The Temperature-Humidity Covariance Budget in the Convective Boundary Layer

We discuss the accuracy requirements for measuring mesoscale (roughly horizontal scales >10 km or 5 to 10 times the planetary boundary-layer (PBL) depth) fluxes in the convective PBL, and the ability of current research aircraft to achieve this accuracy. We conclude that aircraft equipped with inertial nagivation systems capable of < 3 km hr −1 navigational accuracy are able to resolve mesoscale fluctuations in velocity, and thus variances and fluxes on the mesoscale. We then discuss measurements of velocity and scalar spectra, and cospectra of vertical velocity with horizontal velocity components and scalars, obtained from long flight legs with the National Center for Atmospheric Research Electra aircraft over the boreal forest of Canada in summer during the BOreal Ecosystem-Atmosphere Study (BOREAS), over the tropical Pacific Ocean from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and over the East China Sea during wintertime cold-air outbreaks from the Air Mass Transformation Experiment (AMTEX). Each of these studies has somewhat different forcings and boundary conditions, so we can compare their consequences on the spectra and cospectra. On average, we found no significant scalar or momentum fluxes for horizontal scales >10 km. We also develop a simple model based on observed thermal structure to explain the phase angle between vertical velocity and the along-wind horizontal velocity as a function of height, which shows good agreement with the observed phase angle in AMTEX.

Section: Measurement Resultsmentioning

confidence: 99%

The spectral composition of fluxes and variances over land and sea out to the mesoscale

Lenschow

Sun

2007

“…With the assumption of horizontal homogeneity, the budget equation for the variance of ozone is 1 a7 I as 1 aw'/ ---= where es is the rate at which the molecular diffusivity of ozone dissipates ozone fluctuations and Q is the sum of the internal sources and sinks of ozone, i.e., the rate of production or dissipation of ozone within a parcel of air. We can estimate the magnitude of the source term in Equation (4), -w's' S/&r, by analogy with the temperature variance equation in the free convection layer (Wyngaard et al, 1978), They show that the transport term in the surface layer is considerably smaller than the production term. Thus, if there were no other source of ozone variance, production by the gradient term must be approximately equal to the dissipation.…”

Section: Resultsmentioning

confidence: 99%

Airborne measurements of the vertical flux of ozone in the boundary layer

Lenschow

Delany

Stankov

et al. 1980

Self Cite

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.

“…This result is also in qualitative agreement with that predicted by Warhaft's (1976) equation. Using data on 6' 'q' published by Wyngaard et al (1978) and on w", 6' 12, and q12 by , together with mean values of A0 , Aq, and Ah of Table I, one can readily calculate that Warhaft's (1976) equation produces a ratio KN/KE of about 0.5. Nevertheless, the present results shown in Table V forA,/A, imply that K,/K, near the inversion is much smaller than values observed previously in the surface sublayer.…”

Section: Discussionmentioning

confidence: 99%

Nearly steady convection and the boundary-layer budgets of water vapor and sensible heat

Brutsaert

1987

The budgets of water vapor and sensible heat in the convective atmospheric boundary (mixed) layer are analyzed by means of a simple slab approach adapted to steady large-scale advective conditions with radiation and cloud activity. The entrainment flux for sensible heat is assumed to be a linear function of the surface flux. The flux of water vapor at the top of the mixed layer is parameterized by extending the first-order Betts-Deardortfapproach, i.e., by adopting linear changes for both the specific humidity and the flux across the mixed layer and across the inversion layer of finite thickness. In this way the dissimilarity of sensible heat and water vapor transport in the mixed layer can be taken into account. The experimental data were obtained from the Air Mass Transformation Experiment (AMTEX). The entrainment constant for sensible heat at the top of the mixed layer was found to have values similar to those observed in other weakly convective situations, i.e., around 0.4 to 0.6. This appears to indicate that the effect of mechanical turbulence was not negligible; however, the inclusion of this effect in the formulation did not improve the correlation. In contrast to the first-order approach, the zero-order approach, i. e., the jump equation commonly used for the flux of a scalar at the inversion, (w'c' ),, = w,Ac (where w, is the entrainment velocity and AC the concentration jump across the inversion), was found to be invalid and incapable of describing the data.