Wind shear

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

Toward parameterization of the stable boundary layer

Abstract: Wangara data is used to examine the depth of the nocturnal boundary layer (NBL) and the height to which surface-linked turbulence extends. It is noted that a linearity of virtual temperature profiles has been found to extend up to a significant portion of the NBL, and then diverge where the wind shear rides over the surface-induced turbulence. A series of Richardson numbers are examined for varying degrees of turbulence and the significant cooling region is observed to have greater depth than the depth of the linear relationship layer. A three-layer parameterization of the thermodynamic structure of the NBL is developed so that a system of five equations must be solved when the wind velocity profile and the temperature at the surface are known. A correlation between the bulk Richardson number and the depth of the linear layer was found to be 0.89.

A new wind-farm parameterization for large-scale atmospheric models

Abstract: In this article, a new model is developed to parameterize the effect of wind farms in large-scale atmospheric models such as weather models. In the new model, wind turbines in a wind farm are parameterized as elevated sinks of momentum and sources of turbulence. An analytical approach is used to estimate the turbine-induced forces as well as the turbulent kinetic energy (TKE) generated by the turbines inside the atmospheric boundary layer (ABL). In addition, the proposed model can take into account not only the effect of wind-farm density but also the effect of wind-farm layout and wind direction. The performance of the new model is tested with large-eddy simulations of ABL flows over very large wind farms with different turbine configurations. The results show that the new model is capable to accurately predict the turbine-induced forces as well as the TKE generated by the turbines inside the ABL.

Modulation of equatorial subseasonal convective episodes by tropical‐extratropical interaction in the Indian and Pacific Ocean regions

Abstract: Composite relationships among outgoing longwave radiation, sea level pressure, surface winds, and upper tropospheric circulation are examined for northern winter during subseasonal episodes of eastward progression of convection from the Indian Ocean to the western Pacific. This evolution often culminates with westerly wind burst events and strong air-sea interaction associated with regional-scale convective blowups in the western equatorial Pacific. We first document some of these interactions in the composites for two timescales, the submonthly (6–30 days) and that of the Madden-Julian Oscillation (MJO) timescale (30–70 days). We then analyze the December 1992 period during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) to illustrate how these composite relationships are manifested in a case study. Convection in the Indian Ocean for the composites is shown to be associated with a northern hemisphere wave train at 200 mbar, arcing through the midlatitudes, that can contribute to convective blowups farther east on the submonthly (6–30 days) timescale in the Intertropical Convergence Zone (ITCZ) in the eastern Pacific. The eastern Asian trough that is part of this wave train is associated with pressure surges from the northern hemisphere and subsequent convection over Southeast Asia. As the MJO convective envelope moves east to Australasia, midlatitude wave trains in either hemisphere include upper level troughs east of Asia and Australia and pressure surges from either hemisphere that contribute to pressure rises over the Indonesian region and a subsequent shift of the convective envelope to the western Pacific. The vertical wind structure for the December 1992 case study is consistent with the composite surface and upper level winds and also shows strong vertical wind shear in the boundary layer, a sharply defined westerly maximum near 700 mbar and an intensification of the upper level easterlies near 100 mbar. Very deep westerlies (to 200 mbar) are confined to shorter timescales. The case study illustrates the various time and space scale interactions noted in the composites. Reciprocal interactions between the tropics and the midlatitudes on the submonthly and MJO timescales in both the composites and the case study involve pressure surges and wave interaction that influence subsequent convection as the convective envelope migrates eastward from the tropical Indian to Pacific Ocean region.

In a changing climate weakening tropical easterly jet induces more violent tropical storms over the north Indian Ocean

Abstract: [1] Tropical Easterly Jet (TEJ) of summer monsoon over the north Indian ocean is weakening in recent years. The absolute easterly shear shows a strong negative correlation (significant at 99.9% level by students' two sided t-test) with the number of severe storms suggesting that a decrease in easterly shear is favorable for the formation of more severe tropical storms. For the first time in recorded history a category 5 Hurricane formed in June 2007 together with two more severe tropical storms over the north Indian ocean. Thus if the present decreasing trend of TEJ intensity continues there is a strong likelihood of the formation of tropical cyclones of hurricane intensity even during the summer monsoon. Presently these intense systems are known to form only in the pre and post monsoon seasons, when the vertical wind shear is small.

Distribution of Helicity, CAPE, and Shear in Tropical Cyclones

Abstract: The previous study of helicity, CAPE, and shear in Hurricane Bonnie (1998) was extended to all eight tropical cyclones sampled by NASA during the Convection and Moisture Experiments (CAMEX). Storms were categorized as having large or small ambient vertical wind shear, with 10 m s−1 as the dividing line. In strongly sheared storms, the downshear mean helicity exceeded the upshear mean by a factor of 4. As in the previous study, the helicity differences resulted directly from the tropical cyclone response to ambient shear, with enhanced in-up-out flow and veering of the wind with height present downshear. CAPE in strongly sheared storms was 60% larger downshear. Mean inflow near the surface and the depth of the inflow layer each were 4 times larger downshear. At more than 30% of observation points outside the 100-km radius in the downshear right quadrant, midlatitude empirical parameters indicated a strong likelihood of supercells. No such points existed upshear in highly sheared storms. Much small...