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D. H. Lenschow, B. B. Stankov, and L. Mahrt

Abstract

Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s−1 respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites.

A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.

The observed transition in temperature typically precedes the velocity transition by 20–40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.

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P. Guo, Y.-H. Kuo, S. V. Sokolovskiy, and D. H. Lenschow

Abstract

This study presents an algorithm for estimating atmospheric boundary layer (ABL) depth from Global Positioning System (GPS) radio occultation (RO) data. The algorithm is applied to the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) RO data and validated using high-resolution radiosonde data from the island of St. Helena (16.0°S, 5.7°W), tropical (30°S–30°N) radiosondes collocated with RO, and European Centre for Medium-Range Weather Forecasts (ECMWF) high-resolution global analyses. Spatial and temporal variations of the ABL depth obtained from COSMIC RO data for a 1-yr period over tropical and subtropical oceans are analyzed. The results demonstrate the capability of RO data to resolve geographical and seasonal variations of ABL height. The spatial patterns of the variations are consistent with those derived from ECMWF global analysis. However, the ABL heights derived from ECMWF global analysis, on average, are negatively biased against those estimated from COSMIC GPS RO data. These results indicate that GPS RO data can provide useful information on ABL height, which is an important parameter for weather and climate studies.

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A. S. Frisch, D. H. Lenschow, C. W. Fairall, W. H. Schubert, and J. S. Gibson

Abstract

A cloud-sensing Doppler radar is used with a vertically pointing antenna to measure the vertical air motion in clouds during the Atlantic Stratocumulus Transition Experiment. The droplet fall velocity contamination was made negligible by using only measurements during the time the reflectivity was below − 17 dBZ. During one day of measurements, the daytime character of the vertical velocity variance is different than that of the nighttime case. In the upper part of the cloud, the variance had a distinct maximum for both day and night; however, the nighttime maximum was about twice as large as the daytime case. Lower down in the cloud, there was a second maximum, with the daytime variance larger than the nighttime case. The skewness of the vertical velocity was negative near cloud top in both the day and night cases, changing to positive skewness in the lower part of the cloud. This behavior near cloud top indicates that the upper part of the cloud is behaving like an upside-down convective boundary layer, with the downdrafts smaller in area and more intense than the updrafts. In the lower part of the cloud, the behavior of the motion is more like a conventional convective boundary layer, with the updrafts smaller and more intense than the downdrafts. The upside-down convective forcing in the upper part of the cloud is due to radiative cooling, with the daytime forcing less because of shortwave warming.

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R. L. Schwiesow, S. D. Mayor, V. M. Glover, and D. H. Lenschow

Abstract

The NCAR Airborne Infrared Lidar System (NAILS) observed the edge of an extended, sloping aerosol layer that intersected a stratocumulus cloud deck over the Pacific Ocean during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment, 260 km WNW of San Diego. In situ measurements support the interpretation of the lidar observations as arising from a particle-laden layer with relatively clean air above, below, and to the SW. Intersection of these sloping layers with cloud top leads to substantial horizontal variability of boundary-layer structure in the intersection region. The intersection of the aerosol layer with cloud top also corresponded closely to a quasi-linear trough in the cloud top that showed enhanced brightness and an enhanced number of small particles.

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I. R. Paluch, S. McKeen, D. H. Lenschow, R. D. Schillawski, and G. L. Kok

Abstract

A diurnally averaged ozone decay rate of about 0.11 day−1was observed under partly cloudy sky in the boundary layer over the eastern Atlantic during the second Lagrangian experiment in the Atlantic Stratocumulus Transition Experiment. The observed decay rate can be accounted for by the combined effects of ozone loss through photolysis by UV radiation, reaction With H02 radical, and deposition on the sea surface, and effects of ozone production through photooxidation of carbon monoxide and methane in the presence of nitrogen oxides. The main contributor to the ozone decay is photolysis by UV radiation, but the other sources and sinks also make significant contributions.

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I. R. Paluch, D. H. Lenschow, S. Siems, G. L. Kok, R. D. Schillawski, and S. McKeen

Abstract

The mean time rates of change of temperature, total water mixing ratio and ozone along airflow trajectories in the lower troposphere over the eastern Pacific are inferred by comparing aircraft soundings from the First ISCCP Regional Experiment (FIRE) and the Hawaiian Rainband Project (HaRP). Through the use of the estimated mean fluxes of temperature and total water mixing ratio, it is found that the tendency for stratus layers to grow or dissipate is very sensitive to the assumed turbulence structure below the capping inversion. A mixed-layer model that assumes a well-mixed boundary layer up to the capping inversion predicts a solid cloud layer extending all the way to Hawaii, whereas a model that allows decoupling predicts rapid dissipation of the stratus layer. It is concluded that stratus dissipation here is due to the slowdown of turbulent mixing throughout the layer below the capping inversion, not the drying out of a well-mixed layer; hence, the mixed-layer model cannot be expected to predict realistic cloud dissipation. The differences in ozone concentration observed in the boundary layer during HaRP and FIRE suggest a chemical loss of ozone of 3–8ppb day−1, corresponding to a lifetime of 3–9 days. This implies that ozone cannot be treated as a conserved tracer when dealing with ozone budgets over periods of days. The ozone sink is probably of photochemical origin, and it requires further investigation.

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Ming Yu Zhou, D. H. Lenschow, B. B. Stankov, J. C. Kaimal, and J. E. Gaynor

Abstract

Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which is about twice that estimated in a previous experiment. The wavelength is up to 5.5 times the mixed-layer height above the top of the mixed layer. Within the mixed layer, terms in the turbulence kinetic energy equation are similar to previous studies. Above the mixed layer, shear production becomes large, and is approximately balanced by the sum of the buoyancy, dissipation and transport terms. The temperature variance and flux budgets also have large terms and significant residuals in the overlying inversion.

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K. J. Davis, N. Gamage, C. R. Hagelberg, C. Kiemle, D. H. Lenschow, and P. P. Sullivan

Abstract

Wavelet analysis is applied to airborne infrared lidar data to obtain an objective determination of boundaries in aerosol backscatter that are associated with boundary layer structure. This technique allows high-resolution spatial variability of planetary boundary layer height and other structures to be derived in complex, multilayered atmospheres. The technique is illustrated using data from four different lidar systems deployed on four different field campaigns. One case illustrates high-frequency retrieval of the top of a strongly convective boundary layer. A second case illustrates the retrieval of multiple layers in a complex, stably stratified region of the lower troposphere. The method is easily modified to allow for varying aerosol distributions and data quality. Two more difficult cases, data that contain a great deal of instrumental noise and a cloud-topped convective layer, are described briefly. The method is also adaptable to model analysis, as is shown via application to large eddy simulation data.

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Peter P. Sullivan, Chin-Hoh Moeng, Bjorn Stevens, Donald H. Lenschow, and Shane D. Mayor

Abstract

The authors use large-eddy simulation (LES) to investigate entrainment and structure of the inversion layer of a clear convectively driven planetary boundary layer (PBL) over a range of bulk Richardson numbers, Ri. The LES code uses a nested grid technique to achieve fine resolution in all three directions in the inversion layer.

Extensive flow visualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion. It is found that coherent structures in the convective PBL, that is, thermal plumes, are primary instigators of entrainment in the Ri range 13.6 ⩽ Ri ⩽ 43.8. At Ri = 13.6, strong horizontal and downward velocities are generated near the inversion layer because of the plume–interface interaction. This leads to folding of the interface and hence entrainment of warm inversion air at the plume’s edge. At Ri = 34.5, the inversion’s strong stability prevents folding of the interface but strong horizontal and downward motions near the plume’s edge pull down pockets of warm air below the nominal inversion height. These pockets of warm air are then scoured off by turbulent motions and entrained into the PBL. The structure of the inversion interface from LES is in good visual agreement with lidar measurements in the PBL obtained during the Lidars in Flat Terrain field experiment.

A quadrant analysis of the buoyancy flux shows that net entrainment flux (or average minimum buoyancy flux min) is identified with quadrant IV wθ+ < 0 motions, that is, warm air moving downward. Plumes generate both large negative quadrant II w+θ < 0 and positive quadrant III wθ > 0 buoyancy fluxes that tend to cancel.

The maximum vertical gradient in potential temperature at every (x, y) grid point is used to define a local PBL height, z i(x, y). A statistical analysis of z i shows that skewness of z i depends on the inversion strength. Spectra of z i exhibit a sensitivity to grid resolution. The normalized entrainment rate w e/w∗, where w e and w∗ are entrainment and convective velocities, varies as ARi−1 with A ≈ 0.2 in the range 13.6 ⩽ Ri ⩽ 43.8 and is in good agreement with convection tank measurements. For a clear convective PBL, the authors found that the finite thickness of the inversion layer needs to be considered in an entrainment rate parameterization derived from a jump condition.

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J. C. Wyngaard, W. T. Pennell, D. H. Lenschow, and M. A. LeMone

Abstract

The behavior of the temperature-humidity covariance (θq) budget in the convectively driven boundary layer is determined through analysis of data from AMTEX and (to a lesser extent) Kansas and Minnesota. In the near-neutral surface layer a balance is found between production and molecular destruction; in the mixed layer, transport is also important. We extend the Corrsin theory for inertial subrange scalar spectral behavior to the temperature-humidity cospectrum, and thus relate the molecular destruction rate of θq to its inertial range level. Destruction rates inferred from AMTEX cospectra agree with those found from the imbalance of production and transport terms. The budgets within the surface layer and the mixed layer are parameterized separately with appropriate scales.

Both temperature and humidity fluctuations contribute to the small-scale refractive index variations which affect acoustic and electromagnetic wave propagation in the atmosphere. Our results indicate that their joint contribution CTq to the refractive index structure parameter is directly related to the molecular destruction rate of θq. The results provide a basis for understanding and predicting the behavior of CTq in the convective boundary layer.

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