Wind shear

About: Wind shear is a research topic. Over the lifetime, 8023 publications have been published within this topic receiving 185373 citations.

Investigation of transport processes across the sea surface microlayer by infrared imagery

Abstract: [1] Heat is used as a proxy tracer for gases to study the transport processes across the sea surface microlayer. Infrared imaging techniques permit fast measurements of heat transfer velocities and give an insight into the transport mechanisms across the thermal sublayer. The observed fluctuations of the sea surface temperature suggest that surface renewal is the major turbulent transport mechanism at medium and high wind speeds. The scale space analysis of the temperature patterns at the sea surface with respect to their contribution to the skin-bulk temperature difference shows the turbulent nature of the transport process. Large-scale turbulence dominates the transport at low friction velocities, whereas small-scale turbulence is more dominant at higher wind friction. The skin-bulk temperature difference is estimated by fitting the measured sea surface temperature distribution with a PDF function based on a surface renewal model. Periodic heat flux switching in the wind-wave flume delivers independent estimates of surface and bulk temperature and verifies the statistical approach, whereas at very low wind speeds and film-covered surfaces the statistical method underestimates the skin-bulk temperature difference across the thermal sublayer. The large scatter of the transfer velocities when plotted versus wind speed indicates that not only the wind shear but also other processes such as the wave field and surfactants influences near-surface turbulence and thus air-water gas transfer.

Observations of Low-Level Baroclinity Generated by Anvil Shadows

Abstract: Low-level cooling beneath the cirrus anvil canopies of supercell thunderstorms is documented in two Verification of the Origins of Rotation in Tornadoes Experiment cases and in the 17 May 1981 Arcadia, Oklahoma, supercell. Surface temperature decreases of 3°C or more occurred beneath the anvils within 45 min of the onset of overcast conditions. Cooling was confined to the lowest few hundred meters of the boundary layer, and believed to be due mainly to a deficit in the energy budget following a reduction of incoming shortwave radiation. In the three cases studied, the vertical wind shear was strong; thus, mixing prevented the formation of an inversion layer. Strong insolation at the ground outside of the anvil shadows coupled with the cooling beneath the cirrus canopies led to corridors of baroclinity along the shadow edges. It is shown that residence times in these baroclinic zones may be long enough for parcels to acquire considerable horizontal vorticity (e.g., ∼10−2 s−1) en route to a storm u...

A wind comparison study using an ocean general circulation model for the 1997–1998 El Niño

Abstract: Predictions of the 1997–1998 El Nino exhibited a wide range of forecast skill that were dependent, in part, on the wind-driven initial conditions for the ocean. In this study the results of a reduced gravity, primitive equation, sigma coordinate ocean general circulation model are compared and contrasted when forced by several different wind products for the 1997–1998 El Nino/La Nina. The different wind products include atmospheric model winds, satellite wind products, and a subjective analysis of ship and in situ winds. The model results are verified against fields of observed sea level anomalies from TOPEX/Poseidon data, sea surface temperature analyses, and subsurface temperature from the Tropical Atmosphere-Ocean buoy array. Depending on which validation data type one chooses, different wind products provide the best forcing fields for simulating the observed signal. In general, the model results forced by satellite winds provide the best simulations of the spatial and temporal signal of the observed sea level. This is due to the accuracy of the meridional gradient of the zonal wind stress component that these products provide. Differences in wind forcing also affect subsurface dynamics and thermodynamics. For example, the wind products with the weakest magnitude best reproduce the sea surface temperature (SST) signal in the eastern Pacific. For these products the mixed layer is shallower, and the thermocline is closer to the surface. For such simulations the subsurface thermocline variability influences the variation in SST more than in reality. The products with the greatest wind magnitude have a strong cold bias of >1.5°C in the eastern Pacific because of increased mixing. The satellite winds along with the analysis winds correctly reproduce the depth of the thermocline and the general subsurface temperature structure.

The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part A: Case Selection and Set-Up

Abstract: We describe a novel methodology on the selection and composition of a single-case observational dataset from the comprehensive measurement program at the Cabauw observatory field site located in the Netherlands. The case can be regarded as the basis of the third case study conducted within the framework of the GEWEX (Global Energy and Water Exchange) Atmospheric Boundary-Layer Study (GABLS) and is meant to be used for the evaluation of single-column models. The ideal case is supposed to cover a period of at least 24 h with clear skies, moderate near-surface winds and a stable stratification during nighttime. From the multi-year data archive with Cabauw observations data for 1–2 July 2006 were found to best match the requirements, and were consequently selected for analysis. The dates contains a 24-h period with a nearly constant geostrophic wind of $$\approx $$ 7 m $$\hbox {s}^{-1}$$ , and a considerable wind shear in the vertical. It is also characterized by a pronounced low-level jet related to an inertial oscillation that developed around sunset when the atmosphere had decoupled from the surface. Detailed initial conditions, surface conditions and dynamical forcings are derived on the basis of local observations and the outcome of a conceptual and a three-dimensional atmospheric model. It appears that a very precise prescription of the forcings is a prerequisite to enable a meaningful evaluation of models against observational data.

Convective momentum transport and perturbation pressure field from a cloud-resolving model simulation

Abstract: This study uses a 2D cloud-resolving model to investigate the vertical transport of horizontal momentum and to understand the role of a convection-generated perturbation pressure field in the momentum transport by convective systems during part of the Tropical Ocean and Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) Intensive Observation Period. It shows that convective updrafts transport a significant amount of momentum vertically. This transport is downgradient in the easterly wind regime, but upgradient during a westerly wind burst. The differences in convective momentum transport between easterly and westerly wind regimes are examined. The perturbation pressure gradient accounts for an important part of the apparent momentum source. In general it is opposite in sign to the product of cloud mass flux and the vertical wind shear, with smaller magnitude. Examination of the dynamic forcing to the pressure field demonstrates that the linear forcing representing the interaction between the convective updrafts and the large-scale wind shear is the dominant term, while the nonlinear forcing is of secondary importance. Thus, parameterization schemes taking into account the linear interaction between the convective updrafts and the large-scale wind shear can capture the essential features of the perturbation pressure field. The parameterization scheme for momentum transport by Zhang and Cho is evaluated using the model simulation data. The parameterized pressure gradient force using the scheme is in excellent agreement with the simulated one. The parameterized apparent momentum source is also in good agreement with the model simulation. Other parameterization methods for the pressure gradient are also discussed.