Role Of Surface Friction Research Articles

Role of Surface Friction on Shallow Nonprecipitating Convection

The role of surface friction on shallow nonprecipitating convection is investigated using a series of large-eddy simulations with varying surface friction velocity and with a cloud identification algorithm. As surface friction intensifies, convective rolls dominate over convective cells and secondary overturning circulation becomes stronger in the subcloud layer, thus transporting more moisture upward and more heat downward between the subcloud and cloud layers. Identifying individual clouds, using the identification algorithm based on a three-dimensional topological analysis, reveals that intensified surface friction increases the number of clouds and the degree of tilting in the downstream direction. Highly intensified surface friction increases wind shear across the cloud base and induces cloud tilting, which leads to a vertically parabolic profile of liquid water mixing ratio instead of the classical two-layer structure (conditionally unstable and trade inversion layers). Furthermore, cloud tilting induces more cloud cover and more cloud mass flux much above the cloud base (e.g., 0.8 < z < 1.2 km), but less cloud cover and less cloud mass flux in the upper cloud layer (e.g., z > 1.2 km) because of increased lateral entrainment rate. Similarly, profiles of directly measured entrainment and detrainment rates show that detrainment in the lower cloud layer becomes smaller with stronger surface friction.

Pivot Walking of an Inertially Actuated Robot

In this paper, we introduce an inertially actuated robot that moves in the horizontal plane. The robot is actuated by using two spinning masses. The mathematical model of the system is developed and the equations of motion are derived. The role of surface friction in the robot's motion is also investigated. We presented a theorem that proves the accessibility of affine-control nonlinear systems and applied it to our robot. Thereby, we showed that the system is not controllable without surface friction. In addition, we designed a variable structure controller to regulate specific gait pattern that we define as “pivot walking.” In addition, a trajectory generation algorithms is presented for mapping a given desired path to set of kinematic characteristic. Then, we numerically demonstrate that this approach leads to successful locomotion of the robot. Finally, we built a prototype to experimentally verify the theoretical concepts.

The Role of Surface Friction in Tropical Intraseasonal Oscillation

The role of surface friction in the tropical intraseasonal oscillation, or the Madden–Julian oscillation (MJO), is investigated by comparing two 4-yr integrations with the Goddard atmospheric general circulation model: one with the original model design with one added feature to enhance the intensity of the MJO and the other is identical but with surface friction in the Tropics replaced by its zonal mean value. This comparison indicates that in the second experiment the MJO not only still exists but also exists with similar intensity. The oft-cited frictional wave-CISK (FWC) interpretation for the origin of the MJO, which emphasizes the role of frictionally induced convergence in the surface layer, is reassessed in light of these experiments. The possibility of the MJO in the second experiment being forced by the middle latitudes is excluded by a third experiment. These experiments do not support one of the central ideas in FWC that surface friction plays an instability-enhancing role.

Effects of Coastal Orography on Landfalling Cold Fronts. Part II: Effects of Surface Friction

Abstract The role of surface friction in modifying cold fronts as they make landfall in regions of steep coastal orography is examined by means of idealized simulations. Both the effects of the surface-roughness change at the coast in the absence of orography and the effects of orography are considered. Flow over a large and abrupt change in surface roughness generates an inertia–gravity wave above the boundary layer with characteristics similar to that associated with flow over a plateau. Deceleration of the cross-coast flow occurs over land, as well as for a short distance upstream, and causes retardation of frontal motion. A prescribed northerly postfrontal jet weakens rapidly after landfall. Maximum vertical motions are several centimeters per second; however, only small rainfall enhancement is expected since the updraft is very narrow and produces only small vertical displacements. Friction modifies the flow over the orography by increasing the upstream flow deceleration and reducing the magnitude of...

Examples of the role of surface friction in downslope windstorms

Numerical simulations of four mountain wave events over the Colorado Rockies were carried out with a two-dimensional hydrostatic model including a turbulent mixing parameterization in order to investigate the effect of surface friction. Surface friction was found to play a major role in modulating and even in some cases preventing the wave amplification mechanism from producing severe downslope windstorms.

The Role of Surface Friction in Downslope Windstorms

Abstract Numerical simulations of the 11 January 1972 windstorm in Boulder, Colorado, were carded out using a hydrostatic model with a turbulent kinetic energy parameterization to investigate the role of fictional effects in the development of nonlinear mountain waves. Sensitivity tests to the roughness length specification and to the turbulent mixing and dissipation length formulations show that surface friction delays the onset of the strong surface winds and also prevents the downstream propagation of the zone of maximum windspeed. Shear production within convectively stable regions is the dominant mechanism for the production of the turbulent kinetic energy. Moreover, these results are consistent with the hypothesis that a hydrostatic amplification mechanism is capable of accounting for the development of strong downslope winds.

Surface Roughness Effects on Suction Vortex Formation: A Laboratory Simulation

Abstract A vortex simulator is used to study multiple vortices with particular interest on the role of surface friction in their formation. Measurements with a hot-film anemometer show that the maximum velocity in a multiple-vortex system of four is greater than that for a single vortex created with the same volume flow rate. The friction experiments indicate that over a rough surface more swirl is necessary to initiate the transition to a greater number of vortices than over a smooth surface. Thus, for a tornado on the verge of a transition to multiple vortices, translation across a smoother terrain could act as a catalyst for the process.