A Study of the Environmental Influence on the Amplitude of Lee Waves

A three-layer theoretical model is used to calculate the lee wave of a real example occurring over Blue Ridge in Pittsburgh, in which the maximum vertical velocity is 0.11 m s^-1. Based on this, the influence of changes in the thickness and values of the Scorer parameter in each layer are analyzed. It is shown that the influence of each layer parameters on the lee-wave amplitude is different, and the amplitude is more sensitive to the changes in the lower layer. Since the environment changes can affect the Scorer parameter profile, the influence of the environment on the amplitude is studied. The results show that the amplitude will decrease in the daytime because of solar heating, and increase at night because of radiational cooling, according to observational data. The case is also simulated by the Advanced Regional Prediction System （ARPS） model. The simulated amplitude is 0.089 m s^-1, which is close to the calculated result. Numerical sensitivity experiments are performed to test the former calculated experiments. The simulated results are consistent with the analytically calculated results.

The energy conversion rates from eddies and mean fl ow into internal lee waves in the global ocean

Internal lee waves play an important role in transferring energy from eddies to small scale mixing.However,the energy conversion from eddies into lee waves in the global ocean remains poorly understood.Conversion rates from eddies and from mean fl ow in the global ocean were diff erentiated using single beam sounding data,stratifi cation from climatology,eddy velocity,and mean fl ow from a global ocean model.The global integral energy conversion from eddies is estimated to be 0.083 TW and is almost twice as that from the mean fl ow.A new method was developed to study the uncertainties of energy conversion caused by dealing with the topographic data.Results show that diff erent data processing procedures,and the resolution and accuracy of topographic data have a signifi cant impact on the estimated global energy conversion.

INFLUENCE OF THE SCORER PARAMETER PROFILE ON THE WAVELENGTH OF TRAPPED LEE WAVES

A three-layer theoretical model was established, in which the atmosphere is divided into three layers based on the Scorer parameter 12 , which is large in the middle layer and small in the other two layers. The wave number formula of lee waves was deduced with this theoretical model, and a typical example for the lee wave was calculated. Thus, the influence of changes in the thickness of every layer and values of the Scorer parameter in every layer was examined. The results show that the wavelength decreases with an increase in the thickness of the lower and the middle layers and is more sensitive to the changes in the middle layer. Therefore, if the changes in these two layers are different, the changes in the middle layer will dominate the changes in the wavelength. The results also show that the wavelength decreases with the increase in the value of 12 in every layer, among which the sensitivity to the 12 in the upper layer is the most striking. The calculation results reasonably can explain the influence of diurnal changes on the wavelength. The example was simulated using Advanced Regional Prediction System model, and the sensitivity experiments were performed to confirm the effects of the Scorer parameter profiles on the wavelengths. The simulated results are consistent with the calculated results.

Use of Targeted Orographic Smoothing in Very High Resolution Simulations of a Downslope Windstorm and Rotor in a Sub-tropical Highland Location

Nested simulations of a downslope windstorm over Cangshan mountain,Yunnan,China,have been used to demonstrate a method of topographic smoothing that preserves a relatively large amount of terrain detail compared to typical smoothing procedures required for models with terrain-following grids to run stably.The simulations were carried out using the Met Office Unified Model(MetUM)to investigate downslope winds.The smoothing method seamlessly blends two terrain datasets to which uniform smoothing has been applied—one with a minimum of smoothing,the other smoothed more heavily to remove gradients that would cause model instabilities.The latter dataset dominates the blend where the steepest slopes exist,but this is localised and recedes outside these areas.As a result,increased detail is starkly apparent in depictions of flow simulated using the blend,compared to one using the default approach.This includes qualitative flow details that were absent in the latter,such as narrow shooting flows emerging from roughly 1-2 km wide leeside channels.Flow separation is more common due to steeper lee slopes.The use of targeted smoothing also results in increased lee side temporal variability at a given point during the windstorm,including over flat areas.Low-/high-pass filtering of the wind perturbation field reveals that relative spatial variability above 30 km in scale(reflecting the background flow)is similar whether or not targeting is used.Beneath this scale,when smoothing is targeted,relative flow variability decreases at the larger scales,and increases at lower scales.This seems linked to fast smaller scale flows disturbing more coherent flows(notably an along-valley current over Erhai Lake).Spatial variability of winds in the model is unsurprisingly weaker at key times than is observed across a local network sampling mesoscale variation,but results are compromised due to relatively few observation locations sampling the windstorm.Only when targeted smoothing is applied does the model capture the downslope windstorm's extension over the city of Dali at the mountain's foot,and the peak mean absolute wind.

Analysis of a simplification strategy in a nonhydrostatic model for surface and internal wave problems

This paper examines the simplification strategy of retaining only the nonhydrostatic effect of local acceleration in a three-dimensional fully nonhydrostatic model regarding the submesoscale wave phenomenon in the ocean.Elaborate scale analysis of the vertical component of the Reynold-averaged Navier-Stokes(RANS)equation was performed,confirming the rationalization of this simplification.Then,the simplification was implemented in a RANS equation-based nonhydrostatic model NHWAVE(nonhydrostatic WAVE)to make a simplified nonhydrostatic model.Numerical examples were taken to test its performance,including surface sinusoidal waves propagating on an idealized East China Sea topography,tidally induced internal lee waves and small-scale solitary waves.The results show that in a considerably wide range of nonlinear strengths,the simplified nonhydrostatic model can obtain similar results as those in the fully nonhydrostatic model,even for smaller-scale solitary waves.Nonlinearity influences the applicability of the simplification.The stronger the nonlinearity is,the worse the simplified model describes the nonhydrostatic phenomenon.In general,the simplified nonhydrostatic model can simulate surface waves better than internal waves.Improvement of computational efficiency in the simplified nonhydrostatic model is reasonable,reducing the central processing unit time duration in the fully nonhydrostatic model by 16.4%–20.6%.The specially designed algorithm based on the simplified nonhydrostatic equation can remarkably reduce the computational time.

Simulation model of SAR remote sensing of turbulent wake of semi-elliptical submerged body

In terms of the 2-dimensional hydrodynamic simplified model of a semi-elliptical submerged body moving horizontally at high speed, by using the full-spectrum model of SAR（synthetic aperture radar） remote sensing and taking the effect of oceanic interior turbulence on surface gravity capillary waves into account, applying the k-ε model of turbulence with internal wave mixing, and adopthag the Nasmyth spectrum of oceanic turbulence, the 2-dimensional simulation model of SAR remote sensing of this semi-elliptical submerged body is built up. Simulation by using this model at X band and C band is made in the northeastern South China Sea （21°00＇ N, 119°00＇ E）. Satisfactory results of the delay time and delay distance of turbulent surface wake of this semi-elliptical submerged body, as well as the minimum submerged depth at which this submerged body which cannot be discovered by SAR, are obtained through simulation.