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D. H. Lenschow

Abstract

Aircraft measurements of sensible and latent heat fluxes, surface and air temperature, mean wind and humidity were used to examine the boundary layer structure over the Great Lakes for two cases in late fall when the water was warmer than the air above. The sensible and latent vertical turbulent heat fluxes at the surface were in the range 5–10 mW em−2, and 6–15 mW em−3, respectively. Estimates of the mean vertical velocity at the interface between the mixed layer (where the equivalent potential temperature lapse rate was ∼1C km−1 and the mixing ratio was effectively constant) and the stable air above were obtained from the equations for the rate of change of sensible plus latent heat and of water vapor in the boundary layer. The values obtained by this method appear to be reasonable when compared with estimates obtained from the wind field. Using this mean vertical velocity, the vertical fluxes of latent and sensible heat and liquid water in the cloud layer just below the top of the mixed layer can be determined. The downward buoyancy flux at this level was found to be <8% of the upward surface buoyancy flux.

Sensible and latent heat fluxes were found to vary by as much as a factor of 2 along the flight path across Lake Michigan at 147 m because of variation in the lake surface temperature. The maximum wind speed in a set of measurements at four heights, from 30 to 308 m above the lake, near the upwind end was at the lowest level. It is shown that this peculiarity may be due to the thermal wind.

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D. H. Lenschow

Abstract

A model is proposed for the variation with height of the terms in the turbulence kinetic energy budget throughout an unstably stratified barotrople planetary boundary layer. The model is based upon aircraft measurements throughout the boundary layer that are presented here and previous results from surface layer measurements. The model assumes that at the limit of neutral stability, the transport term in the budget equation is at a minimum. When the height above the ground is greater than about ten times the absolute value of the Obukhov length, the shear-generation term is negligible, while the rate of dissipation of turbulence energy becomes almost constant, and the transport term increases almost linearly with height to balance the almost linear decrease of the buoyancy-generation term. Measurements of the ratio of the vertical flux of the horizontal part of the turbulence kinetic energy to the vertical part show good agreement with a model based upon surface layer observations and a laboratory tank experiment.

One set of observations was taken over a lake from just downwind of the shore to about 30 km offshore and the assumption of horizontal homogeneity was found to be unjustified.

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L. Mahrt and D. H. Lenschow

Abstract

A model for the growth of a convectively mixed layer is derived by layer integrating the basic equations and parameterizing unknown terms in the mixed layer turbulence kinetic energy equation by means of free convection similarity theory. When shear generation of turbulence energy is neglected in the turbulent inversion layer capping the mixed layer, the model essentially reduces to that of Tennekes. This shear generation is found to be important only in cases of significant baroclinicity and shallow mixed layer depth or small free flow stratification.

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Niels Otto and D. H. Lenschow

Abstract

Data taken during the Air Mass Transformation Experiment (AMTEX) by the NCAR Electra aircraft have proven useful for investigating the structure of thermals penetrating into the turbulent inversion layer which caps the convective mixed layer. On 16 February 1975, two flight legs, one upwind and one crosswind, and each about 12 min long (∼80 km in length), were flown at a nominally constant altitude at about the level of the turbulent inversion layer. Because of the variations in height of this relatively thin layer, the airplane spent about equal amounts of time above and below the inversion layer. These two legs are further split into six sections for statistical analyses, each with somewhat different characteristics. Variances, co-variances, spectra and cospectra of potential temperature, the three air velocity components, and humidity are computed to illustrate the dynamic processes occurring in this region. Two spectral maxima occur in vertical velocity and temperature: one at a wavelength of about 1.5 times the mixed layer depth and the other at about 200–300 m, which seems to be related to the characteristic size of a penetrating thermal.

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Qing Wang and D. H. Lenschow

Abstract

Isolated cumuli penetrating through marine stratocumulus clouds were documented during the Atlantic Stratocumulus Transition Experiment. This paper aims at understanding the role of the penetrating cumulus in regulating stratocumulus and boundary-layer structure through analysis of data from the NCAR Electra aircraft. When penetrating cumulus clouds are present, the boundary layer is generally decoupled from the near-surface air except in the cumulus region. Therefore, air in the cumulus region includes air entrained at the cloud top, as well as air modified by surface processes. In the stratocumulus region, however, entrained inversion air and moist surface air are confined to separate layers. As a result, large horizontal variations are found in scalars, such as ozone and water vapor. Turbulence statistics and conditional sampling of entrainment events in the cumulus and stratocumulus regions indicate that stronger entrainment may occur at the cumulus top compared to the surrounding stratocumulus. This analysis is, however, complicated by insufficient sampling of cloud-top jump conditions in both regions.

Convergent flow in the lower boundary layer and compensating diverging flow in the upper boundary layer were identified along the flight trark. This flow field, together with the vertical coupling of surface air with the cloud layer in the cumulus region, helps to transport moisture upwards from the sea surface and disperse it to the surrounding stratocumulus sheet, thus helping to maintain the stratocumulus cover.

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I. R. Paluch and D. H. Lenschow

Abstract

Observations of stratiform clouds in a region several hundred kilometers west of the southern California coast were made from the NCAR Electra research aircraft in the summer of 1987 during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). Examples are shown of how heating or cooling of air by the sea and the evaporation of precipitation affect the stability of the temperature profile above the surface layer, which in turn affects the vertical moisture transports and the resulting cloud formation. We expect that sea-surface heating leads to the formation of stratus layers, while sea-surface cooling or cooling from evaporation of precipitation may produce fields of cumuli. The observations lead to a conceptual model of the life cycle of a stratus layer, starting as a thin, rather homogeneous layer, which grows and becomes patchy with time, produces precipitation, followed by formation of small cumuli below, and finally disintegrates, leaving a field of cumuli behind.

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

Abstract

Terms in the budgets of turbulence kinetic energy, temperature and humidity variances, and temperature and humidity fluxes have been evaluated for a baroclinic, convective boundary layer using data obtained from the NCAR Electra aircraft during the Air Mass Transformation Experiment (AMTEX). Although the mean temperature and momentum budgets, which were also evaluated, are strongly influenced by the horizontal temperature gradient, the second-moment budgets are little affected. The mean momentum budget is not well balanced, probably due to a combination of neglect of horizontal advection (aircraft advection measurements are shown to be statistically unreliable) and error in the surface geostrophic wind. For the most part, the measured terms in the second-moment budgets agree with previous estimates. Turbulence dissipation, however, was systematically less than that found in previous tower-based experiments. We find that over most of the mixed layer the temperature variance is maintained by turbulent transport and the temperature flux by buoyant production while, in contrast, the humidity variance and flux are maintained primarily by gradient production. Near the top of the mixed layer both temperature and humidity statistics are strongly affected by entrainment processes.

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R. A. Brost, J. C. Wyngaard, and D. H. Lenschow

Abstract

This paper discusses the turbulence profiles and budgets for two days of radiation, dynamical and thermodynamical observations by the NCAR Electra in shallow marine stratocumulus off the California coast in June 1976.

The boundary layer is characterized by relatively high wind speeds (12–20 m s−1) and low liquid water contents (0.1 g kg−1); the clouds are not very convective and seem to have little influence on the turbulence budgets. In cloud, drizzle has a significant impact on the liquid water budget and occasionally even on the total water budget even though no drizzle is observed at the surface. The stresses, velocity variances, and their budgets behave as in a neutral boundary layer, sometimes with an additional peak in the cross-wind variance at the inversion due to shear production.

There is scant evidence of direct production of vertical velocity variance at cloud top due to radiative cooling or latent heat release; it is maintained principally by the pressure-scrambling terms through redistribution of the shear-produced energy. We find, however, that while the Rotta parameterization for pressure scrambling in the stress budgets works well near the surface and sometimes throughout the layer, it is unsatisfactory in the variance budgets.

Fluctuations of temperature and moisture on a scale of several hundred meters in cloud satisfy the Clausius-Clapeyron equation. When the boundary layer is well mixed in equivalent potential temperature and total water substance, the vertical turbulent fluxes of these quantities are usually almost linear. The efficiency of cloud-top radiative cooling in producing mixed-layer convection is also discussed.

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M. C. vanZanten, B. Stevens, G. Vali, and D. H. Lenschow

Abstract

In situ and radar data from the second field study of the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) have been used to study drizzle in stratocumulus. Measurements indicate that drizzle is prevalent. During five of seven analyzed flights precipitation was evident at the surface, and on roughly a third of the flights mean surface rates approached or exceeded 0.5 mm day−1. Additional analysis of the structure and variability of drizzle indicates that the macroscopic (flight averaged) mean drizzle rates at cloud base scale with H 3/N where H is the flight-averaged cloud depth and N the flight-averaged cloud droplet number concentration. To a lesser extent flight-to-flight variability in the mean drizzle rate also scales well with differences in the 11- and 4-μm brightness temperatures, and the cloud-top effective radius. The structure of stratocumulus boundary layers with precipitation reaching the surface is also investigated, and a general picture emerges of large flight-averaged drizzle rates being manifested primarily through the emergence of intense pockets of precipitation. The characteristics of the drizzle spectrum in precipitating versus nonprecipitating regions of a particular cloud layer were mostly distinguished by the number of drizzle drops present, rather than a change in size of the median drizzle drop, or the breadth of the drizzle spectrum.

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R. A. Brost, D. H. Lenschow, and J. C. Wyngaard

Abstract

The mean radiational, dynamical and thermodynamical structure of the marine stratocumulus-topped mixed layers of the California coast is described for two days in June 1976 using data from the NCAR Electra aircraft. We suggest that the synoptic conditions found may be typical of about half of the shallow stratocumulus-topped boundary layers that occur in this region during summer. The inversion was low near the coast and increased in height to the west, consistent with the average westward increase in sea-surface temperature. North–south inversion height change was largely due to entrainment and mean mesoscale vertical motions. Below the inversion, strong winds (12–20 m s−1 from the north) and horizontal inhomogeneities resulted in large advection terms in mean field equations. The sloping inversion often produced large vertical shears of the actual and geostrophic wind velocities across the inversion. Because of low liquid-water contents (0.1 g kg−1), temperature and water vapor could be measured in cloud with in situ instrumentation without significant errors due to wetting.

The longwave radiative extinction length was found to be relatively short; 63% of the cloud-top jump in radiation flux occurred within 40 m. Radiative heat loss was largely balanced by shear-driven entrainment. Compositing vertical gradients provided by individual aircraft ascents and descents is shown to overestimate vertical gradients at the inversion.

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