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Robert A. Lynch and E. F. Bradley

Abstract

An improved shearing stress meter (drag plate) of 6 ft (183 cm) diameter has been constructed for micro-meteorological work. This size should enable representative sampling of rough surfaces and is intended particularly for farmland experimental sites where, after harvesting, the ground is either left to grain stubble or ploughed.The sensors are inductive proximity probes, whose advantages of robustness and stability enable the instrument to be handled without undue precautions against damage and to withstand gusts of over 20 dyn cm−2 while achieving a resolution of 0.01 dyn cm−2.Comparison experiments involving two drag plates installed in a ploughed field with a sonic anemometer mounted nearby indicate good agreement in stress measurement among all three instruments.

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E. W. Schulz, B. G. Sanderson, and E. F. Bradley

Abstract

A method for removing ship motion from wind measurements using a simple and inexpensive strap-down system of accelerometers is described and analyzed. In slight seas, error analysis indicates that mean root-mean-square uncertainties associated with the motion correction are 0.03 and 0.006 m s−1 for the horizontal and vertical wind, respectively, for all runs analyzed. The mean uncertainty in the wind stress due to motion correction is 8 × 10−4 N m−2. In a shallow coastal sea setting, ship motion appears to almost always be successfully detected and removed from the vertical component of the observed wind. The horizontal wind components appear to be successfully corrected in 86% of the runs analyzed. Motion correction is shown to have a significant influence on the covariance-calculated wind stress. In approximately half of the runs analyzed the wind stress changes by more than 15%. Motion correction has a smaller effect on the heat fluxes.

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C. W. Fairall, E. F. Bradley, J. E. Hare, A. A. Grachev, and J. B. Edson

Abstract

In 1996, version 2.5 of the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk algorithm was published, and it has become one of the most frequently used algorithms in the air–sea interaction community. This paper describes steps taken to improve the algorithm in several ways. The number of iterations to solve for stability has been shortened from 20 to 3, and adjustments have been made to the basic profile stability functions. The scalar transfer coefficients have been redefined in terms of the mixing ratio, which is the fundamentally conserved quantity, rather than the measured water vapor mass concentration. Both the velocity and scalar roughness lengths have been changed. For the velocity roughness, the original fixed value of the Charnock parameter has been replaced by one that increases with wind speeds of between 10 and 18 m s−1. The scalar roughness length parameterization has been simplified to fit both an early set of NOAA/Environmental Technology Laboratory (ETL) experiments and the Humidity Exchange Over the Sea (HEXOS) program. These changes slightly increase the fluxes for wind speeds exceeding 10 m s−1. For interested users, two simple parameterizations of the surface gravity wave influence on fluxes have been added (but not evaluated).

This new version of the algorithm (COARE 3.0) was based on published results and 2777 1-h covariance flux measurements in the ETL inventory. To test it, 4439 new values from field experiments between 1997 and 1999 were added, which now dominate the database, especially in the wind speed regime beyond 10 m s−1, where the number of observations increased from 67 to about 800. After applying various quality controls, the database was used to evaluate the algorithm in several ways. For an overall mean, the algorithm agrees with the data to within a few percent for stress and latent heat flux. The agreement is also excellent when the bulk and directly measured fluxes are averaged in bins of 10-m neutral wind speed. For a more stringent test, the average 10-m neutral transfer coefficients were computed for stress and moisture in wind speed bins, using different averaging schemes with fairly similar results. The average (mean and median) model results agreed with the measurements to within about 5% for moisture from 0 to 20 m s−1. For stress, the covariance measurements were about 10% higher than the model at wind speeds over 15 m s−1, while inertial-dissipation measurements agreed closely at all wind speeds. The values for stress are between 8% (for inertial dissipation) and 18% (for covariance) higher at 20 m s−1 than two other classic results. Twenty years ago, bulk flux schemes were considered to be uncertain by about 30%; the authors find COARE 3.0 to be accurate within 5% for wind speeds of 0–10 m s−1 and 10% for wind speeds of between 10 and 20 m s−1.

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C. W. Fairall, P. O. G. Persson, E. F. Bradley, R. E. Payne, and S. P. Anderson

Abstract

The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the Rapid Radiative Transfer Model (RRTM), from 55 radiosondes ascents in cloud-free conditions during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment field program, gave mean agreement within 2 W m−2 of those measured with a shipborne PIR. PIR data from two sets of instrument intercomparisons were used to demonstrate ways of detecting inconsistencies in thermopile-sensitivity coefficients and dome-heating correction coefficients. These comparisons indicated that pairs of PIRs are easily corrected to yield mean differences of 1 W m−2 and rms differences of 2 W m−2. Data from a previous field program over the ocean indicate that pairs of PIRs can be used to deduce the true surface skin temperature to an accuracy of a few tenths of a kelvin.

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J. A. Businger, M. Miyake, A. J. Dyer, and E. F. Bradley

Abstract

The results of a heat flux comparison experiment carried out at Hay, New South Wales, Australia, during May 1966 using a sonic anemometer thermometer (SAT), Fluxatron and Evapotron are reported. The instruments agree with each other to within a factor of 2 for individual runs. The large fluctuations from run to run of the individual estimates are mainly caused by the fact that the Eulerian point average does not provide an adequate statistical sample of the heat flux. This point is illustrated by the non-stationary behavior of the instantaneous product of vertical wind and temperature. As auxiliary results, values of σ w/u * and σ T/T * have been obtained which are somewhat higher than, but in general agreement with, observations reported by Mordukhovich and Zwang.

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J. A. Businger, J. C. Wyngaard, Y. Izumi, and E. F. Bradley

Abstract

Wind and temperature profiles for a wide range of stability conditions have been analyzed in the context of Monin-Obukhov similarity theory. Direct measurements of heat and momentum fluxes enabled determination of the Obukhov length L, a key independent variable in the steady-state, horizontally homogeneous, atmospheric surface layer. The free constants in several interpolation formulas can be adjusted to give excellent fits to the wind and temperature gradient data. The behavior of the gradients under neutral conditions is unusual, however, and indicates that von Kármán's constant is ∼0.35, rather than 0.40 as usually assumed, and that the ratio of eddy diffusivities for heat and momentum at neutrality is ∼1.35, compared to the often-suggested value of 1.0. The gradient Richardson number, computed from the profiles, and the Obukhov stability parameter z/L, computed from the measured fluxes, are found to be related approximately linearly under unstable conditions. For stable conditions the Richard on number approaches a limit of ∼0.21 as stability increases. A comparison between profile-derived and measured fluxes shows good agreement over the entire stability range of the observations.

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R. P. Cechet, J. Bennett, I. Helmond, P. A. Coppin, E. F. Bradley, I. J. Bapton, and J. S. Godfrey

Abstract

An accurate platinum RTD-based (resistive temperature device) system has been developed to measure the vertical temperature profile in the region of the atmosphere-ocean interface. TASITA, the towed air–sea interaction temperature analyzer, continuously measures the vertical temperature profile using 17 fixed temperature probes mounted on the instrument: 9 in the uppermost meter of the ocean, and 8 in the lowest 2 m of the atmosphere. The absolute accuracy is better than ±0.05°C, and the relative accuracy between RTDs is ±0.01°C. The instrument is designed to be towed beside a research vessel in undisturbed water outside the ship's wake. Towing speeds between 4 and 8 kt are possible. Instrument operational use is aimed specifically at low wind conditions when the sea surface is smooth to slight and mixing in the top few meters of the ocean is inhibited. Under these conditions a diurnal warm surface water layer is often present in which the surface temperature of the water is markedly different to that tens of centimeters below. Data collected in the western equatorial Pacific show variations in the temperature structure of the surface mixed layer caused by solar beating of the ocean surface and freshwater lenses resulting from heavy precipitation.

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Nicholas A. Bond, Clifford F. Mass, Bradley F. Smull, Robert A. Houze, Ming-Jen Yang, Brian A. Colle, Scott A. Braun, M. A. Shapiro, Bradley R. Colman, Paul J. Neiman, James E. Overland, William D. Neff, and James D. Doyle

The Coastal Observation and Simulation with Topography (COAST) program has examined the interaction of both steady-state and transient cool-season synoptic features, such as fronts and cyclones, with the coastal terrain of western North America. Its objectives include better understanding and forecasting of landfalling weather systems and, in particular, the modification and creation of mesoscale structures by coastal orography. In addition, COAST has placed considerable emphasis on the evaluation of mesoscale models in coastal terrain. These goals have been addressed through case studies of storm and frontal landfall along the Pacific Northwest coast using special field observations from a National Oceanic and Atmospheric Administration WP-3D research aircraft and simulations from high-resolution numerical models. The field work was conducted during December 1993 and December 1995. Active weather conditions encompassing a variety of synoptic situations were sampled. This article presents an overview of the program as well as highlights from a sample of completed and ongoing case studies.

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C. J. Donlon, S. J. Keogh, D. J. Baldwin, I. S. Robinson, I. Ridley, T. Sheasby, I. J. Barton, E. F. Bradley, T. J. Nightingale, and W. Emery

Abstract

Satellite sea surface skin temperature (SSST) maps are readily available from precisely calibrated radiometer systems such as the ERS along-track scanning radiometer and, in the near future, from the moderate-resolution imaging spectroradiometer. However, the use of subsurface bulk sea surface temperature (BSST) measurements as the primary source of in situ data required for the development of new sea surface temperature algorithms and the accurate validation of these global datasets is questionable. This is because BSST measurements are not a measure of the sea surface skin temperature, which is actually observed by a satellite infrared radiometer. Consequently, the use of BSST data for validation and derivation of satellite derived “pseudo-BSST” and SSST datasets will limit their accuracy to at least the rms deviation of the BSST–SSST difference, typically about ±0.5 K. Unfortunately, the prohibitive cost and difficulty of deploying infrared radiometers at sea has prevented the regular collection of a comprehensive global satellite SSST validation dataset. In response to this situation, an assessment of the TASCO THI-500L infrared radiometer system as a potential candidate for the widespread validation of satellite SSST observations is presented. This is a low-cost, broadband radiometer that has been commonly deployed in the field to measure SSST by several research groups. A comparison between SSST derived from TASCO THI-500L measurements and contemporaneous scanning infrared sea surface temperature radiometer measurements, which are accurate to better than 0.1 K, demonstrates low bias (0.1 K) and rms (0.08 K) differences between the two instruments. However, to achieve this accuracy, the TASCO THI-500L radiometer must be deployed with care to ensure that the radiometer fore-optics are kept free of salt water contamination and shaded from direct sunlight. When this is done, this type of low-cost radiometer system could form the core of a global SSST validation program.

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Justin A. W. Cox, W. James Steenburgh, David E. Kingsmill, Jason C. Shafer, Brian A. Colle, Olivier Bousquet, Bradley F. Smull, and Huaqing Cai

Abstract

The influence of orographic circulations on the precipitation structure of a Wasatch Mountain winter storm is examined using observations collected during the third intensive observing period (IOP3) of the Intermountain Precipitation Experiment (IPEX). The event featured the passage of a midlevel (700–550 hPa) trough followed 3 h later by a surface trough. Prior to and during the midlevel trough passage, large-scale southwesterly flow impinged on the Wasatch Mountains. Low-level confluence was observed between this southwesterly flow and along-barrier southerly flow within 20–40 km of the Wasatch Mountains. This confluence zone, which moved toward the Wasatch Mountains during and following the passage of the midlevel trough, was accompanied by low-level convergence and precipitation enhancement over the upstream lowlands. Dual-Doppler analysis revealed the presence of a shallow along-barrier jet near the base of the Wasatch Mountains that was surmounted by southwesterly cross-barrier flow at mid- and upper-mountain levels. This cross-barrier flow produced strong (1–2 m s−1) ascent as it interacted with the steep windward slopes of the Wasatch Mountains, where precipitation was roughly double that observed in the lowlands upstream. Flow deflection and splitting were also observed near the highest terrain features. A narrow region of strong subsidence, which at times exceeded 2 m s−1, was found to the lee of the Wasatch and, based on radar imagery, appeared to modulate hydrometeor spillover aloft. Processes contributing to the evolution of the near-barrier flow field, including topographic blocking, diabatic effects, and surface friction contrasts, are discussed.

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