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Michael Tjernström

Abstract

Data from the Arctic Ocean Experiment 2001 (AOE-2001) are used to study the vertical structure and diurnal cycle of the summertime central Arctic cloud-capped boundary layer. Mean conditions show a shallow stratocumulus-capped boundary layer, with a nearly moist neutrally stratified cloud layer, although cloud tops often penetrated into the stable inversion. The subcloud layer was more often stably stratified. Conditions near the surface were relatively steady, with a strong control on temperature and moisture by the melting ice surface.

A statistically significant diurnal cycle was found in many parameters, although weak in near-surface temperature and moisture. Near-surface wind speed and direction and friction velocity had a pronounced cycle, while turbulent kinetic energy showed no significant diurnal variability. The cloud layer had the most pronounced diurnal variability, with lowest cloud-base height midday followed by enhanced drizzle and temporarily higher cloud-top heights in the afternoon. This is opposite to the cycle found in midlatitude or subtropical marine stratocumulus. The cloud layer was warmest (coolest) and more (less) stably stratified midafternoon (midmorning), coinciding with the coolest (warmest) but least (most) stably stratified capping inversion layer.

It is speculated that drizzle is important in regulating the diurnal variability in the cloud layer, facilitated by enhanced midday mixing due to a differential diurnal variability in cloud and subcloud layer stability. Changing the Arctic aerosol climate could change these clouds to a more typical “marine stratocumulus structure,” which could act as a negative feedback on Arctic warming.

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Michael Tjernström and Darko Koračin

Abstract

An ensemble-average closure model intended for mesoscale studies is applied to a marine stratocumulus-capped PBL. The intention is to test this model, in particular, for cases where cloud and subcloud layers are decoupled. The test is based on one case from the First ISCCP Regional Experiment, where solid cloud-capped and clear sky areas were found in close proximity.

The model results compare favorably both with the measurements and with results from more complex model formulations. They show the response of the entire boundary layer dynamic structure to stratocumulus formation as well as longwave and shortwave radiative heat transfers. The net result is that the entire turbulent layer in the cloud-capped case is more vigorously mixed, more neutrally stratified, and deeper compared to a cloud-free PBL developing under similar conditions. Surface fluxes of sensible and latent heat, from the measurements as well as simulations thus vary relatively little between the areas in spite of the observed substantial sea surface temperature difference.

All simulations presented here reveal cloud decoupling during daytime. The multilayer structure is, however, seen almost only in profiles of second-order moments. The mean profiles indicate one single, deep well-mixed layer, while the turbulence profiles clearly show two separate well-mixed layers. The turbulent flux of water vapor from the surface thus generally never penetrates to the cloud layer during daytime but may eventually cause formation of a shallow layer of cumuli below the main cloud layer.

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Michael Tjernström and Branko Grisogono

Abstract

Fully 3D nonlinear model simulations for supercritical flow along locations at the California coast, at Cape Mendocino, and Point Sur, are presented. The model results are objectively and subjectively verified against measurements from the Coastal Waves 1996 experiment with good results. They are then analyzed in terms of the flow structure, the impact of the local terrain, the atmospheric forcing on the ocean surface, and the momentum budgets. It is verified that the flow is supercritical (Fr > 1) within a Rossby radius of deformation from the coast and that it can be treated as a reduced-gravity, shallow water flow bounded by a sidewall—the coastal mountain barrier. As the supercritical flow impinges on irregularities in the coastline orientation, expansion fans and hydraulic jumps appear. The modeled Froude number summarizes well the current understanding of the dynamics of these events. In contrast to inviscid, irrotational hydraulic flow, the expansion fans appear as curved lines of equal PBL depth and “lens-shaped” maxima in wind speed residing at the PBL slope. This is a consequence of the realistic treatment of turbulent friction. Modeled mean PBL vertical winds in the hydraulic features range ±∼1–2 cm s−1, while larger vertical winds (±∼5–10 cm s−1) are due to the flow impinging directly on the mountain barrier. Local terrain features at points or capes perturb the local flow significantly from the idealized case by emitting buoyancy waves. The momentum budget along straight portions of the coast reveals a semigeostrophic balance modified by surface friction. While being geostrophic in the across-coast direction, the along-coast momentum shows a balance between the pressure gradient force and the turbulent friction. In the expansion fans, the flow is ageostrophic, and the imbalance is distributed between turbulent friction and ageostrophic acceleration according to the magnitude of the former. There is also a good correspondence between the magnitude of the local curl of the surface stress vector and the measured depression in sea surface temperature (SST) in areas where the latter is large and the along-coast flow is relatively weak, implying that a substantial portion of the upwelling is driven locally. Supplying the measured SST in the numerical simulations, with a considerable depression along the coast, had only marginal feedback effects on the character of the flow.

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Stefan Söderberg and Michael Tjernström

Abstract

In this study, a three-dimensional hydrostatic mesoscale model is used to address the transient behavior of supercritical along-coast flow. A control experiment and several sensitivity tests are performed in order to investigate the diurnal cycle of flow characteristics. An idealized representation of the northern California terrain is used, and the model results are interpreted within the reduced-gravity shallow water concept. In two preceding studies by the authors, this theory appeared to be violated since the flow accelerated along the coastline upstream of the change in coastline orientation, even though the flow was supercritical. Here, it is shown that the criticality of the flow for typical summertime conditions along the California coast actually varies diurnally. The gradual acceleration of the flow along the upstream coastline is established during a subcritical phase of the simulation; thus, the shallow water concept is not violated. Because the along-coast jet is primarily driven by the cross-coast baroclinicity, there will be a continuous variation in the strength of the jet. This in turn will affect the flow criticality and thus the flow is not only spatially, but also temporally, transcritical. The results here suggest that observations of quasi-steady-state supercritical flow in reality are not likely; transcritical flow along mountainous coastlines should be more prevalent.

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Michael Tjernström and David P. Rogers

Abstract

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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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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Ann-Sofi Smedman, Michael Tjernström, and Ulf Högström

Abstract

Data from a marine coastal experiment over the Baltic Sea, comprising airborne measurements and mast measurements, have been used to highlight the turbulence dynamics of a case with most unusual flow characteristics. The boundary layer had a depth of about 1200 m. The thermal stratification was near neutral, with small positive heat flux below 300 m and equally small negative heat flux above. The entire situation lasted about 6 hours. Turbulence levels were unexpectedly high in view of the fact that momentum flux was negligible (in fact positive) in the layers near the surface, and buoyancy flux was also small. The turbulence was found to scale with the height of the boundary layer, giving rise to velocity spectra having the shape of those characteristic of convectively mixed boundary layers. Analysis of the turbulence budget for the entire planetary boundary layer (PBL) revealed that energy was produced from shear instability in the uppermost parts of the PBL and was distributed to the lower parts of the PBL by pressure transport. Dissipation was found to be evenly distributed throughout the entire PBL. Without data on surface wave characteristics, no firm conclusions concerning air–sea interaction processes can be drawn, but there are clear indications that the dynamical decoupling observed at the surface is due to the effect of decaying sea state conditions (high wave age conditions). In any case, the process of active turbulence production in the layers close to the surface observed in “ordinary” near-neutral boundary layers has been effectively turned off here, leaving only turbulence of the “inactive” kind, imported by pressure transport from layers above. The results strongly support the findings reported in the recent literature on “laboratory turbulence” that the process of strong turbulence and shearing stress production near the wall of boundary layers of very different kinds is virtually independent of forcing from large-scale structures embedded in the flow.

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Michael Tjernström, Ben B. Balsley, Gunilla Svensson, and Carmen J. Nappo

Abstract

The authors report results of a study of finescale turbulence structure in the portion of the nocturnal boundary layer known as the residual layer (RL). The study covers two nights during the Cooperative Atmosphere–Surface Exchange Study 1999 (CASES-99) field experiment that exhibit significant differences in turbulence, as indicated by the observed turbulence dissipation rates in the RL. The RL turbulence sometimes reaches intensities comparable to those in the underlying stable boundary layer.

The commonly accepted concept of turbulence generation below critical values of the gradient Richardson number (Rig) is scale dependent: Ri values typically decrease with decreasing vertical scale size, so that critical Rig values (≈0.25) occur at vertical scales of only a few tens of meters. The very small scale for the occurrence of subcritical Ri poses problems for incorporating experimentally determined Rig -based methods in model closures in models with poor resolution.

There appear to be two distinct turbulence “regimes” in the RL: a very weak but ever-present background turbulence level with minimal temporal and spatial structure and a more intense intermittent regime during which turbulent intensity can approach near-surface nighttime turbulent intensities. It is hypothesized that the locally produced RL turbulence can be related to upward-propagating atmospheric gravity waves generated by flow over the low-relief terrain. The presence of critical layers in the RL, caused by wind turning with height, results in the generation of intermittent turbulence.

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