Marine Atmospheric Boundary Layer Depth Research Articles

Seasonal Variations in Eddy-Induced Atmospheric Perturbations in the South China Sea

Abstract This study investigates the impact of eddy-induced sea surface temperature (SST) anomalies on the overlying atmosphere in the South China Sea, utilizing observational and reanalysis datasets. The results reveal that SST anomalies caused by anticyclonic or cyclonic eddies have a significant impact on the acceleration or deceleration of surface winds, with a stronger response in summer compared to winter. Moreover, atmospheric responses, such as heat flux, precipitation, and marine atmospheric boundary layer (MABL) depth above anticyclonic or cyclonic eddies, exhibit in-phase seasonal variations as surface winds. The study also explores the mechanisms leading to atmospheric response, pointing toward the vertical mixing mechanism as the dominant cause, supported by both the in-phase relationship between SST and surface wind anomalies and the linear relationship between the wind stress divergence anomaly and downwind SST gradient anomaly. Seasonal variations in coupling intensity are attributed to varying background atmospheric conditions, with more effective vertical turbulence mixing and stronger coupling intensity caused by more unstable MABL and enhanced large-scale vertical wind shear during summer than winter. Besides, the atmosphere above eddies is under a quasi-equilibrium condition, in which the surface wind stress increases monotonically with the depth of MABL. Given that the MABL depth response to eddy-induced SST anomaly is stronger in summer than in winter, it is reasonable to expect a more intense wind response during this season. Thus, the MABL depth coupling works together with the vertical mixing mechanism to explain the proportional relationship between SST and wind anomalies, and why the atmospheric response is stronger in summer than in winter. Significance Statement The atmosphere exerts a significant influence on the ocean, with strong winds cooling the sea surface temperature on a large scale (more than 1000 km). Conversely, the ocean tends to drive the atmosphere at a mesoscale level (10–1000 km), whereby a warm sea surface temperature causes surface wind to accelerate. This study aims to better understand how mesoscale eddies in the South China Sea influence the overlying atmosphere in different seasons. Our research explores factors that control mesoscale air–sea coupling, and highlights the importance of stability and depth change of marine atmospheric boundary layer. These results will establish a foundation for future research examining how the atmospheric perturbations caused by mesoscale eddies can in turn impact both ocean circulation and eddies.

The Behavior of Squall Lines in Horizontally Heterogeneous Coastal Environments

Abstract Inland squall lines respond to the stable marine atmospheric boundary layer (MABL) as they move toward a coastline and offshore. As a storm’s cold pool collides with the marine layer, characteristics of both determine the resulting convective forcing mechanism over the stable layer and storm characteristics. Idealized numerical experiments exploring a parameter space of MABL characteristics show that the postcollision forcing mechanism is determined by the buoyancy of the cold pool relative to the MABL. When the outflow is less buoyant, storms are forced by a cold pool within the marine environment. When the buoyancies are equivalent, a hybrid cold pool–internal gravity wave develops after the collision. The collision between a cold pool and less buoyant MABL initiates internal waves along the stable layer, regardless of MABL depth. These waves are inefficient at lifting air into the storm, and ascent from the trailing cold pool is needed to support deep convection. Storm intensity decreases with deeper and less buoyant MABLs, in part due to the reduction in elevated instability. Precipitation is enhanced just prior to the collision between a storm and the deepest marine layers. Storms modify their environment downstream, leading to the development of a moist adiabatic unstable layer and a lowering of the level of free convection (LFC) to below the top of the deepest marine layer. An MABL moving as a sea breeze into the storm-modified air successfully lifts parcels to the new LFC, generating convective towers ahead of the squall line. This mechanism may contribute to increased coastal flash flooding risks during observed events.

Investigation of Turbulence Parametrization Schemes with Reference to the Atmospheric Boundary Layer Over the Aegean Sea During Etesian Winds

The spatial structure of the marine atmospheric boundary layer (MABL) over the Aegean Sea is investigated using the Weather Research and Forecasting (WRF) mesoscale model. Two ‘first-order’ non-local and five ‘1.5-order’ local planetary boundary-layer (PBL) parametrization schemes are used. The predictions from the WRF model are evaluated against airborne observations obtained by the UK Facility for Airborne Atmospheric Measurements BAe-14 research aircraft during the Aegean-GAME field campaign. Statistical analysis shows good agreement between measurements and simulations especially at low altitude. Despite the differences between the predicted and measured wind speeds, they reach an agreement index of 0.76. The simulated wind-speed fields close to the surface differ substantially among the schemes (maximum values range from 13 to $$18\hbox { m s}^{-1}$$ at 150-m height), but the differences become marginal at higher levels. In contrast, all schemes show similar spatial variation patterns in potential temperature fields. A warmer (1–2 K) and drier (2–3 $$\hbox { g kg}^{-1})$$ layer than is observed, is predicted by almost all schemes under stable conditions (eastern Aegean Sea), whereas a cooler (up to 2 K) and moister (1–2 $$\hbox { g kg}^{-1})$$ layer is simulated under near-neutral to nearly unstable conditions (western Aegean Sea). Almost all schemes reproduce the vertical structure of the PBL and the shallow MABL (up to 300 m) well, including the low-level jet in the eastern Aegean Sea, with non-local schemes being closer to observations. The simulated PBL depths diverge (up to 500 m) due to the different criteria applied by the schemes for their calculation. Under stable conditions, the observed MABL depth corresponds to the height above the sea surface where the simulated eddy viscosity reaches a minimum; under neutral to slightly unstable conditions this is close to the top of the simulated entrainment layer. The observed sensible heat fluxes vary from −40 to $$25\hbox { W m}^{-2}$$ , while the simulated fluxes range from −40 to $$40\hbox { W m}^{-2}$$ ; however, all of the schemes’ predictions are close to the observations under unstable conditions. Finally, all schemes overestimate the friction velocity, although the simulated range (from 0.2 to $$0.5\hbox { m s}^{-1})$$ is narrower than that observed (from 0.1 to $$0.7\hbox { m s}^{-1})$$ .

Boundary Layer Convergence Induced by Strong Winds across a Midlatitude SST Front*

Abstract Recent studies indicate that the influence of midlatitude SST fronts extends through the marine atmospheric boundary layer (MABL) into the free atmosphere, with implications for climate variability. To better understand the mechanisms of this ocean-to-atmosphere influence, SST-induced MABL convergence is explored here with the Weather Research and Forecasting mesoscale model in an idealized, dry, two-dimensional configuration, for winds crossing from cold to warm SST and from warm to cold SST. For strong cross-front winds, O(10 m s−1), changes in the turbulent mixing and MABL depth across the SST front lead to MABL depth-integrated convergence in the cold-to-warm case and depth-integrated divergence in the warm-to-cold case. The turbulent stress divergence term changes over a shorter length scale than the pressure gradient and Coriolis terms, such that the MABL response directly above the SST front is governed by nonrotating, internal boundary layer–like physics, which are consistent with the vertical mixing mechanism. An important consequence is that the increment in the cross-front surface stress diagnoses the vertical motion at the top of the MABL. These physics are at variance with some previously proposed SST frontal MABL models in which pressure adjustments determine the MABL convergence. The SST-induced MABL convergence results in vertical motion that excites a stationary internal gravity wave in the free atmosphere, analogous to a mountain wave. For a 15 m s−1 cross-front wind, the gravity wave forced by an SST increase of 3°C over 200 km is comparable to that forced by an 80-m change in topography.

Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability

Abstract. In the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. In the prognostic local MBL height equation the height change, the horizontal MBL height advection, and the large scale vertical velocity at MBL top are calculated explicitly from the simulation. The entrainment velocity is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes.

Variability in the summertime coastal marine atmospheric boundary‐layer off California, USA

AbstractAn analysis of boundary‐layer structure and surface‐layer turbulence from measurements off the California coast is presented from data collected by research aircraft during two field experiments: the Coastal Waves 1996 (CW96) and the Monterey Area Ship Track (MAST) experiments. CW96 covers the near‐coast region, in particular in the vicinity of major headlands, whereas MAST extends offshore.Along the US west coast, coastal modification of the along‐coast flow occurs on two main horizontal scales. Firstly, a large‐scale variability is due to the interplay between the shallow near‐coast marine atmospheric boundary‐layer (MABL) and the coastal terrain, typically higher than the MABL depth. The MABL depth decreases smoothly towards the coast while the wind speed increases to a coastal jet in response to the sloping MABL inversion. Secondly, the flow is affected by supercritical flow dynamics. As the wind speed increases and the MABL depth decreases towards the coast, the MABL flow becomes supercritical in a shallow‐water sense. As supercritical shallow‐water flow interacts with major headlands, expansion fans form, affecting both the wind speed and the MABL depth.The combination of CW96 and MAST data reveals significant differences between the flow along the coast and that far offshore. MABL winds are stronger near the coast whereas aloft the winds are weaker than offshore. The near‐coast MABL is also better mixed. Turbulence increases towards the coast while the sensible‐heat flux decreases and often changes sign, leading to stable stratification near the coast. A length‐scale determining the across‐coast influence of the expansion fans is defined from simple inviscid and irrotational shallow‐water theory. Data from four days of the CW96 experiment show that this simple theory describes the low‐level wind speed adequately. Surface‐layer turbulence also scales with this simple length‐scale, but the results are complicated by upwelling of cold water, giving rise to strong internal boundary‐layers. From terms in the turbulent‐energy budgets it appears that dissipation of turbulence exceeds local production. Copyright © 2004 Royal Meteorological Society

Summertime Marine Refractivity Conditions along Coastal California

Large vertical gradients of temperature and moisture, which are not uncommon at the top of the marine atmospheric boundary layer (MABL), yield strong gradients in microwave refractivity that can result in anomalous electromagnetic (EM) propagation, including ducting wherein energy is strongly channeled horizontally. Of particular importance to surface radars and other microwave transmitters are surface-based ducts in which energy is ducted throughout the entire depth of the MABL. The Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is used to define boundary layer structure during two coastal field experiments, and this model’s ability to forecast refractivity, including surface-based ducting, is assessed. At three marine sites, COAMPS shows considerable skill in MABL forecasts during the Variability of Coastal Atmospheric Refractivity experiment, although it contains biases for the MABL to be somewhat shallow and for the forecast duct strength, measured by the refractivity jump at MABL top, to be too weak. Nevertheless, for a total of 95 forecasts at these sites, COAMPS correctly forecasts surface-based ducting/no surface-based ducting events 82% of the time, with a false alarm rate of only 0.15. Abrupt variations in coastal MABL depth and wind speed have been observed and modeled when supercritical flow (Froude number > 1) interacts with coastal terrain that is uniformly higher than the MABL depth. COAMPS and data from the Coastal Waves 1996 (CW96) experiment conducted along the northern California coast are used to address the implications of MABL variability to the coastal EM propagation environment. Comparison of CW96 research aircraft cross sections and soundings with COAMPS forecast fields indicate that the mesoscale model captures much of the observed MABL vertical structure and horizontal variability, including the temperature inversion and moisture lapse capping the MABL, the along- and cross-shore MABL slopes, and the presence and intensity of a coastal low-level jet. Supercritical expansion fans form in the lee of Cape Blanco, Oregon, and Cape Mendocino, California, on 1 July 1996 during CW96, and COAMPS indicates the presence of a compression jump where the flow is blocked on the upwind (north) side of Cape Mendocino. In conjunction with these MABL features, the EM propagation environment also exhibits substantial alongshore variation. Stronger, near-surface-based ducting occurs in the expansion fans as the marine layer thins and accelerates; weaker, elevated ducting occurs in the blocked flow.

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

Turbulence and heat fluxes in the marine atmospheric boundary layer (MABL) for the roll vortex regime, observed during the Genesis of Atlantic Lows Experiment (GALE) over the western Gulf Stream, have been studied. The spectral 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 circulations contribute significantly 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 thermal-street 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 flux 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 (θ) increases slightly upward while humidity (q) decreases slightly upward above the unstable surface layer. The gradient production (associated with the θ 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 flux 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.

Response of marine atmospheric boundary layer height to sea surface temperature changes: Mixed‐layer theory

The flow of marine atmospheric boundary layer (MABL) air across ocean‐surface temperature fronts subjects the MABL to an abrupt change in surface thermal forcing. The MABL responds to this variable forcing by a readjustment of both the MABL heat and moisture content as well as the MABL depth. This note addresses questions raised in recent publications in this journal with a rigorous theoretical analysis for the simplest case, an equilibrated MABL crossing a very abrupt front at right angles. The MABL's adjustment is found by a solution to the mixed‐layer budget of buoyancy and the entrainment closure based on turbulent energetics of the MABL. Since the solution is analytic, this simple case can be easily extended to circumstances of more realistic forcing. The solution discussed here establishes a firm theoretical basis for previous, partly heuristic and partly empirical, work; highlights weaknesses of that approach, and shows how simplistic engineering solutions to complex geophysical problems can be completely misleading.