Heat fluxes and roll circulations over the western Gulf Stream during an intense cold-air outbreak

Abstract

Turbulence and heat fluxes in the marine atmospheric boundary layer (MABL) far the roll vortex regime, observed during the Genesis of Atlantic Lows Experiment (GALE) over the western Gulf Stream, have been studied. The spqctral analysis suggests that cloud streets (roll vortices) are vertically organized convection in the MABL having the same roll scale for both the cloud layer and subcloud layer, and that the roll spacing is about three times the MABL depth. The roll circu~tions contribute signi~cantly to the sensible (temperature) and latent heat (moisture) fluxes with importance increasing upward. Near the MABL top, these fluxes are primarily due to roll vortices which transfer both sensible heat and moisture upward in the lower half of the convective MABL. Near the MABL top, the roll circulations transfer sensible heat downward and moisture upward in the clear thermalstreet region, but roll vortices influenced by evaporative cooling can transfer sensible heat upward and moisture downward in the cloud-street region. Near the cloud-top, the upward buoyancy Rux due to evaporative cooling is highly related to the roll circulations near the inversion.For the lower half of the MABL, the normalized temperature flux decreases upward more rapidly than the humidity flux, which is mainly because Potential temperature (8) increases slightly upward while humidity (q) decreases slightly upward above the unstable surface layer. The gradient production (associated with the 6 gradient) is a source for the temperature flux in the unstable surface layer but changes to a sink in the mixed layer, while the gradient production (associated with the q gradient) acts as a source for the humidity ilux in both the unstable surface and mixed layers. The results suggest that the entrainment at the MABL top might affect the budgets of temperature and humidity fluxes in the lower MABL, but not in the unstable surface layer.

Effects of Roll Vortices on Turbulent Fluxes in the Hurricane Boundary Layer

“…The low-frequency band ( f < 0.1 Hz) contains larger or mesoscale eddies, the roll-frequency band ( f = 0.1-0.16 Hz) is dominated by roll vortices, and the high-frequency band contains smaller scale eddies and turbulence of the inertial subrange (Chou and Ferguson 1991). The cospectral plots in the lower panels of Fig.…”

“…ABL rolls have also been investigated using research aircraft (LeMone 1976;Chou and Ferguson 1991;Brooks and Rogers 1997;Chen et al 2001;Vandemark et al 2001) and from tower-based anemometer observations (Savtchenko 1999). However, to date no such measurements have been made in the HBL.…”

The Climatology, Meteorology, and Boundary Layer Structure of Marine Cold Air Outbreaks in Both Hemispheres*

“…These have been studied in the Northern Hemisphere (NH) through remote sensing (Brümmer and Pohlmann 2000), in situ observations (Hein and Brown 1988;Chou and Ferguson 1991;Brümmer 1999;Renfrew and Moore 1999), and modeling studies (Stage and Businger 1981;Müller and Chlond 1996;Schröter et al 2005). An extensive body of knowledge on the relationship between meteorology and clouds in these environments now exists based on these studies.…”

Higher Ocean Wind Speeds During Marine Cold Air Outbreaks

“…Even when no polar lows form, strong surface wind speeds can occur along Arctic fronts separating cold air masses from warmer air further south (Grønås and Skeie, 1999). The large vertical temperature differences between the ocean surface and the air during MCAOs can give rise to large surface heat fluxes, as measured by numerous flight campaigns (Grossman and Betts, 1990;Chou and Ferguson, 1991;Brümmer, 1996;Renfrew and Moore, 1999) and with buoys . One of the mechanisms that arise due to these fluxes are roll clouds or cloud streets, with their large wind speed gradients across small distances (Hartmann et al, 1997;Renfrew and Moore, 1999).…”