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1

Electronic Resource

The role of thermals in the convective boundary layer (1980)

Lenschow, D. H. ; Stephens, P. L.

Springer

Boundary layer meteorology 19 (1980), S. 509-532

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Detailed measurements of the structure of thermals throughout the convective boundary layer were obtained from the NCAR Electra aircraft over the ocean during the Air Mass Transformation Experiment (AMTEX). Humidity was used as an indicator of thermals. The variables were first high-pass filtered with a 5 km cutoff digital filter to eliminate mesoscale variations. Segments of the 5 min (30 km length) horizontal flight legs with humidity greater than half the standard deviation of humidity fluctuations for that leg were defined as thermals. This was found to be a better indicator of thermals than temperature in the upper part of the boundary layer since the temperature in a thermal is cooler than its environment in the upper part of the boundary layer. Using mixed-layer scaling, the normalized length and number of thermals were found to scale with the 1/3 and -1/3 powers, respectively, of normalized height, while vertical velocity and temperature scaled according to similarity predictions in the free convection region of the surface layer. The observational results presented here extend throughout the entire mixed layer. Using these results in the equation for mean updraft velocity of a field of thermals, the sum of the vertical pressure gradient and edge-effect terms can be estimated. This residual term is found to be important throughout most of the boundary layer. The magnitude of the divergence of vertical velocity variance within a thermal is found to be larger than the magnitude of the mean updraft velocity term throughout most of the mixed layer.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00122351

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2

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The characteristics of turbulent velocity components in the surface layer under convective conditions (1977)

Panofsky, H. A. ; Tennekes, H. ; Lenschow, D. H. ; [et al.]

Springer

Boundary layer meteorology 11 (1977), S. 355-361

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract It is proposed that the ratios of the standard deviations of the horizontal velocity components to the friction velocity in the surface layer under convective conditions depend only onz i /L wherez i is the height of the lowest inversion andL is the Monin-Obukhov length. This hypothesis is tested by using observations from several data sets over uniform surfaces and appears to fit the data well. Empirical curves are fitted to the observations which have the property that at largez i /-L, the standard deviations become proportional tow *, the convective scaling velocity. Fluctuations of vertical velocity obtained from the same experiments scale withz/L, wherez is the height above the surface, in good agreement with Monin-Obukhov theory.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF02186086

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3

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An observational study of the structure of the nocturnal boundary layer (1979)

Mahrt, L. ; Heald, R. C. ; Lenschow, D. H. ; [et al.]

Springer

Boundary layer meteorology 17 (1979), S. 247-264

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract In an effort to describe the basic vertical structure of the nocturnal boundary layer, observations from four experiments are analyzed. During the night, the depth of significant cooling appears to increase with time while the depth of the turbulence and height of the low level wind maximum tend to remain constant or decrease with time. Since the inversion layer extends above the low level wind maximum and shear is small in the region of the low level jet, the Richardson number reaches a maximum at the jet level and then decreases again with height. As a result, turbulence is observed to be a minimum at the height of the low level wind maximum and then increases again above this height.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00117983

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4

Electronic Resource

Reply (1978)

Panofsky, H. A. ; Wyngaard, J. C. ; Lenschow, D. H.

Springer

Boundary layer meteorology 15 (1978), S. 259-260

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00121927

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5

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The spectral velocity tensor for homogeneous boundary-layer turbulence (1989)

Kristensen, L. ; Lenschow, D. H. ; Kirkegaard, P. ; [et al.]

Springer

Boundary layer meteorology 47 (1989), S. 149-193

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract We have postulated a simple model for the spectral tensor Φ ij (k) of an anisotropic, but homogeneous turbulent velocity field. It is a simple generalization of the spectral tensor Φ inf ij piso(k) for isotropic turbulence and we show how in the limit of isotropy, Φ ij (k) becomes equal to Φ inf ij piso(k). Whereas Φ inf ij piso(k) is determined entirely by one scalar function of k = ¦k¦, namely the energy spectrum, we need three independent scalar functions of k to specify Φ ij (k). We show how it is possible by means of the three stream-wise velocity component spectra to determine the three scalar functions in Φ ij (k) by solving two uncoupled, ordinary linear differential equations of first and second order. The analytic form of the component spectra each has a set of three parameters: the variance and the integral length scale of the velocity component and a dimensionless parameter, which governs the curvature of the spectrum in the transition domain from the inertial subrange towards lower wave numbers. When the three sets of parameters are the same, the three spectra correspond to isotropic turbulence and they are all interrelated and related to the energy spectrum. We show how it is possible to obtain these spectral forms in the neutral surface layer and in the convective boundary layer from data reported in the literature. The spectral tensor is used to predict the lateral coherences for all three velocity components and these predictions are compared with coherences obtained in two experiments, one using three masts at a horizontally homogeneous site in Denmark and one employing two aircraft flying in formation over eastern Colorado. Comparison shows reasonable agreement although with considerable experimental scatter.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00122327

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6

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Airborne measurements of the vertical flux of ozone in the boundary layer (1980)

Lenschow, D. H. ; Delany, A. C. ; Stankov, B. B. ; [et al.]

Springer

Boundary layer meteorology 19 (1980), S. 249-265

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ISSN: 1573-1472

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract A fast-response chemiluminescent ozone sensor was mounted in an aircraft instrumented for air motion and temperature measurements. Measurements of the vertical flux of ozone by the eddy correlation technique were obtained after correcting for time delay and pressure sensitivity in the ozone sensor output. The observations were taken over eastern Colorado for two days in April, one a morning and the other an afternoon flight. Since the correlation coefficient of ozone and vertical velocity is small compared to, for example, temperature and vertical velocity in the lower part of the convective boundary layer, an averaging length of the order of 100 km was required to obtain a reasonably accurate estimate of the ozone flux. The measured variance of ozone appeared to be too large, probably mainly due to random noise in the sensor output, although the possibility of the production of ozone fluctuations by chemical reactions cannot be dismissed entirely. Terms in the budget equation for ozone were estimated from the aircraft measurements and the divergence of the ozone flux was found to be large compared to the flux at the surface divided by the boundary-layer height.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00117223

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7

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A study of ozone in the Colorado mountains (1983)

Fehsenfeld, F. C. ; Bollinger, M. J. ; Liu, S. C. ; [et al.]

Springer

Journal of atmospheric chemistry 1 (1983), S. 87-105

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ISSN: 1573-0662

Keywords: Tropospheric ozone ; ozone precursors ; photochemistry ; nitrogen oxides ; rural ozone

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Geosciences

Notes: Abstract The seasonal and diurnal variations of ozone mixing ratios have been observed at Niwot Ridge. Colorado. The ozone mixing ratios have been correlated with the NO x (NO+NO2) mixing ratios measured concurrently at the site. The seasonal and diurnal variations in O3 can be reasonably well understood by considering photochemistry and transport. In the winter there is no apparent systematic diurnal variation in the O3 mixing ratio because there is little diurnal change of transport and a slow photochemistry. In the summer, the O3 levels at the site are suppressed at night due to the presence of a nocturnal inversion layer that isolated ozone near the surface, where it is destroyed. Ozone is observed to increase in the summer during the day. The increases in ozone correlate with increasing NO x levels, as well as with the levels of other compounds of anthropogenic origin. We interpret this correlation as in-situ or in-transit photochemical production of ozone from these precursors that are transported to our site. The levels of ozone recorded approach 100 ppbv at NO x mixing ratios of approximately 3 ppbv. Calculations made using a simple clean tropospheric chemical model are consistent with the NO x -related trend observed for the daytime ozone mixing ratio. However, the chemistry, which does not include nonmethane hydrocarbon photochemistry, underestimates the observed O3 production.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00113981

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8

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Direct measurements of nitrogen oxides and ozone fluxes over grassland (1986)

Delany, A. C. ; Fitzjarrald, D. R. ; Lenschow, D. H. ; [et al.]

Springer

Journal of atmospheric chemistry 4 (1986), S. 429-444

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ISSN: 1573-0662

Keywords: Biosphere-atmosphere exchange ; biogenic emission ; eddy correlation ; dry deposition ; nitrogen oxides fluxes ; ozone deposition to grassland ; turbulent exchange

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Geosciences

Notes: Abstract Using the eddy correlation method, fluxes of nitric oxide, nitrogen dioxide, ozone, water, and sensible heat were measured at a site 20 km north of Denver, Colorado over mature crested wheat grass, 0.75 m high in late June and early July. During this period the weather was fair with no synoptic disturbances. In the early morning a well-mixed diluted urban pollution plume traversed the site, by late morning aged pollution had mixed downward into the local boundary layer, and by afternoon the air came from a relatively unpolluted area of the high plains. The mean trace gas concentrations reflect this repeated pattern of local air flow. The fluxes of the trace gases were influenced both by the variation of the means and by other factors including temperature and biological activity. Ozone fluxes were found to be always negative and proportional to the mean, with an average deposition velocity for this case of about 0.006 m s-1. For the oxides of nitrogen this simple treatment was not appropriate. Both deposition and emission were observed, generally deposition predominated in the morning and emission in the afternoon with observed variations in the fluxes of NOx=NO+NO2 from −0.3 to +0.2 ppbv m s-1.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00053844

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