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David P. Rogers

Abstract

Aircraft observations obtained during the Frontal Air–Sea Interaction Experiment (FASINEX) are used to investigate the structure of the marine atmospheric boundary layer in the vicinity of an ocean front. A quasi-stationary sea surface temperature (SST) discontinuity of 2°C was maintained across the frontal zone throughout the duration of the experiment The primary response of the atmosphere to changes in the SST was observed in the surface-related turbulence fluxes. In the case of warm air flowing over cold water, the boundary layer appears to develop an internal boundary layer (IBL) in response to the sudden change in the sea surface temperature. The organized updrafts and downdrafts within this layer collapse with entrainment–detrainment processes in these cells dominating the turbulence statistics. The IBL grows in response to the wind shear in this layer, although the surface shear stress is much smaller on the colder side of the front than on the warm. The depth of the IBL, and, in the absence of the IBL, the mixed layer are found to scale with the friction velocity and the Coriolis parameter.

The IBL confines the surface-related turbulent mixing and shear-driven processes to the lower layers of the atmosphere. Thus. the shallow boundary layer cloud field appears to be maintained primarily by radiative transfer within the cloud layer. Multiple cloud-capped mixed layers were frequently observed throughout the experiment. They appear to be directly related to the horizontal variation of the SST with deeper boundary layer and higher cloud levels formed over warmer water.

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Michael Tjernström and David P. Rogers

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The average and turbulence structure of two marine stratocumulus layers, from the Atlantic Stratocumulus Transition Experiment, are analyzed. These layers ware in adjacent air masses with different histories: one cloud layer was in a clean air mass with a marine history and the other was in a continental air mass, which had a higher aerosol content. The air masses were brought together by synoptic-scale flow and are separated by a semiclear transition zone.

The clouds were decoupled from the marine surface mixed layer in both air masses. In the transition zone, the marine mixed layer was deeper than that under either cloud layer. The total depth below the main inversion, including the cloud layer was, however, substantially greater than in the semiclear transition zone. The western clean cloud layer was more well mixed than the eastern aerosol-rich cloud layer, and the turbulence analysis shows that the western cloud layer complies to convective scaling. Buoyancy production of turbulence was also positive in the eastern cloud, but here shear production was larger than the buoyancy production by a factor of 4, and convective scaling fails.

One cause of the stability differences may lie in differences in radiative forcing, both external and internal. The difference in external forcing is due to higher humidity aloft to the east, reducing the net cloud-top longwave cooling. The difference in internal forcing is due to differences in the cloud microphysics. The larger number of smaller drops in the eastern cloud, arising from the abundance of aerosol particles, increases both albedo and absorption, and thus solar heating. Increased solar heating balances the longwave cooling at the cloud top, warms the cloud interior, and decreases the depth of the net-cooled layer in the cloud. All these effects decrease the buoyancy in the eastern cloud layer in comparison to the western one.

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Darko Koračin and David P. Rogers

Abstract

The effect of a stable internal boundary layer (IBL) on the cloud-capped marine boundary layer is investigated using a one-dimensional second-order closure model. A stable IBL forms if there is a reversal in the surface buoyancy flux when warm air flows over colder water. These conditions exist in the vicinity of Ocean fronts where sea surface temperature discontinuities of about 2°C in 5 km have been observed. There is a balance between the buoyant consumption and inertial production of kinetic energy so that the layer remains weakly turbulent and can deepen due to shear-driven mixing. The stability of the layer limits momentum exchange with the air above so that there is a significant reduction in the surface stress in the IBL and an acceleration of the flow aloft. There are important implications for cloud development in regions of large ocean temperature gradients because a stable IBL can limit the vertical transfer of moisture from the surface to the upper part of the boundary layer.

In addition, solar radiation is found to heat the cloud layer sufficiently to cause decoupling between the cloud and subcloud layers during the daytime. This effect is important in determining the rate at which the cloud layer evaporates.

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David P. Rogers and Darko Korac̆in

Abstract

The effects of longwave and shortwave radiative heating on the coupling between stratocumulus clouds and the boundary layer is investigated using a one-dimensional second-moment turbulence-closure model. The decoupling of a stratiform cloud from the subcloud layer is often a precursor to cloud break up and the transition to scattered cumulus clouds or clear sky. Coupling between cloud and subcloud layers is found to be very sensitive to cloud depth and subcloud layer sensible and latent heat fluxes. In particular, a strong moisture flux can maintain weak coupling between the cloud and subcloud layers so that the lower part of the cloud layer may continue to develop despite the formation of a stable temperature gradient between the top of the subcloud layer and cloud base.The effect of shortwave heating on decoupling is threefold. First, shortwave heating directly offsets the net longwave cooling at cloud top by as much as 30% (in February at latitude 29°N), reducing or eliminating the overall cooling of the cloud layer during part of the day. Second, shortwave heating decreases exponentially from a maximum at cloud top, which tends to stabilize and evaporate the cloud layer. In a deep cloud layer radiative heating is restricted to the upper part of the cloud, which warms at a faster rate than the lower part of the cloud; hence, decoupling can occur within the cloud layer. Vertical mixing in the cloud is limited, and multiple cloud layers may form. Third, the maximum shortwave heating is displaced below the maximum longwave cooling, creating a divergent flux that generates convection in the upper part of the cloud layer that, in turn, promotes entrainment. In a deep cloud layer, shortwave radiative heating can affect the decoupling of a cloud and subcloud layer only if longwave cooling is reduced sufficiently to allow longwave radiative heating of cloud base to warm the lower part of the cloud at a faster rate than the subcloud layer is heated by the sea surface. In a shallow cloud layer, shortwave radiation may penetrate to cloud base to provide an additional heat source to decouple the cloud from the subcloud layer.These results highlight the difficulty of predicting the formation, evolution, and dissipation of marine stratocumulus clouds.

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Barry A. Gardiner and David P. Rogers

Abstract

Results are presented from the 1981 Convective Cloud Precipitation Experiment (CCOPE). Aircraft data are analyzed from different flight levels within a convective cloud using the ωq Q mixing diagram (Paluch). Points on this diagram are characterized by updrafts, downdrafts and the variation of the virtual potential temperature at the penetration level. New growth appears to have occurred on the upshear side of the cloud and mixed with all levels in the environment as it ascended. Maintenance of the cloud appears to be depend on a balance between entrainment instability and dynamically forced airflow through cloud base. The results are discussed within the framework of a conceptual model for cloud evolution.

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Ian M. Brooks and David P. Rogers

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Large-scale horizontal rolls can have a significant influence on turbulent transport across the atmospheric boundary layer. The formation and maintenance of such rolls is dependent on the thermal and dynamic stability of the boundary layer (BL). The authors present aircraft observations of boundary layers, both with and without roll circulations, off the coast of California. The contribution of the rolls to the turbulent fluxes of heat, moisture, and momentum, and the variances of the three velocity components are determined for four cases. The fractional roll contributions to the u and w variances, and the sensible heat and along-wind momentum fluxes, show a near linear increase with altitude, from less than 10% at 30 m to more than 70% at the top of the BL. The variance in υ and crosswind momentum flux are more scattered, although the variance shows a slight increase with altitude from about 40% to 60%. The latent heat flux also shows a great deal of scatter, especially in the lower third of the BL where the total flux is small; above this, values range between about 40% and 85% but show no clear trends. A stability parameter in the form of a bulk Richardson Ri number is calculated for each of 13 profiles through the boundary layer; it is found that the Richardson number successfully identifies those cases where rolls are present, and its value corresponds to some extent with the strength of the rolls. Values close to zero correspond to cases with well-defined rolls; for 0.1 < Ri < 0.25 rolls are found to exist, but they tend to be weak and patchy; and no rolls are found where Ri is greater than the critical value of approximately 0.25. Reynolds numbers are calculated from a number of different definitions and indicate the dynamic instability of the shear dominated boundary layers.

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Linda Ström, Michael Tjernström, and David P. Rogers

Abstract

Airborne in situ and remote meteorological measurements from around Cape Mendocino, California, sampled during the Coastal Waves 1996 field program are analyzed for three different days: 7, 12, and 26 June 1996. Two days conformed to typical summertime conditions, with a strong northerly downcoast flow, while the wind on the third day was weaker. On the first 2 days, the flow was supercritical in the sense that the Froude number was larger than unity and these 2 days feature expansion fans in the lee of the cape. On the third day, no such phenomenon was observed. All 3 days had a strong thermal wind caused by the marine inversion sloping down toward the coast. On the first 2 days, the flow aloft was westerly or northerly, so that the thermal wind added to the background flow results in a strong jet. On the third day the flow aloft was southerly, and consequently even with the added thermal wind, the northerly flow in the marine layer was too weak to be supercritical. The main difference between the first 2 days was the fact that 7 June is cloud free, while 12 June had a stratocumulus cover.

The 3 days are analyzed as composites, and several scales are identified and described: 1) the large-scale synoptic forcing, determined from the wind aloft and synoptic conditions; 2) blocking by the coastal topography, the thermal wind balance in the near-coast zone, and nondimensional properties; 3) the hydraulic properties in the sub- and supercritical flows as they pass the change in coastline orientation at Cape Mendocino; 4) the impact of the local blocking by the terrain at the cape itself, generating a lee-wave phenomena for the high-wind days, which was the actual cause of the collapse of the marine layer in the lee of the cape; 5) the boundary layer interaction, apparently generating SST anomalies, but with little or no feedback to the wind field.

A momentum budget analysis for the two high-wind days show a significant difference between one day (7 June) when mesoscale perturbation dominated the flow and the other day (12 June), when large-scale forcing dominated and the mesoscale perturbation was smaller. The importance of the cloud layer on 12 June is illustrated, using lidar data.

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David P. Rogers, James W. Telford, and Steven K. Chai

Abstract

This paper addresses the question of time changes in the cloud parcels comprising a cumulus cloud. Observations are analyzed which show how a cloud parcel begins its life on the upshear side of the cloud. As the cloud as a whole continues to develop upshear, the particular parcel is diluted by entrainment of dry air from above it, and the vertical cycling of cooled parcels formed in this way produces complex modifications of the cloud droplet spectra which can be traced in the observations. As this process continues, the parcel is found closer and closer to the downshear evaporating side of the cloud as the parcel is modified and ages. When sufficiently diluted, the whole cloud column subsides and evaporates.

The largest cloud drops, and the ice particles, are found in the older, diluted, decaying side of the cloud.

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David P. Rogers, Douglas W. Johnson, and Carl A. Friehe

Abstract

Observations of the mean and turbulent structure of the marine atmospheric boundary layer (MABL). obtained using the U.K. Meteorological Research Flight C-130 Hercules aircraft are used to investigate the momentum balance over the Irish Sea when warm air is advected offshore. The marine boundary layer is made up of two layers: a strongly stable internal boundary layer (IBL). and a stable residual layer located between the top of the IBL and the base of the planetary boundary layer inversion.

Measurements obtained near the upwind coast indicate that the flow is highly ageostrophic. Downwind of the Irish coast, there is a transition toward equilibrium between the geostrophic, Coriolis. and friction components of the flow along part of the flight track. However, another segment of the flight track indicates an imbalance between the pressure gradient and the other measured terms, which may be attributable to gravity waves affecting the adjustment process. This is more apparent in the leg perpendicular to the coast where the pressure gradient is balanced by the observed acceleration with negligible contributions from the Coriolis and friction terms.

Gravity waves associated with mountain lee waves propagate along the direction of the mean wind shear in the IBL, which is directed to the right of the wind measured along the flight track perpendicular to the coast at 30-m altitude. The dominant wavelength is about 19 km, which corresponds with the buoyancy frequency of the MABL new the Irish coast and is supported by satellite images of the cloud structure. Farther downstream the buoyancy frequency increases, but the longer wavelength signal remains dominant. An important result of the gravity waves is the modification of the wind field and wind stress within the IBL. The largest effect is observed in the stress direction, but large changes in magnitude are also observed. The results indicate that the direction of the wind stress corresponds to a large degree with the direction of the mean horizontal wind sheer.

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Steven K. Chai, David P. Rogers, and James W. Telford

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Abstract not available

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