Dean Vickers

Quality Control and Flux Sampling Problems for Tower and Aircraft Data

Abstract A series of automated tests is developed for tower and aircraft time series to identify instrumentation problems, flux sampling problems, and physically plausible but unusual situations. The automated procedures serve as a safety net for quality controlling data. A number of special flags are developed representing a variety of potential problems such as inconsistencies between different tower levels and the flux error due to fluctuations of aircraft height. The tests are implemented by specifying critical values for parameters representing each specific error. The critical values are developed empirically from experience of applying the tests to real turbulent time series. When these values are exceeded, the record is flagged for further inspection and comparison with the rest of the concurrent data. The inspection step is necessary to either verify an instrumentation problem or identify physically plausible behavior. The set of tests is applied to tower data from the Riso Air Sea Experiment and...

The Cospectral Gap and Turbulent Flux Calculations

Abstract An alternative method to Fourier analysis is discussed for studying the scale dependence of variances and covariances in atmospheric boundary layer time series. Unlike Fourier decomposition, the scale dependence based on multiresolution decomposition depends on the scale of the fluctuations and not the periodicity. An example calculation is presented in detail. Multiresolution decomposition is applied to tower datasets to study the cospectral gap scale, which is the timescale that separates turbulent and mesoscale fluxes of heat, moisture, and momentum between the atmosphere and the surface. It is desirable to partition the flux because turbulent fluxes are related to the local wind shear and temperature stratification through similarity theory, while mesoscale fluxes are not. Use of the gap timescale to calculate the eddy correlation flux removes contamination by mesoscale motions, and therefore improves similarity relationships compared to the usual approach of using a constant averaging timesc...

On the Exchange of Momentum over the Open Ocean

AbstractThis study investigates the exchange of momentum between the atmosphere and ocean using data collected from four oceanic field experiments. Direct covariance estimates of momentum fluxes were collected in all four experiments and wind profiles were collected during three of them. The objective of the investigation is to improve parameterizations of the surface roughness and drag coefficient used to estimate the surface stress from bulk formulas. Specifically, the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk flux algorithm is refined to create COARE 3.5. Oversea measurements of dimensionless shear are used to investigate the stability function under stable and convective conditions. The behavior of surface roughness is then investigated over a wider range of wind speeds (up to 25 m s−1) and wave conditions than have been available from previous oversea field studies. The wind speed dependence of the Charnock coefficient α in the COARE algorithm is modified to , where m = 0.017 m−1 ...

SHALLOW DRAINAGE FLOWS

Two-dimensional sonic anemometers and slowresponse thermistors were deployedacross a shallow gully during CASES99. Weak gully flow of a few tenths of m s-1 anda depth of a few metres develops in the earlyevening on most nights with clear skies.Flow down the gully developed sometimes evenwhen the opposing ambient wind exceeded10 m s-1 at the top of the60–m tower. Cold air drainage fromlarger-scale slopes flows over the top ofthe colder gully flow. The gully flowand other drainage flows are generally eliminated in the middle of the night in conjunctionwith flow acceleration abovethe surface inversion layer and downwardmixing of warmer air and highermomentum. As the flow decelerates later inthe night, the gully flow may re-form.The thin drainage flows decouple standard observational levels of3–10 m from the surface.Under such common conditions, eddy correlationflux measurements cannot be used toestimate surface fluxes nor even detect thethin gully and drainage flows. The gentlegully system in this field program is typical ofmuch of the Earth’s land surface.

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Observations of Fluxes and Inland Breezes over a Heterogeneous Surface

Abstract Repeated aircraft runs at about 33 m over heterogeneous terrain are analyzed to study the spatial variability of the mesoscale flow and turbulent fluxes. An irrigated area, about 12 km across, generates a relatively cool moist inland breeze. As this air flows out over the warmer, drier surrounding land surface, an internal boundary layer develops within the inland breeze, which then terminates at a well-defined inland breeze front located about 1½ km downstream from the change of surface conditions. This front is defined by horizontal convergence, rising motion, and sharp spatial change of moisture, carbon dioxide, and ozone. Both a scale analysis and the observations suggest that the overall vertical motion associated with the inland breeze is weak. However, the observations indicate that this vertical motion and attendant vertical transport are important in the immediate vicinity of the front, and the inland breeze does lead to significant modification of the turbulent flux. In the inland breez...

Fetch Limited Drag Coefficients

Measurements made at a tower located 2 km off the coast of Denmark inshallow water during the Risø Air Sea Experiment (RASEX) are analyzedto investigate the behaviour of the drag coefficient in the coastal zone.For a given wind speed, the drag coefficient is larger during conditions ofshort fetch (2-5 km) off-shore flow with younger growing waves than it isfor longer fetch (15-25 km) on-shore flow. For the strongest on-shorewinds, wave breaking enhances the drag coefficient. Variation of the neutral drag coefficient in RASEX is dominated byvariation of wave age, frequency bandwidth of the wave spectra and windspeed. The frequency bandwidth is proportional to the broadness of the waveheight spectra and is largest during conditions of light wind speeds. Usingthe RASEX data, simple models of the drag coefficient and roughness length are developed in terms of wind speed, wave age and bandwidth. An off-shoreflow model of the drag coefficient in terms of nondimensional fetch isdeveloped for situations when the wave state is not known.

The Very Stable Boundary Layer on Nights with Weak Low-Level Jets

Abstract The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very weak turbulence and negligible turbulent mixing. The focus of this paper i...

Sea surface drag coefficients in the Risø Air Sea Experiment

This study examines the dependence of the computed drag coefficient on wind speed, stability, fetch, flux sampling problems, and method of calculation of the drag coefficient. The analysis is applied to data collected at a tower 2 km off the coast of Denmark during the Riso Air Sea Experiment (RASEX). Various flux sampling problems are evaluated to eliminate unreliable fluxes. Large drag coefficients are observed with weak large-scale flow. However, the value of the computed drag coefficient at weak wind speeds is sensitive to flux sampling problems and the method of calculation of the drag coefficient, which might be a general characteristic of weak winds. The drag coefficient is significantly larger for short fetch conditions, particularly at strong wind speeds.

A New Drag Relation for Aerodynamically Rough Flow over the Ocean

From almost 7000 near-surface eddy-covariance flux measurements over the sea, the authors deduce a new air‐sea drag relation for aerodynamically rough flow: u * 5 0:0583U N10 2 0:243: Here u * is the measured friction velocity, and UN10 is the neutral-stability wind speed at a reference height of 10 m. This relation isfitted toUN10valuesbetween 9 and24 m s 21 . A drag relationformulatedasu * versus UN10 has several advantages over one formulated in terms of C DN10 5(u * /U N10 ) 2 . First, the multiplicative coefficient on UN10 has smaller experimental uncertainty than do determinations of CDN10. Second, scatterplots of u * versus UN10 are not ill posed when UN10 is small, as plots of CDN10 are; u * ‐UN10 plots presented here suggest aerodynamically smooth scaling for small UN10. Third, this relation depends only weakly on Monin‐Obukhovsimilaritytheoryand,consequently,reducestheconfoundingeffectsofartificialcorrelation. Finally, with its negative intercept, the linear relation produces a CDN10 function that naturally rolls off at high wind speed and asymptotically approaches a constant value of 3.40 3 10 23 . Hurricane modelers and theair‐sea interactioncommunityhave been tryingtorationalize such behavior in thedragcoefficient for at least 15 years. This paper suggests that this rolloff in CDN10 results simply from known processes that influence wind‐wave coupling.