Convective Internal Boundary Layer Research Articles

Mechanistic study on modification of convective internal boundary layer by spanwise surface motion

Modification of a convective internal boundary layer (IBL) by spanwise motion of a warm surface is investigated by imposing different surface moving speeds in the present study. Our analysis shows that the spanwise surface motion reduces the Reynolds shear stress right after the increase in the surface temperature in the convective IBL. The maximum decreasing rate of the Reynolds shear stress is found to be approximately 75% at the largest moving speed of the warm surface considered in the manuscript. Due to the reduction of the Reynolds shear stress, the vertical momentum transport is fundamentally altered, and the mean flow accelerates immediately after the increase in the surface temperature. By scrutinizing the instantaneous and conditional averaged flow fields as well as the pre-multiplied energy spectra, we have attributed the reduction of the Reynolds shear stress to the suppression of the near-surface velocity streaks and quasi-streamwise vortices, and the delayed growth of the convective structures, such as thermal plumes. Our investigation suggests that the developments of the convective IBL can be influenced by a strong spanwise motion of the warm surface, which should be taken into consideration in the prediction model for practical applications.

Coral Reef Coupling to the Atmospheric Boundary Layer Through Exchanges of Heat, Moisture, and Momentum: Case Studies From Tropical and Desert Fringing Coral Reefs

Coral reefs represent abrupt changes in surface roughness, temperature, and humidity in oceanic and coastal locations. This leads to formation of internal atmospheric boundary layers that grow vertically in response to turbulent mixing which conveys changes in surface properties into the prevailing wind. As a result, coral reefs under favourable conditions couple to the overlying atmosphere. Here we present rare observations of coral reef – atmospheric interactions during summer monsoon conditions on the Great Barrier Reef, Australia, and a desert fringing coral reef in the Gulf of Eilat, Israel. We show that in the hyper-arid location of the Gulf of Eilat where air temperatures are greater than water temperatures, a stable atmospheric boundary inhibits coupling of the reef to the atmosphere. In contrast, under monsoon conditions on the Great Barrier Reef, the coral reef is shown to couple to the overlying atmosphere leading to the formation of a convective internal boundary layer in which convective exchange of heat and moisture influences cloud and possibly precipitation. We conclude that understanding these processes is essential for determining the role of coral reefs in coastal meteorology, and whether through coupling with the overlying atmosphere coral reefs may regulate their meteorology by triggering formation of cloud and/or vertical exchange on aerosols and their precursor gases such as Dimethyl sulphide.

The Saharan convective boundary layer structure over large scale surface heterogeneity: A large eddy simulation study

Extreme atmospheric and surface conditions over the vast Saharan desert result in the development of one of the deepest atmospheric boundary layers (Saharan Atmospheric Boundary Layer, SABL) on the planet. The land cover in the Sahara mainly consists of wide interchanging areas of stony and sandy desert intercepted by narrower bare rock formations. Significant changes in surface albedo, surface temperature, turbulence fluxes and wind speed have been observed with remote measurement platforms like aircraft and satellites. This was a motivation to simulate the convective boundary layer (CBL) that develops over the heterogeneous Saharan surface with a two-way coupled system of a large eddy simulation code (LES) and a land surface model (LSM). Initial and boundary conditions are provided by airborne observations and mesoscale atmospheric simulations. In order to investigate the effect of surface heterogeneity on the vertical structure of the SABL, a large scale (larger than 10 km) idealized warm surface anomaly is simulated and analyzed. A land strip with a low surface albedo produces almost doubled fluxes in density and stronger convergence near the ground than over the surrounding brighter surfaces. First and second order turbulence statistics reveal that strong thermals penetrate the inversion layer above the CBL producing a vigorous exchange between unstable and overlying stable air. Furthermore, a convective internal boundary layer (CIBL) is formed over the warm strip. The downwind development of the CIBL is studied and CBL depth equations from literature are revised. The average turbulent structure of the SABL reveals flow changes locally and downwind of the surface heterogeneity (strip), which can be more significant than the dispersive fluxes due to surface heterogeneity, and may have an important effect on processes such as dust uplift and its long range transport that influence weather and climate globally.

Turbulence Adjustment and Scaling in an Offshore Convective Internal Boundary Layer: A CASPER Case Study

AbstractThe characteristics of a convective internal boundary layer (CIBL) documented offshore during the East Coast phase of the Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER-EAST) field campaign has been examined using field observations, a coupled mesoscale model (i.e., Navy’s COAMPS) simulation, and a couple of surface-layer-resolving large-eddy simulations (LESs). The Lagrangian modeling approach has been adopted with the LES domain being advected from a cool and rough land surface to a warmer and smoother sea surface by the mean offshore winds in the CIBL. The surface fluxes from the LES control run are in reasonable agreement with field observations, and the general CIBL characteristics are consistent with previous studies. According to the LESs, in the nearshore adjustment zone (i.e., fetch < 8 km), the low-level winds and surface friction velocity increase rapidly, and the mean wind profile and vertical velocity skewness in the surface layer deviate substantially from the Monin–Obukhov similarity theory (MOST) scaling. Farther offshore, the nondimensional vertical wind shear and scalar gradients and higher-order moments are consistent with the MOST scaling. An elevated turbulent layer is present immediately below the CIBL top, associated with the vertical wind shear across the CIBL top inversion. Episodic shear instability events occur with a time scale between 10 and 30 min, leading to the formation of elevated maxima in turbulence kinetic energy and momentum fluxes. During these events, the turbulence kinetic energy production exceeds the dissipation, suggesting that the CIBL remains in nonequilibrium.

Insights From a Decade of Research on Coral Reef—Atmosphere Energetics

AbstractCoral reefs cover approximately 0.10 to 0.25% of the marine environment and yet are home to around 25% of marine species and support the livelihoods of more than 500 million people. They face a wide range of threats, with the impact of global warming gaining most attention due to its frequently claimed causal link to coral bleaching. Here we review a decade of research into the micrometeorology of Heron Reef, a lagoonal platform coral reef in the southern Great Barrier Reef, Australia. Using novel pontoon‐mounted eddy covariance systems, we show that often >80% of net radiation is partitioned into heating the water overlying the reef, the reef benthos, and substrate. Significant spatial variability in energy and trace gas exchanges occurs over the reef in response to different geomorphic and hydrodynamic conditions. Synoptic weather patterns that bring light winds, clear skies, and high humidity, result in reef scale meteorology that appears to have a greater influence on coral bleaching events than the background oceanic warming trend. The reef develops its own convective internal boundary layer, with potential to influence cloud development and therefore the surface energy balance. Knowledge of such local effects is lacking, so it is recommended that future research is needed into reef scale processes and how they interact with larger‐scale forcing.

Ocean‐atmosphere heat fluxes at the Ronne Polynya, Antarctica

The Ronne Polynya is a coastal polynya, a region of thin ice or open water in sea ice, caused by the offshore transport of the ice by strong winds from the land (Figure 1). As soon as the ice is transported offshore, new ice forms on the exposed ocean surface and is also advected offshore in a continual process, earning this type of polynya the nickname ‘ice factory’. These polynyas have an important impact on the regional meteorology and oceanography of the high latitudes as well as on the global ocean circulation. The exposed ocean surface is relatively warm compared to the cold polar atmosphere, and the large temperature and humidity differences result in large sensible and latent heat fluxes from the ocean to the atmosphere. This leads to a warming and moistening of the atmospheric boundary layer above and downwind of the polynya and, through vigorous convective mixing, the formation of a CIBL (convective internal boundary layer) (Figure 1). A decrease in the ocean-atmosphere temperature and humidity gradients is caused by this warming and moistening, which results in a decrease in the surface heat fluxes with fetch from the shore, or ice shelf front (Renfrew and King, 2000). The depth of the CIBL increases with fetch due to the warming and also the entrainment of warm air from above the CIBL (Garratt, 1992). As well as this turbulent heat transfer, polynyas can also influence the balance of radiative heat transfer through the generation of ice fog (Smith et al., 1990) and convective clouds or plumes (Pinto and Curry, 1995). Polynyas, therefore, have the potential to modify and induce mesoscale atmospheric motion, impacting on regional climate (Pinto et al., 1995). High rates of ice production due to the large ocean-atmosphere heat fluxes and the continual removal of the newly formed ice by the wind result in extensive brine rejection, whereby sea water rejects salt on freezing, leaving the sea ice relatively fresh and the modified water column relatively salty and therefore dense. This dense water sinks, as shown in Figure 1, accumulating on the continental shelf and forming a water mass, which eventually contributes to the temperature- and salinity-driven global ocean circulation, known as the thermohaline circulation (THC). Therefore the ice-formation mechanism within polynyas is important for the ventilation of deep and bottom water in both the Southern and Arctic Oceans (Morales Maqueda et al., 2004). It follows that, in order to accurately model the response of both the high latitudes and global THC to a changing climate, processes occurring within polynyas must be investigated. I was lucky enough to be given the opportunity to participate in a British Antarctic Survey (BAS) fieldwork campaign at the end of the Antarctic summer field season in February 2007. Using an instrumented Twin Otter aircraft belonging to the BAS, three flights were conducted over the Ronne Polynya between 25 and 28 February 2007 to investigate ocean-atmosphere heat fluxes. Quantification of these heat fluxes is a step towards quantifying the surface heat budget, which, together with the surface salinity budget, will aid understanding of the key processes governing deep-water formation within polynyas. Aircraft observations of the

A study of potential bluetongue vectors and meteorology in Jersey

Garratt JR. 1992. The atmospheric boundary layer. Cambridge University Press: Cambridge. Liu AK, Martin S, Kwok R. 1997. Tracking of ice edges and ice floes by wavelet analysis of SAR images. J. Atmos. Ocean. Tech. 14: 1187–1198. Morales Maqueda MA, Wilmott AJ, Biggs NRT. 2004. Polynya dynamics: A review of observations and modeling. Rev. Geophys. 42: RG1004, doi:10.1029/ 2002RG000116. Pinto JO, Curry JA, McInnes KL. 1995. Atmospheric convective plumes emanating from leads 1. Thermodynamic structure. J. Geophys. Res. 100: 4621–4631. Pinto JO, Curry JA. 1995. Atmospheric convective plumes emanating from leads 2. Microphysical and radiative processes. J. Geophys. Res. 100: 4633–4642. Renfrew IA, King JC. 2000. A simple model of the convective internal boundary layer and its application to surface heat flux estimates within polynyas. Bound.-Lay. Meteorol. 94: 335–356. Renfrew IA, King JC, Markus T. 2002. Coastal polynyas in the southern Weddell Sea: Variability of the surface energy budget. J. Geophys. Res. 107: doi: 10.1029/2000JC000720. Smith SD, Muench RD, Pease CH. 1990. Polynyas and leads: An overview of physical processes and environment. J. Geophys. Res. 95: 9461–9479. Spreen G, Kaleschke L, Heygster G. 2008. Sea ice remote sensing using AMSR-E 89 GHz channels. J. Geophys. Res. 113: doi:10.1029/2005JC003384. Stull RB. 1988. An introduction to boundary layer meteorology. Kluwer Acad.: Norwell, Mass. pp 666. K. Nicholls, as well as A. Weiss and R. Ladkin and also the support team at Rothera for their help with this research. My PhD is funded by the Natural Environment Research Council with a CASE studentship at the BAS. My participation in the fieldwork was made possible through the Antarctic Funding Initiative Collaborative Gearing Scheme.

Dispersion from ground-level sources in a shoreline urban area

We present results from a field study conducted in Wilmington, a suburb of Los Angeles, during 8 days of the period 26 August–10 September 2004. The tracer, sulfur hexafluoride, was released at a height of 3 m from a power plant site on the shoreline, and the concentrations of the tracer were sampled on five arcs at 100, 400, 1000, 3000, and 5000 m from the source during 6 h of the day starting at 7 a.m. This resulted in 40-h-long experiments, out of which, 21 had concentration measurements that could be interpreted with models. The meteorological conditions that governed dispersion were measured using sonic anemometers and sodars. The data analysis indicates that even during summer, the stability of the onshore flow is strong enough to keep the height of the convective internal boundary layer below 150 m at distances of 5000 m from the shoreline. However, the turbulence levels in the stable boundary layer are not smaller than those in the surface convective layer suggesting the presence of a shear generated boundary layer, which is advected with the onshore flow. A simple Gaussian dispersion model was used to interpret the ground-level concentrations measured in the experiment. The model uses expressions for plume spreads that depend on meteorological information at a height of 50 m from the surface. The vertical spread of the plume is limited to the height of the shear generated boundary layer. The height of this boundary layer is proportional to σ w / N , where σ w is the standard deviation of the vertical velocity fluctuations, and N is the Brunt–Vaisala frequency of the stable layer capping the surface-based convective layer. This result is based on indirect evidence: model performance improves significantly when vertical plume spread is limited to this height.

Conditional Analysis of an Internal Boundary Layer

Data from a convective internal boundary layer (IBL) are analyzed by focusing on the instantaneousstructure of the top of the IBL instead of the time-average structure.A conditional averaging technique is developed todiscriminate between air from above the IBL and air from below the IBL , which alternately invade some instrument levels due tosubstantial variation of the top of the convective IBL.Sensitivity to the conditional sampling criteria is examined. Inside the IBL , buoyant and mechanicalproduction and dissipation dominate the turbulent kinetic energy budget.The horizontal advection and turbulent transport terms are smaller, but not negligible. The inferred pressure correlation term is negligible.Above the IBL , buoyant production and dissipation, although weak,dominate the turbulent kinetic energy budget. Shear generation andturbulent transport are smaller but significant.

Application of a Perturbation Recycling Method in the Large-Eddy Simulation of a Mesoscale Convective Internal Boundary Layer

An arrangement of boundary conditions is described and demonstrated that facilitates the large-eddy simulation (LES) of inhomogeneous boundary layers such as internal boundary layers. In addition to the domain where the internal boundary layer develops, the method requires a section of domain over the upwind surface that is of the order of 10 boundary layer thicknesses and thus similar in size to that needed for the LES of the upwind boundary layer. In addition to periodic lateral, closed upstream, and open downstream boundary conditions, a simple and efficient perturbation recycling technique, which follows from one of Lund, Wu, and Squires, is used to generate a steady supply of fully developed turbulence from the inflow boundary. The arrangement is used to simulate a convective internal boundary layer during a cold-air outbreak over water. Results show the method consistently produces a solution that is homogeneous over the upwind surface and inhomogeneous over the downwind surface, and that the statistics are stationary after spinup. The sensitivity to the placement of the outflow boundary is tested, and examples of instantaneous and mean fields generated by the simulations are shown.

Experimental Study of the Vertical Structure of the Lower Troposphere over a SmallGreek Island in the Aegean Sea

Abstract An experimental campaign was carried out on a small Greek island that is characterized by complex terrain; its aim was to study the local characteristics of the vertical structure of the atmospheric boundary layer (ABL). The instrumentation was installed close to the shoreline and consisted of a 13-m-high meteorological mast instrumented at three levels, and a high-range vertical monostatic sodar. Tethered balloon flights were carried out for 3 days under different atmospheric conditions. The analysis of the available data revealed interesting features of the vertical structure of the atmosphere over the island, with the development of a convective internal boundary layer (IBL) within the first 150 m above the ground, while the marine boundary layer (MBL) formed at higher altitudes, up to 450 m. Buoyant oscillations appear within the MBL in the form of gravity waves with frequencies of 7 min. Theoretical calculations of the IBL height verified the experimental results. During the night, a complex...

Two-Dimensional Vector Wind Fields from Volume Imaging Lidar Data

Spatially resolved wind fields are derived by cross correlation of aerosol backscatter data from horizontal and vertical scans of the University of Wisconsin volume imaging lidar during the 1997/98 Lake-Induced Convection Experiment. Data from three cases are analyzed. The first two cases occurred on 10 and 13 January 1998 during cold-air outbreaks. Horizontal scans at 5 m above the lake reveal cellular structure of the steam fog. Vector winds are derived with 250-m spatial resolution over 60 and 36 km2 areas. These wind fields show acceleration and veering of offshore flow in the convective internal boundary layer along the upwind edge of Lake Michigan. The wind fields are used to compute divergence and vorticity. Effects of shoreline shape and topography are evident in the data. Horizontal wind speeds derived from vertical scans show the effects of convection on the vertical distribution of momentum. In the third case, 21 December 1997, a well-defined, shallow density current flowing offshore at ≈1 m s−1 is observed in the presence of larger-scale (3–4 m s−1) onshore flow. Winds on both sides of the land-breeze boundary as well as the three-dimensional structure of the event were recorded and analyzed.

Vertical Structure Of Turbulence In Offshore Flow During Rasex

The adjustment of the boundary layer immediately downstream froma coastline is examined based on two levels of eddy correlation data collected on a mast at the shore and six levels of eddy correlation data and profiles of mean variables collected from a mast 2 km offshore during the Riso Air-Sea Experiment. The characteristics of offshore flow are studied in terms of case studies and inter-variable relationships for the entire one-month data set. A turbulent kinetic energy budget is constructed for each case study. The buoyancy generation of turbulence is small compared to shear generation and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly weaken downstream from the coast. The reduction of downward mixing of momentum by the stratification leads to smaller roughness lengths compared to the unstable case. Shear generation at higher levels and advection of stronger turbulence from land often lead to an increase of stress and turbulence energy with height and downward transport of turbulence energy toward the surface. With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained by shear generation in the accelerating offshore flow.

Mean circulations of boundary-layer rolls in lake-effect snow storms

An observational study of the roll-average flow fields of boundary-layer rolls is presented. Data used for this purpose were collected by dual-Doppler radar and aircraft measurements taken over southern Lake Michigan during the 1983/1984 field operations of Project Lake Snow. The roll circulations agreed well with findings of past observational, numerical and theoretical studies, with cross-roll components roughly 10% of the convective internal boundary layer (CIBL)-mean wind speeds and weaker vertical components. Along-roll winds were systematically stronger in the rollupdraft regions than in the roll-downdraft regions, probably due to distortion of the along-roll wind profile by the rolls. Comparison of observed wind profiles to those required by roll formation mechanisms found by past numerical and theoretical studies suggested that the observed rolls were formed by the along-roll wind shear (Asai, 1970) or wind shear curvature (Kuettner, 1971) in the lowest 0.2Z i, whereZ i is the height of the top of the CIBL.

Observational Study of a Convective Internal Boundary Layer over Lake Michigan

Using aircraft data collected during the University of Chicago Lake-Effect Snow Storm project, the results of a case study of the convective thermal internal boundary layer (TIBL) over Lake Michigan are presented. An intense cold air outbreak on 20 January 1984 featured a rapid growth of the convective TIBL thickness and the concurrent development of cloud and snow. The average slope of the TIBL top over a fetch of 123.7 km was 1.0%. Microphysical characteristics of cloud and snow along with the TIBL development are also presented. Results of the TIBL integrated budgets of heat and total water (including cloud and snow water) are given in detail. Over the surface of Lake Michigan the average downward snow flux (snow precipitation rate) was 0.79 mm (water) per day. The average sensible and latent heat fluxes at the water surface were 323 and 248 W m−2, respectively. About 13 percent of the total warming of this cloud-topped TIBL was due to radiation.

A note on estimating the height of the convective internal boundary layer near shore

When cool air flows from the sea over a warm coast, the air is thermally modified. It is shown that h = cX1/2, where h is the height (in meters) of this thermal or convective internal boundary layer (CIBL) over the coast, X is the distance downwind (in meters) from the shoreline (i.e., the fetch), and c is a coefficient that relates to the shear velocity and wind speed inside the CIBL, potential temperature difference and entrainment coefficient across the CIBL, and the lapse rate outside the CIBL. This equation is a simplification of a theoretical equation and is supported by three similar formulations based on thermodynamic and dimensional analyses. Pertinent field experiments conducted near shorelines in France, Sweden, and Japan indicate that c is approximately 1.91, with a standard deviation of 0.38. All observations are within 95% confidence limits.

Turbulent characteristics of a shallow convective internal boundary layer

Turbulent characteristics of a 50 to 100 m deep convective internal boundary layer (I.B.L.) have been studied. The data were gathered at a flat coastal site (Nasudden on the island of Gotland, Sweden) during three consecutive days in May 1980 which were characterized by a steady, very stable stratified marine approach flow. The site is situated on a flat area ca. 1500 m from the shoreline. Only daytime runs have been analysed in the present paper. The sensible heat flux at the ground was typically 200 W m-2 and was found to decrease more or less linearly with height throughout the I.B.L., being slightly negative at greater heights. The momentum flux was also found to decrease with height, but nevertheless shear production of turbulent kinetic energy was found to be large throughout the entire I.B.L.

海岸地域上空に発達する自由対流内部境界層の構造

海岸地域上空に発達する自由対流内部境界層の構造

海風時に発達する自由対流内部境界層に関する飛行機観測

海風時に発達する自由対流内部境界層に関する飛行機観測

A similarity model of shoreline fumigation

Elements of plume rise in a stably stratified onshore flow, entrainment growth of an overland convective internal boundary layer, and the ultimate pollutant downwash in a fumigation situation are examined in a similarity format. In combination, there results a simple mathematical model for the location and magnitude of the maximum ground level concentration for continuous shoreline fumigation. Data from the Nanticoke shoreline fumigation study, the wind tunnel fumigation simulation of Meroney et al ( Boundary Layer Met. 9, 69–90, 1975) and the computations of Misra ( Atmospheric Environment 14, 397–400, 1980, Misra and Onlock ( Atmospheric Environment 16, 479–489) are combined using the similarity structure outlined in the development. Experimental verification of the plume rise formulation, boundary layer growth and the effect of source height on the maximum concentration in the fumigation zone is included.