Fluxes over Varanasi during intensive observation period of MONTBLEX-90

ABSTRACT. MONTBLEX-90 data for an Intensive Observational Period (IOP) was extracted to investigate the thunderstorm and its impacts on surface layer at Varanasi on 27 July 1990. Sensible heat flux has been computed by profile, aerodynamic and eddy correlation methods. In addition to that. momentum and moisture fluxes have been computed for comparative diagnosis of situations before, at the time and after thunderstorm, Monin-Obukhov similarity theory has been used for quantification of the fluxes. Findings indicate that surface is more buoyant at the time of thunderstorm. Under this influence, maxima of moisture and momentum fluxes occur at the tin1e of thunderstorm. However, heat flux was found to be maximum before the thunderstorm, The results provide an understanding of surface layer turbulent transfer during stable and unstable conditions.

Wave drag by two-dimensional mountain lee waves

lkj & Å¡pkbZ ds lkFk ok;q ds jsf[kdh; :Ik ls c<+us vkSj fLFkjrk dks vifjorZuh; ekurs gq, f}foeh; ioZrh; vojks/k esa fLFkj Lrfjr ok;q&izokg okys jsf[kdh; nzoLFkSfrd fun’kZ dk bl 'kks/k&i= esa mi;ksx fd;k x;k gSA vle&cekZ dh igkfM+;ksa ds vkSj Hkkjr ds if’peh ?kkV ds ioZrh; ok;qjks/k vkSj ioZrh; vfHkokgksa ds fo’ys"k.kkRed vk¡dM+s izkIr fd, x, gSaA vle&cekZ dh igkfM+;ksa ds nksuksa fjtksa ds ioZrh; ok;qjks/k ds lkekU; vk¡dM+s Hkh izkIr fd, x, gaSA A linear hydrostatic model of a stably stratified air-stream flow over a two-dimensional orographic barrier is considered assuming wind increases linearly with height and stability is constant. Analytical expressions for mountain drags and momentum fluxes are obtained for Assam-Burma hills as well as Western Ghats of India. The general expression for mountain drag also obtained for both the ridges of Assam-Burma hills.

BMS flux algebra in celestial holography

Starting from gravity in asymptotically flat spacetime, the BMS momentum fluxes are constructed. These are non-local expressions of the solution space living on the celestial Riemann surface. They transform in the coadjoint representation of the extended BMS group and correspond to Virasoro primaries under the action of bulk superrotations. The relation between the BMS momentum fluxes and celestial CFT operators is then established: the supermomentum flux is related to the supertranslation operator and the super angular momentum flux is linked to the stress-energy tensor of the celestial CFT. The transformation under the action of asymptotic symmetries and the OPEs of the celestial CFT currents are deduced from the BMS flux algebra.

Ocean model response to stochastically perturbed momentum fluxes

In climate model configurations, standard approaches to the representation of unresolved, or subgrid scales, via deterministic closure schemes are being challenged by stochastic approaches inspired by statistical dynamical theory. Despite gaining popularity, studies of various stochastic subgrid scale parameterizations applied to atmospheric climate and weather prediction systems have revealed a diversity of model responses, including degeneracy in the response to different forcings and compensating model errors, with little reduction in artificial damping of the small scales required for numerical stability. Due to the greater range of spatio-temporal scales involved, how to best sample subgrid fluctuations in a computationally inexpensive manner, with the aim of reduced model error and improvements to the simulated climatological state of the ocean, remains an open question. While previous studies have considered perturbations to the surface forcing or subsurface temperature tendencies, we implement an energetically consistent, simple, stochastic subgrid eddy parameterization of the momentum fluxes in regions of the three-dimensional ocean typically associated with high eddy variability. We consider the changes in the modelled energetics of low-resolution simulations in response to stochastically forced velocity tendencies whose perturbation statistics and amplitudes are calculated from an eddy resolving ocean configuration. Kinetic energy spectra from a triple-decomposition reveal a systematic redistribution from the seasonal (climatological minus mean) potential energy to preferentially generate small scale transient kinetic energy while the total energy spectra remains largely unchanged. We show that stochastic parameterization generally improves model biases, noticeably so for the simulated energetics of the Southern Oceans.

The Impacts of Gustiness on Air–Sea Momentum Flux

The exchange of momentum across the air–sea boundary is an integral component of the earth system and its parametrization is essential for climate and weather models. This study focuses on the impact of gustiness on the momentum flux using three months of direct flux observations from a moored surface buoy. Gustiness, which quantifies the fluctuations of wind speed and direction, is shown to impact air–sea momentum fluxes. First, we put forward a new gustiness formula that simultaneously evaluates the impact of fluctuations in wind direction and speed. A critical threshold is established using a cumulative density function to classify runs as either gusty or non-gusty. We find that, during runs classified as gusty, the aerodynamic drag coefficient is increased up to 57% when compared to their non-gusty counterparts. This is caused by a correlated increase in vertical fluctuations during gusty conditions and explains variability in the drag coefficient for wind speeds up to 20 m/s. This increase in energy is connected with horizontal fluctuations through turbulent interactions between peaks in the turbulent spectra coincident with peaks in the wave spectra. We discus two potential mechanistic explanations. The results of this study will help improve the representation of gustiness in momentum flux parameterizations leading to more accurate ocean models.

Nonlinear generation of long waves and the reversal of eddy momentum fluxes in a two-layer quasi-geostrophic model

Although classical theories of midlatitude momentum fluxes focus on the wave-mean flow interaction, wave-wave interactions may be important for generating long waves. It is shown in this study that this nonlinear generation has implications for eddy momentum fluxes in some regimes. Using a two-layer quasi-geostrophic model of a baroclinic jet on a β-plane, statistically steady states are explored in which the vertically integrated eddy momentum flux is divergent at the center of the jet, rather than convergent as in Earth-like climates. One moves towards this less familiar climate from more Earth-like settings by reducing either β, frictional drag, or the width of the baroclinic zone, or by increasing the upper bound of resolvable wavelengths by lengthening the zonal channel. Even in Earth-like settings, long waves diverge momentum from the jet, but they are too weak to compete with short unstable waves that converge momentum. We argue that long waves are generated by breaking of short unstable waves near their critical latitudes, where long waves converge momentum while diverging momentum at the center of the jet. Quasi-linear models with no wave-wave interaction can qualitatively capture the Earth-like regime but not the regime with momentum flux divergence at the center of the jet, because the nonlinear wave breaking and long wave generation processes are missing. Therefore, a more comprehensive theory of atmospheric eddy momentum fluxes should take into account the nonlinear dynamics of long waves.

Observations of Boundary Layer Wind and Turbulence of a Landfalling Tropical Cyclone

This study analyzed the atmospheric boundary layer characteristics based on the multiple level observations by a 350-m height tower during the landfall of Super Typhoon Mangkhut (1822). Mean wind profiles showed logarithmic wind profiles at different wind speed ranges suggesting nearly constant flux layers. The height of the constant layer increased with the wind speed and deceased with the radial distance from the storm centre. This behaviour was supported by flux observations. Momentum fluxes and turbulent kinetic energy increased with the wind speed at all flux measurement levels. The drag coefficient (surface roughness) estimated was nearly a constant with a value of 8´10-3 (0.09 m). Both the estimated eddy diffusivity and mixing length varied with height. The eddy diffusivity also varied with the wind speed. Our results supported that the eddy diffusivity is larger over land than over ocean in a same wind speed range.