Investigations of Heat and Mass Exchange over the Ocean

Investigations of heat and mass exchange are being conducted using data taken from several recent field programs including he ONR sponsored CBLAST program and the NSF sponsored CLIMODE and GASEX programs. The CBLAST-LOW experiments were primarily conducted in low to moderate winds while the CLIMODE and GASEX experiments focused on air-sea interactions at moderate to high winds. The combined data set therefore covers a wide range of wind and stability conditions. For example, near surface winds of 15 m/s were commonly encountered over the North Atlantic during CLIMODE and the data set includes wind events with speeds over 25 m/s. These high wind events drive surface stresses that routinely exceed 1.0 N/m2 and combined latent and sensible heat fluxes from the ocean into the atmosphere that exceed 1200 W/m2. These enormous heat fluxes are driven by high winds and large air-sea temperature and humidity differences encountered over the Gulf Stream during cold air outbreaks.

The combined observations from the 3 programs show that the transfer coefficient for buoyancy, which includes contributions from the latent and sensible heat fluxes, are in reasonable agreement with the COARE 3.0 algorithm for wind speeds less than 15 m/s. However, there are significant differences with C3.0 at higher winds. Therefore, the data is now being used to reduce the uncertainty in the transfer coefficients for wind speeds greater than 15 m/s. Under these conditions, a number of studies have shown that evaporating sea-spray begins to have a noticeable impact on the heat exchange. For example, the reduction of the buoyancy transfer coefficient could be related to the effects of sea spray. However, the contribution of both latent and sensible heat to this flux makes it difficult to determine the impact of sea-spray.

Fortunately, the inclusion of infrared hydrometers allows us to separate the fluxes into the latent and sensible heat flux, so we can investigate their respective transfer coefficients directly. These investigations show that transfer coefficients for sensible and latent heat fluxes differ from each other and argue against a single parameterization for both sensible and latent heat. For example, the CBLAST measurements indicate that the directly measured fluxes are somewhat lower than C3.0 when the latent heat flux is positive (corresponding to an upward moisture flux), but are significantly different than C3.0 when the latent heat flux is negative (corresponding to a downward moisture flux). The downward latent heat flux is often associated with fog and stable conditions. However, the CBLAST data indicates that the Dalton number (i.e., the transfer coefficient for latent heat) is still smaller that C3.0 even after removal of downward fluxes and foggy periods. Therefore, the CBLAST algorithm proposes a neutral Dalton number that is 20% lower than the COARE algorithm at low to moderate wind speeds. On the other hand, the Stanton number (i.e., the transfer coefficient for sensible heat) is in reasonable agreement with COARE 3.0 below 15 m/s. This result argues against the commonly held assumption that the neutral transfer coefficients for heat and mass are equal. Separate latent and sensible heat flux estimates from the CLIMODE and GASEX experiments are being combined with the CBLAST data to investigate these processes – including spray.