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P. A. Taylor

Abstract

A mixing length model is used to relate the turbulent shear stress to the mean velocity field within the planetary boundary layer above a change in surface roughness under conditions of neutral thermal stability. This model gives rise to a parabolic system of partial differential equations. Numerical solutions are given for the case of flow above a step change in surface roughness across a line perpendicular to the geostrophic wind direction. These results show that a very long fetch is required for a true equilibrium flow to exist above the new, downwind surface. In particular, the surface wind direction adjusts only slowly to the new conditions. This suggests that experimental observations of the angle between the surface and geostrophic wind directions in supposedly nondeveloping flows may well have been affected by surface roughness changes well upstream of the experimental site. Some comparisons are made with numerical results for internal boundary layers within the shallower surface layer of the atmosphere.

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P. A. Taylor

Abstract

A wind spiral model, similar to that proposed by Blackadar, is used to represent the flow above a surface of uniform roughness in the planetary boundary layer (extending up to ˜1 km). An attempt is made to determine the applicability of the mixing length model used and to evaluate an empirical parameter used in the model. This attempt, using existing experimental observations of surface shear stress and wind direction, is inconclusive and leads us to suspect that surface inhomogeneity has played a role in some of the experimental data.

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J. P. Taylor and A. McHaffie

Abstract

The sensitivity of warm stratocumulus cloud albedo to changes in droplet concentration, termed “cloud susceptibility,” is calculated using data from the UKMO Meteorological Research Flight. Stratocumulus clouds in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and around the British Isles are studied. The range of susceptibility measured is large and maritime clouds are shown to have the largest susceptibility. Numerical simulations of the changes in cloud radiative and microphysical properties with increasing droplet concentration are carried out. These highlight the high sensitivity of maritime clouds to changes in droplet concentration and the rapid reduction in sensitivity as the cloud droplet concentration increases.

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Jonathan A. Smith and Jonathan P. Taylor

Abstract

Measured cloud spectral signatures in high-resolution infrared interferometer data have been separated from the clear-air signatures using singular value decomposition. Sets of empirical orthogonal functions (EOFs) have then been created from these signatures to investigate the possibility of cloudy view discrimination without the use of any background data. The measured data have been taken by the Airborne Research Interferometer Evaluation System (ARIES), which is specifically designed to gather data from an aircraft that are representative of the forthcoming Infrared Atmospheric Sounding Interferometer (IASI). EOF sets were based on 78 diverse modeled clear-air spectra, supplemented by selected measured spectra. Video data gave independent verification of cloudy and cloud-free views. The development of a cloud-detection scheme is detailed, and several possible cloud-detection procedures were tested. The most promising procedure is presented. Comparative tests are made with cloud-detection algorithms developed for earlier satellite instruments. The results are encouraging; clouds were detected in the measured test data with similar success to other schemes but without requiring prior information or even the uncompressing of transmitted data. With the prospect of IASI (and similar) data being compressed for transmission using EOFs, the procedure here could be implemented in NWP centers as an initial very inexpensive but accurate method to create a cloud-filtering mask.

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S. D. Bachman, J. R. Taylor, K. A. Adams, and P. J. Hosegood

Abstract

Submesoscale dynamics play a key role in setting the stratification of the ocean surface mixed layer and mediating air–sea exchange, making them especially relevant to anthropogenic carbon uptake and primary productivity in the Southern Ocean. In this paper, a series of offline-nested numerical simulations is used to study submesoscale flow in the Drake Passage and Scotia Sea regions of the Southern Ocean. These simulations are initialized from an ocean state estimate for late April 2015, with the intent to simulate features observed during the Surface Mixed Layer at Submesoscales (SMILES) research cruise, which occurred at that time and location. The nested models are downscaled from the original state estimate resolution of 1/12° and grid spacing of about 8 km, culminating in a submesoscale-resolving model with a resolution of 1/192° and grid spacing of about 500 m. The submesoscale eddy field is found to be highly spatially variable, with pronounced hot spots of submesoscale activity. These areas of high submesoscale activity correspond to a significant difference in the 30-day average mixed layer depth between the 1/12° and 1/192° simulations. Regions of large vertical velocities in the mixed layer correspond with high mesoscale strain rather than large . It is found that is well correlated with the mesoscale density gradient but weakly correlated with both the mesoscale kinetic energy and strain. This has implications for the development of submesoscale eddy parameterizations that are sensitive to the character of the large-scale flow.

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H. Bonekamp, G. J. Komen, A. Sterl, P. A. E. M. Janssen, P. K. Taylor, and M. J. Yelland

Abstract

The surface-drag coefficients of two versions of the ECMWF's atmosphere–wave model are compared with those of uncoupled model versions and with those of inertial-dissipation measurements in the open ocean made by the RRS Discovery. It is found that the mean drag resulting from the latest coupled version is on average equal to that of the uncoupled version. However, both have a positive bias when compared with the RRS Discovery observations. This bias is discussed, also in the light of other observational open ocean data. In the second part of the paper, bulk parameterizations with and without parameters of collocated sea-state data are validated against the Discovery observations. Using published estimates of the error in friction velocity and the neutral 10-m winds, all bulk parameterizations score low on goodness-of-fit tests. The lowest scores are obtained for the constant Charnock parameter case, whereas the highest scores are obtained for a wave-age-dependent parameterization. On–off experiments are made for the corrections to the inertial-dissipation data that have been proposed in previous studies. These corrections concern the measurement height and the direct wave-induced turbulence in the lower atmosphere. The first correction results in a slightly better agreement, but the second reduces the goodness-of-fit of the bulk parameterizations.

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R. L. H. Essery, M. J. Best, R. A. Betts, P. M. Cox, and C. M. Taylor

Abstract

A land surface scheme that may be run with or without a tiled representation of subgrid heterogeneity and includes an implicit atmospheric coupling scheme is described. Simulated average surface air temperatures and diurnal temperature ranges in a GCM using this surface model are compared with climatology. Surface tiling is not found to give a clear improvement in the simulated climate but offers more flexibility in the representation of heterogeneous land surface processes. Using the same meteorological forcing in offline simulations using versions of the surface model with and without tiling, the tiled model gives slightly lower winter temperatures at high latitudes and higher summer temperatures at midlatitudes. When the surface model is coupled to a GCM, reduced evaporation in the tiled version leads to changes in cloud cover and radiation at the surface that enhance these differences.

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A. K. Vance, J. P. Taylor, T. J. Hewison, and J. Elms

Abstract

Results are presented from the Measurement of Tropospheric Humidity (MOTH) Tropic and MOTH Arctic airborne field experiments, comparing a number of in situ humidity measurements. Good agreement is shown between the Total Water Content probe on board the C-130 aircraft, and the Vaisala RS90 and “new” Vaisala RS80 radiosondes; “old” Vaisala RS80 radiosondes and Vaisala RD93 dropsondes show the dry bias noted by others. An empirical correction for RD93 dry bias is presented and is shown to produce good results with both MOTH and non-MOTH data. It was concluded that the aircraft and corrected dropsonde data agree (1σ) to ±1 g kg−1; these limits are due to atmospheric variability. The possibility of temperature measurement errors producing errors in RD93 relative humidities is not significant compared to atmospheric variability. Meteolabor Snow White radiosondes are shown to exhibit a wet bias at high and low mixing ratios and possible reasons are discussed. Intercomparisons between the RS90s and other instruments, partitioned by day–night and by experiment, suggest deficiencies in RS90 daytime radiation corrections.

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S. Platnick, P. A. Durkee, K. Nielsen, J. P. Taylor, S.-C. Tsay, M. D. King, R. J. Ferek, P. V. Hobbs, and J. W. Rottman

Abstract

The authors investigate the extent to which the contrast brightness of ship tracks, that is, the relative change in observed solar reflectance, in visible and near-infrared imagery can be explained by the microphysics of the background cloud in which they form. The sensitivity of visible and near-infrared wavelengths for detecting reflectance changes in ship tracks is discussed, including the use of a modified cloud susceptibility parameter, termed the “contrast susceptibility,” for assessing the sensitivity of background cloud microphysics on potential track development. It is shown that the relative change in cloud reflectance for ship tracks is expected to be larger in the near-infrared than in the visible and that 3.7-μm channels, widely known to be useful for detecting tracks, have the greatest sensitivity. The usefulness of contrast susceptibility as a predictor of ship track contrast is tested with airborne and satellite remote sensing retrievals of background cloud parameters and track contrast. Retrievals are made with the high spatial resolution Moderate Resolution Imaging Spectroradiometer Airborne Simulator flown on the National Aeronautics and Space Administration’s high-altitude ER-2 aircraft, and with the larger-scale perspective of the advanced very high resolution radiometer. Observed modifications in cloud droplet effective radius, optical thickness, liquid water path, contrast susceptibility, and reflectance contrast are presented for several ship tracks formed in background clouds with both small and large droplet sizes. The remote sensing results are augmented with in situ measurements of cloud microphysics that provide data at the smaller spatial scales.

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K. J. Evans, P. H. Lauritzen, S. K. Mishra, R. B. Neale, M. A. Taylor, and J. J. Tribbia

Abstract

The authors evaluate the climate produced by the Community Climate System Model, version 4, running with the new spectral element atmospheric dynamical core option. The spectral element method is configured to use a cubed-sphere grid, providing quasi-uniform resolution over the sphere and increased parallel scalability and removing the need for polar filters. It uses a fourth-order accurate spatial discretization that locally conserves mass and total energy. Using the Atmosphere Model Intercomparison Project protocol, the results from the spectral element dynamical core are compared with those produced by the default finite-volume dynamical core and with observations. Even though the two dynamical cores are quite different, their simulated climates are remarkably similar. When compared with observations, both models have strengths and weaknesses but have nearly identical root-mean-square errors and the largest biases show little sensitivity to the dynamical core. The spectral element core does an excellent job reproducing the atmospheric kinetic energy spectra, including fully capturing the observed Nastrom–Gage transition when running at 0.125° resolution.

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