2 and Cn2. Sensible heat as calculated via the Bowen Ratio inherently accounts for moisture content and evaporation as it apportions the balance of sensible heat to latent heat in the ratio. Thus it better permits the calculation of CT2 and Cn2 via a single equation only dependent on temperature and sensible heat in any stability condition as compared to “ground truth” sonic anemometer turbulence values during daylight and nighttime hours at various land sites. The Bulk Aerodynamic method relies on standard meteorological observations but requires stability corrections based on underlying assumptions with this approach. Researchers have shown success of Bulk Aerodynamic methods and similarity theory to predict Cn2 in the maritime surface layer, but many adjustments for weakness in stable conditions (air warmer than the water) are necessary. In this study, field data from a marine wave boundary layer test site allow for assessments of both the EBBR and Aerodynamic methods to quantify maritime surface layer turbulence, and the results compared to sonic anemometer and DELTA Cn2 values.
Surface layer optical turbulence values in the form of CT2 or Cn2 are often calculated from surface layer temperature, moisture, and wind characteristics and compared to measurements from sonic anemometers, differential temperature sensors, and imaging systems. A key derived component needed in the surface layer turbulence calculations is the “Sensible Heat” value. Typically, the sensible heat is calculated using the “Bulk Aerodynamic Method” that assumes a certain surface roughness and a “friction velocity” that approximates the turbulence drag on temperature and moisture mixing from the change in the average surface layer vertical wind velocity. These assumptions/approximations generally only apply in free convection conditions. A more robust method, that applies when free convection conditions are not occurring, to obtain the sensible heat is via the Energy Balance or Bowen Ratio method. The use of the Bowen ratio – the ratio of sensible heat flux to latent heat flux – allows a more direct assessment of the optical turbulence-driving surface layer sensible heat flux than do more traditional assessments of surface layer sensible heat flux. This study compares surface layer CT2 and Cn2 values using sensible heat values from the bulk aerodynamic and energy balance methods to measurements from instruments such as sonic anemometers, differential temperature sensors, and time-lapse imagery. This research further compares improvements to the calculations gained by using sonic anemometer eddy covariance values to obtain the friction velocity, and including humidity effects via covariance methods or simply using virtual temperature from the sonic anemometers.
Psychrometric measurements via sling psychrometers have long been the standard for quantifying thermodynamics of near-surface atmospheric gas-vapor mixtures, specifically moisture parameters. However, these devices are generally only used to measure temperature and humidity at one near-surface level. Multiple self-aspirating psychrometers can be used in a vertical configuration to measure temperature and moisture gradients and fluxes in the first 1-2 meters of the surface layer. This study evaluates the feasibility of a method using infrared (IR) imagery, and a mini-tower of wet and dry paper towels to psychometrically obtain surface layer temperature and moisture gradients and fluxes. First, the possible utility of using a single IR thermometer/detector to evaluate moisture and heat fluxes near the surface was explored, and it was found that the single IR sensor could be used to sense wet- and dry-bulb temperature changes of 0.7 K and 0.6 K respectively over vertical distances as small as 50 cm, thus allowing surface layer temperature and moisture gradients/fluxes to be quantified. The feasibility of this single IR detector method to provide with reasonable certainty values of surface layer heat and moisture fluxes suggests the technique could be exploited with more efficiency and accuracy with a calibrated imaging IR camera or sensor array. The surface layer dry- and wet-bulb temperatures obtained using an MWIR camera system are compared to Kestrel 4000 Weather Meter and Bacharach sling psychrometer measurements under various atmospheric conditions and surface types to test the viability of the method. Uncertainty statistics are calculated and evaluated to quantify effectiveness.
Sonic detection and ranging (SODAR) is a technique for measuring wind speed and turbulence parameters from backscattered sound waves. The SODAR projects a beam of sound straight up, as well as at angles slightly off vertical. Sound waves are scattered by variations in the density of the air and are then received back at the SODAR, the time of flight giving the height being probed. Doppler shifts provide information about the wind velocity. Since larger variations in the local density of the atmosphere imply higher turbulence, backscatter strength is related to turbulence. The instrument used here was a Scintec MFAS flat array SODAR. While the backscatter strength thus appears to be a direct indicator of the turbulence strength, calibration and an estimate of the variation of temperature with height is needed to process this strength into values for CT2 and Cn2. Consequently, it is interesting to compare measurements from this technique with results from other turbulence measurement approaches. A sonic anemometer measures the wind velocity and temperature over the volume of air between its probes. From this instrument, turbulence is estimated by the temperature variations in the air moved past the instrument by the wind. The sonic anemometer measures turbulence at a single location, while the SODAR measures turbulence as a function of height (up to about 400 meters above ground). Thus these comparisons aren’t really looking at the same thing. By mounting the sonic anemometer on a small UAV, this difficulty can be overcome.
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