Characteristics of Surface Layer Scalar Profiles Using In-Situ Measurements in an Undisturbed Marine Environment

The marine atmospheric surface layer (MASL) is the lowest few tens of meters of the atmosphere immediately adjacent to the ocean surface. The MASL plays an important role in the exchange of heat, momentum and water vapor at the air-sea interface. Of particular interest in the MASL is the flux-profile relationship, which is generally assumed to follow the Monin-Obukhov Similarity Theory (MOST) under the assumption of stationarity and horizontal homogeneity. The most frequently used empirical non-dimensional relationships were, however, derived from measurements over land. Their applicability to the marine atmospheric surface layer has not been validated, especially in regard to the profiles of temperature and humidity. The effects of waves on the vertical gradient of the profiles are also largely unknown. In this context, direct measurements of the profiles of temperature and humidity from the lowest few tens of meters above the ocean surface would be of great importance to evaluate MOST applied to the MASL. Such measurements also provide a direct measurement of the evaporation duct, an important duct type affecting electromagnetic wave propagation over the ocean. This type of profile measurement should be done away from the immediate ship environment to avoid flow and thermal distortion by the ship. During the Coupled Air Sea Processes and Electromagnetic ducting Research (CASPER) pilot experiment (offshore Moss Landing, CA, April-May 2015) and later the CASPER-East experiment (offshore Duck, NC, October-November 2015), repeated measurements of the lowest 50 m of marine atmospheric surface layer were made from a small Rigid Hull Inflatable Boat (RHIB) or a small work boat equipped with a tethered profiling system and a small meteorological mast. The tethered profiling system consists of a 2 or 3 m3 Helikite tethered balloon, a radiosonde hanging below the Helikite, a radio signal receiver, and a tablet computer. The mast was equipped with mean sensors for temperature, humidity, wind, pressure, and bulk water temperature a few centimeters below the water surface. The small boats were used to minimize environmental distortions. For each profiling set at a given location, about 10-15 sounding profiles were made to allow sufficient samples to derive a mean profile. Several methods have been used to obtain a ‘mean' profile from the scattered profiling data in the surface layer. In addition to simple bin averages or a polynomial fit to the original data, an optimization method was also developed to derive mean profiles using a least-squares optimization method and the assumption that data were sampled from a population that follows MOST at each level. In this presentation, we will present the characteristics of surface layer profiles of temperature and humidity under various thermal stability and wind conditions. A comparison of these profiles with those generated from single level measurements using the modified COARE bulk flux algorithm will also be provided. Surface layer fluxes estimated from the profile optimization method will also be compared with those using the direct eddy covariance method