Evaporation Duct Height over the Arabian Sea Estimated from Surface-Layer Profiles Measurements

Accurate evaporation duct height (EDH) estimates are imperative for maritime radio communications as well as for Naval applications such as target detection and ranging. In general, the near-surface moisture gradients produced from surface evaporation gives rise to evaporation ducts, which are nearly always present over the oceans. Evaporation ducts trap radio and microwave EM energy from a transmitting source resulting in enhanced propagation range if both transmitter and receiver are in the duct.

Evaporation duct heights are estimated from the modified refractivity (M) profiles as the altitude where M is a local minimum in the surface layer. Due to the inherent difficulty in making profile measurements over the ocean surface, direct measurements of M-profiles are scarce. Therefore, Monin-Obukhov Similarity Theory (MOST) based surface layer models are usually employed to estimate M-profiles from the single level measurements of pressure, temperature, and humidity.

In this study, we used a published dataset of marine atmospheric surface layer profiles made over the Arabian Sea during the International Indian Ocean Experiment (IIOE) from 22 February 1964 to 09 March 1964. The data were taken from a floating instrument carrier named “Mentor.” Multiple level measurements of wind, temperature, and humidity were made in the lowest ~ 8 m of the surface layer. Sea surface temperature (SST) measurements were also available. For this analysis, we used a total of 118 quality-controlled observations.

Initial processing involved the manual conversion of the data in the published documents into digital format. The profiles clearly indicate the near-surface vertical gradient, although the 8-m maximum profiles do not always directly provide evaporation duct properties. We employed the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM) with the highest level measurements as inputs. This procedure yielded the M-profiles and evaporation duct heights. Under the stable conditions, the evaporation duct height varied from 7.7 to 73.8 m with a mean value of 28.5 m. In comparison, the EDH varied from 7.6 to 18.4 m under unstable surface layer conditions with a mean value of 12.5 meters. In addition, a comparison of modeled and observed near-surface gradient and EDH for stable and unstable conditions will be presented.