Strong Capping Inversion Research Articles

Planetary Boundary Layer Flow over Complex Terrain during a Cold Surge Event: A Case Study

Planetary boundary layer (PBL) flow over complex terrain during a cold surge event was investigated using 3-hourly radiosonde measurements in upwind, near ridge, and downwind locations of mountains in the northeastern part of Republic of Korea and using a high-resolution (333-m) numerical simulation. A cold surge occurred on 23 January 2018 and lasted for 4 days. We analyzed onset day of the cold surge when air temperature dropped rapidly. Analysis of the radiosonde data shows that the PBL was characterized by an adiabatic layer with strong capping inversion in early morning and evening as well as during daytime in the upwind and near-ridge sites. The PBL flow at the near-ridge site was strongest among three sites except at 0600 local standard time (LST) when the PBL flow in the lee was strongest. We performed high-resolution (333-m) numerical simulations using the Weather Research and Forecasting (WRF) model. The adiabatic PBL in the upwind site at 0600 LST was simulated, although its depth was underestimated. The model reproduced the strong low-level wind at 0600 LST and large wind shear during the daytime in the lee, but it did not capture the exact timing of the large wind shear. The model showed overall good performance in simulating the vertical profile of the virtual potential temperature and wind below 2 km above ground level at the three sites, with a high index of agreement (IOA) except for the wind at 1200 and 1500 LST in the lee. To examine the cause for the different behavior of PBL flow in the lee of mountains between 0600 LST and the daytime, we calculated the Froude number for PBL flow using radiosonde measurements based on reduced gravity shallow water (RGSW) theory. At 0600 LST, the upwind Froude number F0 was close to 1, while during the daytime, it was much lower than 1. The observed lee flow behavior was consistent with the flow regime change of a single layer over an obstacle with changing F0; the flow with a propagating lee jump changes into that with a stationary lee jump with decreasing F0. Numerical simulation shows that the steepening of streamlines of lee-wave field leads to a jump-like structure in the lee of mountains during the daytime.

Subseasonal Variation Characteristics of Low-Cloud Fraction in Southeastern and Northwestern North Pacific

The subseasonal variability of the low-cloud fraction (LCF) over the southeastern North Pacific (SENP) and northwestern North Pacific (NWNP) was studied using satellite observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is found that subseasonal variability of the LCF was closely related to variations in the estimated inversion strength (EIS), sea surface wind speed (SSW), sensible heat flux (SHF), sea surface temperature (SST), surface temperature advection (Tadv), relative humidity (RH), surface level pressure (SLP) and surface air temperature (SAT). An increase in the LCF over the SENP is associated with the development of an anomalous anticyclonic circulation, which is located on the west coast of America. The cold advection, together with the subsidence warming associated with the anticyclonic circulation, strengthens the temperature inversion, favoring the development of the LCF. In the NWNP, the maximum LCF anomaly was also correlated with the stable boundary layer. The southerly wind blows airflow over the Kuroshio Extension from the subtropics, which brings warm and moist air. When air flows to the colder sea surface, it is cooled and condensed by the intensified heat exchange. A lead-lag composite analysis indicates that the mechanisms are different between the SENP and the NWNP, possibly due to the different types of low-level clouds over these two regions. In the SENP, the trade cumulus dominates under a strong capping inversion over the subtropics, whereas fog and stratus often occur under a shallow capping inversion in the NWNP. The effects of atmospheric circulation are also discussed.

Influence of Lead width on the Turbulent Flow Over Sea Ice Leads: Modeling and Parametrization

AbstractA new turbulence parametrization is developed for a non‐eddy‐resolving microscale model to study the effects of leads (elongated open‐water channels in sea ice) of different width on the polar atmospheric boundary layer (ABL). Lead‐dominated sea ice regions are characterized by large horizontal inhomogeneities of the surface temperature causing strong convection. Therefore, the new parametrization is based on a previous formulation where inhomogeneous conditions of dry convection over leads and nonlocal effects on heat fluxes had already been taken into account for a fixed lead width. A nonlocal lead width dependent approach is applied now for both heat fluxes and momentum fluxes in the convective region. Microscale model results obtained with the new, the previous nonlocal, and a local parametrization are shown, where 10 idealized cases of a lead‐perpendicular, near‐neutral ABL‐flow below a strong capping inversion are considered. Furthermore, time‐averaged large eddy simulation (LES) results of those cases are considered for analyzing the integrated effects of the dry convection on ABL characteristics. Microscale model results obtained with the new nonlocal parametrization agree well with the LES for variable lead widths and different atmospheric forcing although there is a room for further improvement. Furthermore, several features obtained with a local closure clearly disagree with LES. Thus, the microscale study also points to difficulties that might occur in mesoscale studies over regions where leads dominate the flow regime when local closures are applied.

Characteristics of aerosol vertical profiles in Tsukuba, Japan, and their impacts on the evolution of the atmospheric boundary layer

Abstract. Vertical profiles of the aerosol physical and optical properties, with a focus on seasonal means and on transport events, were investigated in Tsukuba, Japan, by a synergistic remote sensing method that uses lidar and sky radiometer data. The retrieved aerosol vertical profiles of the springtime mean and five transport events were input to our developed one-dimensional atmospheric model, and the impacts of the aerosol vertical profiles on the evolution of the atmospheric boundary layer (ABL) were studied by numerical sensitivity experiments. The characteristics of the aerosol vertical profiles in Tsukuba are as follows: (1) the retrieval results in the spring showed that aerosol optical thickness at 532 nm in the free atmosphere (FA) was 0.13, greater than 0.08 in the ABL owing to the frequent occurrence of transported aerosols in the FA. In other seasons, optical thickness in the FA was almost the same as that in the ABL. (2) The aerosol optical and physical properties in the ABL showed a dependency on the extinction coefficient. With an increase in the extinction coefficient from 0.00 to 0.24 km−1, the Ångström exponent increased from 0.0 to 2.0, the single-scattering albedo increased from 0.87 to 0.99, and the asymmetry factor decreased from 0.75 to 0.50. (3) The large variability in the physical and optical properties of aerosols in the FA were attributed to transport events, during which the transported aerosols consisted of varying amounts of dust and smoke particles depending on where they originated (China, Mongolia, or Russia). The results of the numerical sensitivity experiments using the aerosol vertical profiles of the springtime mean and five transport events in the FA are as follows: (1) numerical sensitivity experiments based on simulations conducted with and without aerosols showed that aerosols caused the net downward radiation and the sensible and latent heat fluxes at the surface to decrease. The decrease in temperature in the ABL (−0.2 to −0.6 K) and the direct heating of aerosols in the FA (0.0 to 0.4 K) strengthened the capping inversion around the top of the ABL. Consequently, the ABL height was decreased by 133 to 208 m in simulations with aerosols compared to simulations without aerosols. (2) We also conducted simulations in which all aerosols were compressed into the ABL but in which the columnar properties were the same and compared with the simulation results for uncompressed aerosol profiles. The results showed that the reductions in net downward radiation and in sensible and latent heat fluxes were the same in both types of simulations. However, the capping inversion in the simulations with compression was weakened owing to aerosol direct heating in the ABL and the lack of direct heating in the FA. This resulted in an increase in the ABL height, compared with that in the simulations without compression. (3) The dependencies of the 2 m temperature and ABL height on the optical thickness and Ångström exponent in the FA were investigated using the results of the numerical sensitivity tests. The 2 m temperature and ABL height was decreased with an increase in the optical thickness, and their reduction rates increase with a decrease in the Ångström exponent because the optical thickness in the near-infrared wavelength region was large when the Ångström exponent was small. However, there was a case in which the Ångström exponent was large but the decrease in the ABL height was the largest of all the simulation results. In this case, the strong capping inversion due to the large extinction coefficient around the top of the ABL was an import factor. These results suggest that the vertical profiles of the aerosol physical and optical properties, and the resulting direct heating has important effects on the ABL evolution.

Lagrangian simulations of the plume rise in strong capping inversion

We have performed new investigations applying our Lagrangian algorithm described by Alessandrini et al. (2013) to simulate the plume rise in a convective boundary layer capped by a strong inversion layer. We tested our model with the results of a water tank experiment (Weil et al., 2002). For each case, we compared the simulated and measured mean height, horizontal and vertical plume standard deviations and the entrapment (the fraction of the plume that remains captured above the temperature inversion layer located at the top of the boundary layer). The results show that the model is able to reproduce the main characteristics of the plume accurately.

Lagrangian simulations of the plume rise in strong capping inversion

Lagrangian simulations of the plume rise in strong capping inversion

Observations of Marine Atmospheric Boundary Layer Transitions across the Summer Kuroshio Extension*

Abstract The baiu and Kuroshio Extension (KE) fronts, both zonally oriented and nearly collocated east of Japan, are the dominant summertime features of the atmosphere and ocean, respectively, over the midlatitude northwest Pacific. An atmospheric sounding campaign was conducted on board the R/V Roger Revelle during the 2005 summer. Transects of soundings across the KE front are analyzed to study its effects on the atmosphere, along with continuous surface meteorological and ceilometer cloud-base observations. While the KE front remained nearly stationary during the cruise, the baiu front displayed large meridional displacements that changed wind direction across the KE front. The presence of sharp sea surface temperature (SST) gradients anchored by the KE enhanced the thermal and moisture advection, causing substantial changes in the marine atmospheric boundary layer (MABL) structure. When the baiu front was displaced north of the KE front, southwesterly winds advected warm, humid air from the subtropics over the cold water, producing a surface inversion favorable to fog formation. When the baiu front was to the south, on the other hand, northerly winds across the KE front destabilized the MABL, leading to the formation of a solid low-cloud deck beneath a strong capping inversion. The wind changes with the meridional displacement of the baiu front thus caused large variations in near-surface atmospheric stability and surface turbulent heat fluxes, with potential feedback on deep convection and fog/low-cloud formation around the front.

On the Decay of Supercells through a “Downscale Transition”: Visual Documentation

Photographic and mobile-radar documentation of the dissipation of a supercell and a severe convective storm that had not yet developed into a mature supercell are discussed. It is hypothesized, based on these cases and on others, that when a low-precipitation or classic supercell and/or a developing supercell moves into an environment of cooler surface temperatures and a strong capping inversion, it eventually dissipates through a process of “downscale transition,” in which vertical shear tilts the updraft more in the downshear direction as the CAPE decreases, and the updraft becomes narrower as the storm dissipates. During the downscale transition, it is possible that a cold pool or lack thereof may play a role, but the documentation in the cases detailed herein is not adequate to address this issue.

Modeling convection over arctic leads with LES and a non‐eddy‐resolving microscale model

Turbulent heat transport over inhomogeneous surfaces with sharp temperature discontinuities is investigated with a focus on the flow over leads in sea ice. The main goal consists in the development of a turbulence closure for a microscale atmospheric model resolving the integrated effect of plumes emanated from leads, but not the individual convective eddies. To this end, 10 runs are carried out with a large eddy simulation (LES) model simulating the flow over leads for springtime atmospheric conditions with near‐neutral inflow and a strong capping inversion. It is found that leads contribute to the stabilizing of the polar atmospheric boundary layer (ABL) and that strong countergradient fluxes of heat exist outside a core region of the plumes. These findings form the basis for the development of the new closure. It uses a new scaling with the internal ABL height and the characteristic vertical velocity for the plume region as the main governing parameters. Results of a microscale model obtained with the new closure agree well with the LES for variable meteorological forcing in case of lead orthogonal flow and for a fixed ABL height and lead width. The good agreement concerns especially the plume inclination, temperature, and heat fluxes as well as the relative contributions of gradient and countergradient transport of heat. A future more general closure should account, for example, for variable lead widths and wind directions. Results of the microscale model could be used to derive a future parameterization of the lead effect in large‐scale models.

Strong capping inversion over the Tokyo metropolitan area associated with airborne Asian dust

[1] Lidar measurements in the Tokyo metropolitan area showed that the optical thickness of airborne Asian dust rapidly increased on the morning of April 28, 2005, owing to the arrival of a thick Asian dust layer (ADL), which settled on the planetary boundary layer (PBL). Radiosonde measurements showed that atmospheric temperature in the lower troposphere increased considerably after the arrival of the ADL to the extent that the increment rate reached 12 K/day. Strong capping inversion (CI) was thus formed between the ADL and the PBL with the maximum rate of 0.1 K/m. It was mainly caused by the appearance of a deep isentropic layer that well corresponded to the ADL, indicating that the Asian dust-laden heated and well-mixed air layer led to the temperature increment and consequently induced the CI. The CI inhibited the PBL evolution and caused a higher aerosol concentration in the PBL in the metropolitan area.

High resolution studies of the Gibraltar Levanter validated using sun‐glint anemometry

AbstractThe Levanter is an easterly wind which frequently blows for several hours through the Strait of Gibraltar in summer. In the presence of a strong capping inversion, a synoptic or local scale pressure gradient forces the air through the gap between the orography on either side of the Strait. This flow cannot be resolved with either a global NWP model or a simplified linear model. Results from two Levanter cases, run using the Met Office Unified Model Version 5.3 at 1 km horizontal resolution over a 300×250 km limited area domain covering the Strait of Gibraltar, are presented, together with an unexpected source of verification. Copyright © 2006 John Wiley & Sons, Ltd.

The Dryline on 22 May 2002 during IHOP_2002: Convective-Scale Measurements at the Profiling Site

Abstract A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment. Moisture and temperature jumps of about 3 g kg−1 and 1–2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1–2 g kg−1) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s−1 and variable horizontal widths of 2–5 km, although some were as wide as 7–8 km. These updrafts were up to 1.5 g kg−1 moister than the surrounding environment. Although models suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms over the intensive observation region (IOR). Possible reasons for this lack of convection are discussed. Strong capping inversion and moisture detrainment between the lifting condensation level and the level of free convection related to an overriding drier air, together with the relatively small near-surface moisture values (less than 10 g kg−1), were detrimental to CI in this case.

Observations of marine stratocumulus in SE Pacific during the PACS 2003 cruise

In November 2003, the NOAA Environmental Technology Laboratory (ETL) conducted measurements of Marine Boundary Layer (MBL) stratocumulus clouds, thermodynamic structure, surface fluxes and meteorology in the southeastern Pacific stratocumulus region. The observations were part of a field program to replace the WHOI Ocean Reference Station buoy at 20 S Latitude 85 W Longitude. During the cruise the MBL structure was characterized by a strong capping inversion, periods with well mixed conditions and marine stratus, clear sky periods and periods with moderate vertical gradients of potential temperature and mixing ratio that overlap with periods of small cloud fractional coverage, decoupled layers and low cloud base shallow cumuli clouds. The lifting condensation level (LCL) showed strong variability consistent with the variability of the MBL. Clouds with thickness more that 250 m had drizzle below the cloud base especially during nighttime. Large periods of clear skies were observed at the buoy location, especially just after the solar flux maximum. The aerosol size distribution measurements generally exhibited a bimodal structure. However, abrupt changes in the aerosol size distribution were also recorded, corresponding either with the presence of drizzle (and a depletion of the accumulation mode) or the presence of clear skies (and an increase in the Aitken mode). The stratocumulus observed during the 5‐day station at the buoy location revealed a far more complex picture from the one captured during the East Pacific Investigation of Climate (EPIC) cruise to this same location in 2001.

Meteorological Processes Affecting the Evolution of a Wintertime Cold Air Pool in the Columbia Basin

Meteorological mechanisms affecting the evolution of a persistent wintertime cold air pool that began on 2 January and ended on 7 January 1999 in the Columbia basin of eastern Washington were investigated using a mesoscale numerical model together with limited observations. The mechanisms include surface radiative cooling and heating, large-scale subsidence, temperature advection, downslope warming in the lee of a major mountain barrier, and low-level cloudiness. The cold pool began when cold air accumulated over the basin floor on a clear night and was maintained by a strong capping inversion resulting from a rapid increase of air temperatures above the cold pool. This increase of temperatures aloft was produced primarily by downslope warming associated with strong westerly winds descending the lee slopes of the north‐south-oriented Cascade Mountains that form the western boundary of the Columbia basin. While the inversion cap at the top of the cold pool descended with time as the westerly flow intensified, the air temperature inside the cold pool exhibited little variation because of the fog and stratus accompanying the cold pool. Although the low-level clouds reduced the diurnal temperature oscillations inside the pool, their existence was not critical to maintaining the cold pool because surface radiative heating on a midwinter day was insufficient to completely destroy the temperature deficit in the persistent inversion. The presence of low-level clouds becomes much more critical for the maintenance of persistent cold pools in the spring and, perhaps, the fall seasons when insolation is much stronger than in midwinter. The cold pool was destroyed by cold air advection aloft, which weakened and eventually removed the strong inversion cap, and by an unstable boundary layer that grew upward from the heated ground after the dissipation of low-level clouds. Finally, erosion of the cold pool from above by turbulent mixing produced by vertical wind shear at the interface between quiescent air within the pool and stronger winds aloft was found to be insignificant for this case.

Small-amplitude coastally trapped disturbances and the reduced-gravity shallow-water approximation

Solutions are obtained for linear hydrostatic disturbances propagating parallel to the face of an uninterrupted topographic step in an infinitely deep, stably stratified fluid on an β-plane. These waves are vertically trapped because their frequencies are smaller than the Coriolis parameter and the height of the topographic step is finite. These waves are referred to as step-trapped Kelvin waves, because they are dynamically similar to internal Kelvin waves throughout the layer of fluid below the top of the topographic step. These waves appear to provide an idealized, semi-analytic model for the coastally trapped disturbances observed to propagate parallel to mountainous coastlines in several parts of the world. Computations are performed for a basic state with uniform static stability and for a three-layer basic state in which the two lowest layers represent the marine boundary layer and a strong capping inversion. One might suppose that the linear dynamics of hydrostatic disturbances in the three-layer basic state could be well approximated by a reduced-gravity shallow-water model, but this is not the case. In particular, the reduced-gravity shallow-water model does not provide reliable estimates for the phase speed of linear step-trapped Kelvin waves. This defect suggests that detailed quantitative comparisons between marine boundary-layer flows and the reduced-gravity shallow-water system may not have any intrinsic physical significance. Nevertheless, these results do not preclude the possibility of constructing useful qualitative analogies between marine boundary-layer flows and the reduced-gravity shallow-water model.

An Examination of Local versus Nonlocal Aspects of a TKE-Based Boundary Layer Scheme in Clear Convective Conditions

In this study, the ability of a turbulent kinetic energy (TKE)–based boundary layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme. The two cases are chosen from the Montreal-96 Experiment on Regional Mixing and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong capping inversion, and weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near the PBL top. With the original local version of the turbulence scheme, the model reproduces the vertical structures and turbulent quantities observed in the well-developed boundary layer for the first case. For the second case, the model fails to reproduce the rapid evolution of the boundary layer even though the TKE and sensible heat fluxes are greatly overpredicted. Some nonlocal aspects of the turbulence scheme are tested for these two cases. Inclusion of nonlocal (countergradient) terms in the vertical diffusivity equation has little impact on the simulated PBL. In contrast, alternative formulations of the turbulent length scales that follow the strategy proposed by Bougeault and Lacarrère have a greater influence. With the new turbulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of the local simulations even though the turbulent sensible heat fluxes are smaller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To improve the performance and reduce somewhat the arbitrariness in the Bougeault–Lacarrère technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the particular formulations, the simulated boundary layer agrees better with observations.

Carbon dioxide measurements in the nocturnal boundary layer over Amazonian forest

Abstract. Measurements of carbon dioxide concentration, temperature and windspeed were made in the nocturnal boundary layer over a tropical forest near Manaus, Brazil using a tethered balloon system. The measurements were made up to a maximum height of 300 m on ten consecutive nights in November 1995. Simultaneous surface flux and in-canopy concentration measurements were made at the surface close to the site. The observation period included several different types of conditions. Generally strong windshear and relatively weak temperature gradients prevented the formation of a strong capping inversion to the nocturnal boundary layer. On some nights, however, the inversion was sufficiently strong that the CO2 concentration at 100 m above the surface exceeded 400 ppm. The concentration within the canopy was largely controlled by the presence of an inversion very close to the canopy surface. The temperature and wind profiles are contrasted with conditions in Randônia, Brazil, where the windshear was found to be weaker and higher carbon dioxide concentrations were observed in the early morning. The difference in carbon dioxide concentrations in the nocturnal boundary layer between dusk and dawn is used to estimate the regional nighttime flux of carbon dioxide. The value obtained generally exceeds the measured surface flux and sometimes exceeds the sum of the surface flux and the in-canopy storage made at the tower site. The reasons for the discrepancy are not clear; either one of the methods is in error or the regional carbon dioxide budget differs significantly from the local budget measured at the tower site.

Low Cloud Type over the Ocean from Surface Observations. Part I: Relationship to Surface Meteorology and the Vertical Distribution of Temperature and Moisture

Surface cloud observations and coincident surface meteorological observations and soundings from five ocean weather stations are used to establish representative relationships between low cloud type and marine boundary layer (MBL) properties for the subtropics and midlatitudes by compositing soundings and meteorological observations for which the same low cloud type was observed. Physically consistent relationships are found to exist between low cloud type, MBL structure, and surface meteorology at substantially different geographical locations and seasons. Relative MBL height and inferred decoupling between subcloud and cloud layers are increasingly greater for stratocumulus, cumulus-under-stratocumulus, and cumulus, respectively, at midlatitude locations as well as the eastern subtropical location during both summer and winter. At the midlatitude locations examined, cloudiness identified as fair-weather stratus often occurs in a deep, stratified cloud layer with little or no capping inversion. This strongly contrasts with cloudiness identified as stratocumulus, which typically occurs in a relatively well-mixed MBL under a strong capping inversion at both midlatitude and eastern subtropical locations. At the transition between subtropics and midlatitudes in the western North Pacific, cloudiness identified as fair-weather stratus occurs in a very shallow layer near the surface. Above this layer the associated profile of temperature and moisture is similar to that for cumulus at the same location, and neither of these cloud types is associated with a discernible MBL. Sky-obscuring fog and observations of no low cloudiness typically occur with surface-based inversions. These observed relationships can be used in future studies of cloudiness and cloudiness variability to infer processes and MBL structure where above-surface observations are lacking.

Observations and Numerical Modeling of an Elevated Mixed Layer

Abstract Observations and a numerical model have been used to investigate the structure of an elevated mixed layer (EML) that formed in the lee of the Rocky Mountains over eastern Colorado. The EML formed as a dry convective boundary layer over the higher mountainous terrain of western Colorado, and then advected eastward, producing upper-level warming over the eastern plains. This upper-level warming generated a strong capping inversion at the top of a surface-based plains convective boundary layer that formed concurrently with the EML. A model trajectory analysis indicated that air from the plains boundary layer was detrained into the EML in a zone of convergence along the foothills of the Front Range of the Rocky Mountains. Specific physical processes responsible for meso-β-scale EML development were examined using a two-dimensional version of the mesoscale numerical model. Blocking of the plateau-level winds by the plains breeze proved to be the primary constraint on advection of the EML and its under...

Numerical simulation of turbulent convection over wavy terrain

Thermal convection of a Boussinesq fluid in a layer confined between two infinite horizontal walls is investigated by direct numerical simulation (DNS) and by large-eddy simulation (LES) for zero horizontal mean motion. The lower-surface height varies sinusoidally in one horizontal direction while remaining constant in the other. Several cases are considered with amplitude δ up to 0.15H and wavelength λ of H to 8H (inclination up to 43°), where H is the mean fluid-layer height. Constant heat flux is prescribed at the lower surface of the initially at rest and isothermal fluid layer. In the LES, the surface is treated as rough surface (z0 = 10−4H) using the Monin-Oboukhov relationships. At the flat top an adiabatic frictionless boundary condition is applied which approximates a strong capping inversion of an atmospheric convective boundary layer. In both horizontal directions, the model domain extends over the same length (either 4H or 8H) with periodic lateral boundary conditions.We compare DNS of moderate turbulence (Reynolds number based on H and on the convective velocity is 100, Prandtl number is 0.7) with LES of the fully developed turbulent state in terms of turbulence statistics and Characteristic large-scale-motion structures. The LES results for a flat surface generally agree well with the measurements of Adrian et al. (1986). The gross features of the flow statistics, such as profiles of turbulence variances and fluxes, are found to be not very sensitive to the variations of wavelength, amplitude, domain size and resolution and even the model type (DNS or LES), whereas details of the flow structure are changed considerably. The LES shows more turbulent structures and larger horizontal scales than the DNS. To a weak degree, the orography enforces rolls with axes both perpendicular and parallel to the wave crests and with horizontal wavelengths of about 2H to 4H. The orography has the largest effect for λ = 4H in the LES and for λ = 2H in the DNS. The results change little when the size of the computational domain is doubled in both horizontal directions. Most of the motion energy is contained in the large-scale structures and these structures are persistent in time over periods of several convective time units. The motion structure persists considerably longer over wavy terrain than over flat surfaces.