Investigations of scalar transfer coefficients in fog during the Coupled Boundary Layers and Air Sea Transfer Experiment : a case study

Abstract

The uncertainty in the determination of the momentum and scalar fluxes remains one of the main obstacles to accurate numerical forecasts in low to moderate wind conditions. For example, latent heat fluxes computed from data using direct covariance and bulk aerodynamic methods show that there is good agreement in unstable conditions when the latent heat flux values are generally positive. However, the agreement is relatively poor in stable conditions, particularly when the moisture flux is directed downward. If the direct covariance measurements are indeed accurate, then they clearly indicate that the bulk aerodynamic formula overestimate the downward moisture flux in stable conditions. As a result, comparisons of the Dalton number for unstable and stable conditions indicate a marked difference in value between the two stability regimes.Investigations done for this thesis used data taken primarily at the Air-Sea Interaction Tower (ASIT) during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment 2003 from the 20-27 August 2003. Other data from the shore based Martha's Vineyard Coastal Observatory (MVCO) and moored buoys in the vicinity of the ASIT were also incorporated. During this eight day period, the boundary layer was often characterized by light winds, a stably stratified surface layer and a swell dominated wave field. Additionally, the advection of warm moist air over cooler water resulted in fog formation and a downward flux of moisture on at least three occasions. Therefore, a primary objective of this thesis is to present a case study to investigate the cause of this shortcoming in the bulk formula under these conditions by examining the physical processes that are unique to these boundary layers. Particular attention will be paid to the behavior of the Dalton number in a stable marine atmospheric boundary layer under foggy conditions using insights derived from the study of fog formation and current flux parameterization methods.

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

“…The ABL over the CBLAST region was typically stable during the first half of the summer resulting from the combination of advection and a slowly warming coastal ocean (Crofoot 2004). This is a common occurrence along the East Coast during the late spring and early summer.…”

Flux measurements in the surface marine Atmospheric Boundary Layer over the Aegean Sea, Greece

“…Under weak stable conditions, moisture flux was mostly negative and varied proportionally with sensible heat flux (linear correlation R 2 = 0.68). The observed downward fluxes corresponded to air flows from warmer towards cooler waters upwind the south measuring site and it should be mentioned that downward latent heat fluxes with values one order of magnitude greater have been observed to respond to sea fog events (Crofoot 2004). It is also worth to mention that similar studies of turbulent measurements of moisture under relevant conditions are not often found in the international literature.…”

A new roughness parameterization accounting for wind-wave (mis)alignment

“…measurement mast unstable conditions are often present due to cold air advecting above the much warmer ocean, this is consistent with other locations in the North-Sea (Patton et al, 2015;Barthelmie, 1999). However, for the ASIT measurement mast stable conditions do occur often in late spring to early summer when the ocean is slowly warming by warm ocean water flowing over the colder ocean, typically identified with cool summer weather and fog (Edson et al, 2007;Crofoot, 2004). The integrated stability function can be calculated by Eq.…”