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The classical theory of homogeneous isotropic turbulence predicts that the ratio of transverse to longitudinal structure functions or power spectra is equal to 4/3 in the inertial subrange. For the typical turbulence cascade in the inertial subrange, it also predicts a power law scaling with an exponent of +2/3 and -5/3 for the structure functions and the power spectra, respectively. The goal of this study is to document the statistics of those ratios and exponents derived from aircraft observations, quantify their departures from theoretical predictions, and point out the differences among the aircraft. We estimate the transverse-to-longitudinal ratios and the scaling exponents from in situ high-rate turbulence measurements collected by three research aircraft during four field experiments in two regimes of the marine atmospheric boundary layer: shallow trade-wind convection and subtropical stratocumulus. The bulk values representing the inertial subrange were derived by fitting power law formulae to the structure functions and to the power spectra computed separately for the three components of the turbulent wind velocity measured in horizontal flight segments. The composite scale-by-scale transverse-to-longitudinal ratios were derived by averaging over the segments at common non-dimensional scales. The variability in the results can be attributed to how the wind velocity components are measured on each aircraft. The differences related to environmental conditions, e.g. between characteristic levels and regimes of the boundary layer, are of secondary importance. Experiment-averaged transverse-to-longitudinal ratios are 23 %–45 % smaller than predicted by the theory. The deviations of average scaling exponents with respect to the theoretical values range from - 34 % to + 47 % for structure functions and from - 24 % to + 22 % for power spectra, depending on experiment and velocity component. The composite scale-by-scale transverse-to-longitudinal ratios decrease and increasingly depart from 4/3 with decreasing scale, in contrast to previous experimental studies on local isotropy. The reason for the disagreement in transverse-to-longitudinal ratios between the observations and the theory remains uncertain. [ABSTRACT FROM AUTHOR]

Airborne estimates of greenhouse gas emissions are becoming more prevalent with the advent of rapid commercial development of trace gas instrumentation featuring increased measurement accuracy, precision, and frequency, and the swelling interest in the verification of current emission inventories. Multiple airborne studies have indicated that emission inventories may underestimate some hydrocarbon emission sources in US oil-and gas-producing basins. Consequently, a proper assessment of the accuracy of these airborne methods is crucial to interpreting the meaning of such discrepancies. We present a new method of sampling surface sources of any trace gas for which fast and precise measurements can be made and apply it to methane, ethane, and carbon dioxide on spatial scales of ĝ1/4 1000ĝ€m, where consecutive loops are flown around a targeted source region at multiple altitudes. Using Reynolds decomposition for the scalar concentrations, along with Gauss's theorem, we show that the method accurately accounts for the smaller-scale turbulent dispersion of the local plume, which is often ignored in other average mass balance methods. With the help of large eddy simulations (LES) we further show how the circling radius can be optimized for the micrometeorological conditions encountered during any flight. Furthermore, by sampling controlled releases of methane and ethane on the ground we can ascertain that the accuracy of the method, in appropriate meteorological conditions, is often better than 10ĝ€%, with limits of detection below 5ĝ€kgĝ€hĝ'1 for both methane and ethane. Because of the FAA-mandated minimum flight safe altitude of 150ĝ€m, placement of the aircraft is critical to preventing a large portion of the emission plume from flowing underneath the lowest aircraft sampling altitude, which is generally the leading source of uncertainty in these measurements. Finally, we show how the accuracy of the method is strongly dependent on the number of sampling loops and/or time spent sampling the source plume.

The implementation and accuracy of a low-rate (~1 Hz) horizontal wind measurement system is described for a fixed-wing aircraft without modification to the airframe. The system is based on a global positioning system (GPS) compass that provides aircraft heading and a ground-referenced velocity, which, when subtracted from the standard true airspeed, provides estimates of the horizontal wind velocity. A series of tests was performed flying "L"-shaped patterns above the boundary layer, where the winds were assumed to be horizontally homogeneous over the area bounded by the flight (approximately 25km2). Four headings were flown at each altitude at a constant airspeed. Scaling corrections for both heading and airspeed were found by minimizing the variance in the 1-s wind measurements; an upper limit to the error was then computed by calculating the variance of the corrected wind measurements on each of the four headings. A typical uncertainty found in this manner tends to be less than 0.2ms-1. The measurement system described herein is inexpensive and relatively easy to implement on single-engine aircraft. © 2014 American Meteorological Society.

The Canopy Horizontal Array Turbulence Study (CHATS) experimental study was conducted at a Cilker Orchards's walnut blocks in Dixon, California, before and after leaves emerged, to help improve understanding, simulation capabilities, and modeling of coupled vegetation-atmosphere-land surface interactions. The campaign took place over 12 weeks in the spring of 2007 and was broken into three 4-week phases. Canopy-induced mechanical and thermodynamical vertical variation of turbulence transport characteristics and the linkages between canopy-scale motions and the larger scale planetary boundary layer (PBL) turbulence were measured. The observations focused on measurements characterizing stratification influences on the spatial structure of canopy-induced turbulence, the trace gas source/sink distribution associated with vegetation, and the overall impact of canopy-induced processes on trace gas transport.

We discuss two techniques based on mixed-layer scaling for estimating trace gas surface emission fluxes from aircraft using instruments that do not have sufficient frequency response for direct eddy correlation measurements. The first is the mixed-layer gradient technique, which requires accurate measurements of mean concentrations at several heights in the clear convective planetary boundary layer (CBL) to resolve gradients from even strong surface sources of short-lived trace gases. The flux-gradient relationship is obtained from large-eddy numerical simulations of the CBL. We show that this technique is limited to trace gases with lifetimes of about a day or less. An example is dimethylsulfide (DMS), which is emitted from the ocean and has a lifetime of about a day. Surface DMS flux was estimated from data collected from the NASA P-3B research aircraft during the Pacific Exploratory Mission-Tropics (PEM-Tropics) flight 7 (August 24, 1996), when the aircraft flew a sequence of constant altitude circles about 50 km in diameter at different heights in and above the boundary layer, following the boundary layer air trajectory. The flight took place between 0530 and 1330 local solar time, providing a good opportunity to observe diurnal changes within a Lagrangian framework under nearly clear-sky conditions. The resulting DMS flux of 2.5 ± 0.8 pptv m s-1 (6.1 ± 1.9) × 1013 molecules m-2 s-1, or 8.8±2.8 μmols m-2 d-1) is on the high end of previous measurements in this milieu. The second technique is the mixed-layer variance technique, which uses measurements of the variance at several heights in the CBL to estimate the surface flux. A major problem with the variance technique is accounting for the contribution of mesoscale variability to the measured variance. Several sources of mesoscale variability were identified: clusters of small cumulus clouds rising through the top of the boundary layer, mesoscale variations in the horizontal wind leading to inaccurate tracking of the air mass and, to a lesser extent, the presence of horizontal roll vortices in some areas of the boundary layer. We show that the variance technique should be applicable to estimating surface fluxes of short-lived trace gases in cumulus-free boundary layers that are horizontally uniform, if sample-collection times of about 10 s or less are used. We also show that it may be possible to utilize mesoscale variance measurements to estimate surface fluxes and lifetimes of species which have lifetimes of perhaps a week or more. Copyright 1999 by the American Geophysical Union.

Fast measurements of three scalars, ozone, dimethyl sulfide (DMS), and total water, are used to investigate the entrainment process in the stratocumulus-topped boundary layer (STBL) observed over the eastern subtropical Pacific during the second Dynamics and Chemistry of Marine Stratocumulus Experiment (DYCOMS-II). Direct measurement of the flux profiles by eddy covariance is used to estimate the entrainment velocity, the average rate at which the boundary layer grows diabatically via incorporation of overlying free tropospheric air. The entrainment velocities observed over the course of the mission, which took place during July 2001, ranged from 0.12 to 0.72 cm [s.sup.-1] and appear to outpace the estimated large-scale subsidence as the boundary layer advects over warmer sea surface temperatures. Observed entrainment velocities display only a weak correlation with the buoyancy Richardson number defined at the inversion, which suggests that processes other than inversion strength, such as wind shear, might play a larger role in driving entrainment in the STBL than previously recognized. This study is the first to use DMS as an entrainment tracer because the high-rate mass spectrometric technique has only recently been developed. The biogenic sulfur compound shows great promise for such investigations in marine environments because the free tropospheric concentrations are virtually nonexistent, and it therefore serves as an unambiguous marker of boundary layer air. As such, individual mixing events can be analyzed to determine the mixing fraction of boundary layer and free tropospheric air, and in several such cases buoyancy reversal was observed despite the absence of large-scale dissipation of the cloud field as postulated by cloud-top entrainment instability. Moreover, the redundancy attained in using three separate scalars allows for an investigation of the average height scales above the inversion from where air is blended into the STBL, and this tends to be less than 80 m above the mean inversion height, implying that the entrainment process occurs on very small scales.

We have developed a high-speed sensor for dimethyl sulfide (DMS) based on its fast chemiluminescent reaction with molecular fluorine. Emission in the wavelength range 450-650 nm is monitored via photon counting. The instrument can continuously measure DMS with a response time of 0.1 s and is highly linear and sensitive. Limits of detection (S/N = 1) are 39, 12, and 4 pptv DMS for 0.1-, 1-, and 10-s integration times, respectively. Sensitivity and response time allow the direct measurement of DMS fluxes in the marine atmospheric boundary layer by the eddy correlation technique. Selectivity has previously been measured and is sufficient for monitoring DMS in the marine boundary layer without significant interferences.

The structure and evolution of stratocumulus cloud decks have long been recognized to depend upon a balance of numerous processes including the downward entrainment of the overlying free tropospheric air into the cloud deck. While the cloud is understood to have an impact on the overlying air as it subsides toward the inversion, the nature of the interaction between the stratocumulus and the overlying air has not been studied in detail. We have developed a simple, one-dimensional model of the air in the lower free troposphere as it subsides to the stratocumulus cloud deck. First, we fixed the mixed layer in order to determine how the thermodynamic structure of the overlying air impacts the cloud. Our model shows that this overlying air can have a large impact on the net longwave flux across the cloud top and thus the longwave cloud-top cooling. The overlying air, which may require days to subside through a couple of kilometers, undergoes cooling at a rate that depends on its vapor content. The total cooling is thus strongly dependent on the large-scale divergence. This cooling is greatest immediately above the cloud top and can substantially change the strength of the inversion and the potential for a buoyancy reversal upon entrainment. We then coupled this model of the overlying air to a mixed-layer model. We find that the temperature at which air is entrained into the cloud, and thus the strength of the inversion, is not a direct function of the altitude of cloud top as is normally modeled. The entrainment rate and the strength of the inversion are found to be in a loose balance. If the entrainment rate is too great, the strength of the inversion increases, which reduces the entrainment rate.