Generation mechanisms of convectively induced internal gravity waves in a three-dimensional framework

The generation mechanisms of convective gravity waves in the stratosphere are investigated in a three-dimensional framework by conducting numerical simulations of four ideal storms under different environmental conditions: one un-sheared and three constant low-level sheared basic-state winds with the depth of the shear layer of 6 km and the surface wind speeds (U_s) of 8, 18, and 28 m s^?1, using the Advanced Regional Prediction System (ARPS) model. The storms simulated under the un-sheared (U_s = 0 m s^?1), weakly sheared (U_s = 8 and 18ms^?1), and strongly sheared (U_s = 28ms^?1) basicstate winds are classified into single-cell, multicell, and supercell storms, respectively. For each storm, the wave perturbations in a control simulation, including nonlinearity and microphysical processes, are compared with those in quasi-linear dry simulations forced by diabatic forcing and nonlinear forcing that are obtained from the control simulation. The gravity waves generated by the two forcing terms in the quasi-linear dry simulations are out of phase with each other for all of the storms. The gravity waves in the control simulation are represented by a linear sum of the wave perturbations generated by the nonlinear forcing and diabatic forcing. This result is consistent with the results of previous studies in a two-dimensional framework. This implies that both forcing mechanisms are important for generating the convective gravity waves in the three-dimensional framework as well. The characteristics of the three-dimensional gravity waves in the stratosphere were determined by the spectral combination of the forcing terms and the wave-filtering and resonance factor that is determined from the basic-state wind and stability as well as the vertical structure of the forcing.     

Model multi-cloud parameterizations for convectively coupled waves: Detailed nonlinear wave evolution

Recent observational analysis reveals the central role of three cloud types, congestus, stratiform, and deep-convective cumulus clouds, in the dynamics of large scale convectively coupled Kelvin waves, westward propagating 2-day waves, and the Madden–Julian oscillation. Recently, a systematic model convective parametrization highlighting the dynamic role of the three cloud types has been developed by the authors involving two baroclinic modes of vertical structure: a deep-convective heating mode and a second mode with low level heating and cooling corresponding, respectively, to congestus and stratiform clouds. The model includes a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep-convective precipitation and also a nonlinear switch which favors either deep or congestus convection depending on the relative dryness of the middle troposphere. The detailed nonlinear evolution of large scale convectively coupled waves in the model parametrization is studied here in a chaotic intermittent regime of the nonlinear dynamics associated with weaker mean radiative cooling where such waves are isolated in space and time. This regime is utilized to elucidate in a clean fashion several novel features of the model parametrization. In particular, four stages of nonlinear wave evolution occur: in the preconditioning and birth stages, the role of congestus moistening and second baroclinic convergence are crucial while in the dying stage of the large scale convectively coupled wave, the role of the nonlinear switch, and the drying of the troposphere are essential. In the mature phase, the large scale features of the convectively coupled waves resemble those in observations of convectively coupled Kelvin waves including the propagation speed, wave tilt, temperature, heating, and velocity structure.     

Model multi-cloud parameterizations for convectively coupled waves: Detailed nonlinear wave evolution

Recent observational analysis reveals the central role of three cloud types, congestus, stratiform, and deep-convective cumulus clouds, in the dynamics of large scale convectively coupled Kelvin waves, westward propagating 2-day waves, and the Madden–Julian oscillation. Recently, a systematic model convective parametrization highlighting the dynamic role of the three cloud types has been developed by the authors involving two baroclinic modes of vertical structure: a deep-convective heating mode and a second mode with low level heating and cooling corresponding, respectively, to congestus and stratiform clouds. The model includes a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep-convective precipitation and also a nonlinear switch which favors either deep or congestus convection depending on the relative dryness of the middle troposphere. The detailed nonlinear evolution of large scale convectively coupled waves in the model parametrization is studied here in a chaotic intermittent regime of the nonlinear dynamics associated with weaker mean radiative cooling where such waves are isolated in space and time. This regime is utilized to elucidate in a clean fashion several novel features of the model parametrization. In particular, four stages of nonlinear wave evolution occur: in the preconditioning and birth stages, the role of congestus moistening and second baroclinic convergence are crucial while in the dying stage of the large scale convectively coupled wave, the role of the nonlinear switch, and the drying of the troposphere are essential. In the mature phase, the large scale features of the convectively coupled waves resemble those in observations of convectively coupled Kelvin waves including the propagation speed, wave tilt, temperature, heating, and velocity structure.     

The activity of convectively coupled equatorial waves in CMIP3 global climate models

This study evaluates the convectively coupled equatorial waves in ten coupled general circulation models (GCMs) in the twentieth century experiment from the Coupled Model Intercomparison Project phase 3 of the World Climate Research Programme. The antisymmetric bands in all GCMs are weaker than in observations, and the mixed Rossby-gravity (MRG) wave seems to be a mixture of the equatorial Rossby (ER) and tropical depression-type (TD-type) waves rather than a mixture of the ER and inertiogravity waves found in observations. The simulated TD-type wave is more organized than in observations with a quasilinear wavenumber–frequency relationship. In most GCMs, the two observed activity centers of the MRG and TD-type waves over the southern Indian Ocean and the southwestern Pacific cannot be separated; only one wave activity center is found over the Maritime Continent. The observed northwestward propagation of the TD-type wave over the western North Pacific is also not well simulated in the GCMs. The simulated active season of the MRG and TD-type waves over the northern hemisphere during the boreal summer and fall is much shorter than in observations. The models from CCSR utilizing the Pan and Randall scheme with the convection suppression simulate the realistic Kelvin wave activity with the maximum activity near the equator, while the wave activities filtered for the Kelvin wave in the other GCMs are similar to the extratropical Rossby wave with the maximum activity at higher latitudes. Likewise, only these two models produce a realistic seasonal cycle of the Kelvin wave activity.     

非静力平衡层结大气中的重力惯性波和惯性对流

本文根据一些新的流体力学实验结果及某些天气事实,建立了一个非静力平衡条件下旋转地球大气中重力惯性波和惯性对流的理论模型,并分别求出它在稳定层结,中性层结及不稳定层结三种情况下的点源扰动函数形式的解。 分析解的性质得知,中性及弱不稳定大气中可以激发一种主要由地球旋转惯性决定的大幅度垂直速度振动,其周期大于(至少等于)地球在该纬度的惯性周期2π/f。这也许能说明文献[1]所发现的那种准周期性惯性对流的某些动力性质。在稳定层结流体中,地球旋转参数对流体重力振荡的频率影响不大,层结越稳定,越类似于单纯的重力振荡,只有在层结接近中性时,f对重力振荡的频率修正作用才明显起来。 最后给出不同纬度处不稳定层结大气中所能产生的惯性对流的最大临界尺度。     

水汽和潜热释放对背风波影响的数值试验

利用中尺度数值模式ARPS进行了理想场的数值模拟,分析研究了水汽和潜热释放对大气层结稳定度的影响以及其在背风波的发展和演变过程中的作用,研究发现,潜热释放对大气层结分布的影响要远大于水汽对大气层结分布的直接影响,如果没有潜热的释放,水汽对背风波的发展和演变的作用非常小,而潜热释放可以使湿层结稳定度急剧下降,迅速破坏原有的层结分布,使这个区域出现非拦截的强烈的垂直运动,波动的崩溃更加迅速和明显。但需要说明的是在试验中,将数值模式里控制潜热释放的参数设为:0、1/2、2的假定情况,则在实际的大气运动过程中是不可能存在的。     

A case study of penetrative convection and gravity waves over the PROFS mesonetwork on 23 July 1983

Summary This paper describes the evolution of two thunderstorms which developed over northeastern Colorado on 23 July 1983, and more significantly discusses the possible causal relationship between them. In particular, a disturbance apparently created by the first thunderstorm, which developed over the eastern slopes of the Rocky mountains, seems to have triggered the second thunderstorm, which developed further east over the high plains. We present evidence that suggests that the disturbance is a rapidly propagating gravity wave (possibly a solitary wave of depression) that occupied most of the troposphere and was generated by the explosive convective development of the first thunderstorm. Detailed observations of the interactions between these two storms were possible because both storms developed over a dense network of automated weather stations that provided high temporal and spatial resolution surface measurements of pressure, temperature, precipitation, and horizontal wind velocity. Also located within this mesonetwork was a high power 915 MHz wind profiler that provided radial velocities throughout most of the troposphere. These measurements were supplemented with GOES visible and infrared satellite imagery and operational data from National Weather Service rawinsondes and weather radars.With 15 Figures     

Effect of basic state on seasonal variation of convectively coupled Rossby wave

Convectively coupled equatorial Rossby (ER) waves display maximum varability over the northern hemisphere during boreal summer and over the southern hemisphere during boreal winter. It suggests that the seasonal variation of ER waves is significantly affected by the annual cycle of basic state. However, which specific environmental factor plays a determining role remains obscure. This study investigates the background influence on the seasonal variation of ER wave by employing an intermediate anomaly atmospheric model. By prescribing boreal summer/winter seasonal mean state as the model’s basic state, the authors found that the model is able to simulate the trapping of the ER wave purtrubation over the northern/southern hemisphere as in observation. Further sensitivity experiments suggest that the moisture distribution plays a major role in modulating the ER wave structure while the mosoonal flows play a minor role.     

Effects of the earth's rotation on convection: Turbulent statistics,scaling laws and Lagrangian diffusion

The effects of Earth's rotation on convection into stratified fluid under uniform surface cooling are investigated using a large-eddy simulation (LES) model. The initial mixed layer depth varies by a factor of 40 and temperature gradient below the mixed layer varies by three orders of magnitude. At the end of integration (typically 20–40 inertial periods), the so-called natural Rossby number for the rotating experiments varies from 0.06 to 2. The wide range of conditions used is designed to extract scaling laws of rotating convection and to shed light on the importance of Earth's rotation on convection. It is found that the effects of rotation can be characterized by a series of hyperbolic tangent functions of the natural Rossby number. The effects of rotation are most pronounced when Ro is order 0.1 or less. For Ro ≥ 1, the effects of rotation become small. Comparison of Lagrangian statistics of numerical floats reveals that horizontal mixing is suppressed in the presence of rotation. This result is consistent with the finding that integral length scale and turbulent intensity decrease when rotation is included, in contrast to the conclusion of an early study that argued for increased horizontal mixing in the presence of rotation.     

Adjoint of a parameterized moisture convection model

Summary Adjoint models have found use as dynamical tracers, helping to track a feature or phenomenon back to its origin. Their application to the study of atmospheric convection, however, is challenged by the complexity and nonlinearity of diabatic processes. Herein, the adjoint of a significantly simpler parameterized moisture (PM) model is described and tested. The PM model eliminates explicit moisture by making latent heating conditionally proportional to updraft velocity and providing a lower tropospheric heat sink mimicking rainwater evaporation.The PM adjoint, of course, is useful only if the parameterization can produce realistic results. Earlier work suggested that the PM framework possessed a fundamental flaw that made its storms have an excessive impact on their upstream environments. In fact, the adjoint was used to identify the origin of the discrepancies between PM and traditional cloud model storms, thereby leading to the parameterization improvements and dynamical insights recently discussed in Fovell (2002). The present paper is a companion to that study, describing how the adjoint model was constructed, tested and utilized. In addition, an even more realistic adjoint framework is described.     

Test of a convectively forced gravity wave drag parameterization in a general circulation model

The influence of gravity wave drag induced by cumulus convection (GWDC) on a simulated boreal summer climate was evaluated in a general circulation model. For this, the GWDC scheme developed by Chun and Baik was implemented into a version of the National Centers for Environmental Prediction (NCEP) global spectral model (GSM). Ensemble simulations with the two different convection schemes, the simplified Arakawa-Schubert (SAS) scheme and Community Climate Model (CCM) convection scheme, were conducted for the boreal summer of 1996. A cloud factor to modulate the stress intensity with respect to the cloud type was introduced in this study, in order to prevent unrealistic behaviors of the GWDC scheme in GSM. The effect of gravity wave drag on the zonal mean of wind and temperature fields was focused. On the whole, the effect of GWDC in this study is positive on the simulated seasonal climate. It is evident that biases in temperature in the polar region as well as in the zonal and meridional winds in the upper atmosphere are reduced. The percentage of reduction of the bias in zonal winds is about 10–20%. Such a response of the GWDC forcing widely appears not only in tropical regions but also in mid-latitude regions. These characteristics are prominent in the case of the SAS scheme, which is due to the various convective cloud types. The magnitude of GWDC forcing is generally small, but still positive, in the case of the CCM scheme, which is due to rather homogeneous cloud types. It is also found that the role of a particular GWDC forcing depends upon the inherent systematic biases of a particular model. It is concluded that incorporation of the GWDC parameterization in GCMs should be taken into account to improve the seasonal prediction.     

Some evidence of organized flow over natural waves

Measurements of the flow characteristics at 2 m over unobstructed wave surfaces on Lake Michigan were made using an anemometer-bivane as a velocity sensor. During one 40-min period of measurement, significant energy concentration was observed at the frequency of dominant surface waves in the vertical and cross wind spectra. Cross spectra between the surface elevation and vertical motions in the flow indicate that the surface lags the vertical motions by about 55 ° at the frequency of dominant waves.     

Observation of gravity waves in a boreal forest

In this paper we report the results of the analysis of two 60-min wave events that occurred in a boreal aspen forest during the 1994 BOREAS (Boreal Ecosystems-Atmosphere Study) field experiment. High frequency wind and temperature data were provided by three 3-D sonic anemometer/thermometers and fourteen fine-wire thermocouples positioned within and above the forest. Wave phase speeds, estimated from information revealed by spectral analysis and linear plane wave equations, are 2.2 and 1.3 m s^-1 for the two events. The wavelengths are 130 m and 65 m respectively and are much larger than the vertical wave displacements. There is strong evidence from the present analysis and from the literature supporting our postulate that these waves are generated by shear instability. We propose that wind shear near the top of the stand is often large enough to reduce the gradient Richardson number below the critical value of 0.25 and thus is able to trigger the instability. When external conditions are favorable, the instability will grow into waves.     

Effect of atmospheric stability on the growth of surface gravity waves

In this paper we study the effect of atmospheric stability on the growth of surface gravity waves. To that end we numerically solved the Taylor-Goldstein equation for wind profiles which deviate from a logarithmic form because stratification affects the turbulent momentum transport. Using Charnock's relation for the roughness height z ₀ of the wind profile, it is argued that the growth rate of the wave depends on the dimensionless phase velocity c/u _* (where u _* is the friction velocity) and a measure of the effect of atmospheric stability, namely the dimensionless Obukhov length gL/u _* ², whereas it only depends weakly on gz _t /u _* ² (where z _t is the roughness height of the temperature profile). Remarkably for a given value of u _* /c, the growth rate is larger for a stable stratification (L > 0) than for an unstable one (L < 0). We explain why this is the case. If, on the other hand, one considers the growth rate as a function of c/U ₁₀ (where U ₁₀ is the windspeed at 10 m), the situation reverses for c/U ₁₀ < 1. For practical application in wave prediction models, we propose a new parameterization of the growth rate of the waves which is an improvement of the Snyder et al. (1981) proposal because the effect of stability is taken into account.     

一次中层低涡的发展及其MCC结构

1982年7月29日至8月2日黄河中游大暴雨过程,是继1958年7月特大暴雨过程以来最强的一次。其间洛阳地区的陆浑水库站29日12h降水量达544mm。本文对造成该过程后一段强降水的低涡系统进行了研究。用中尺度滤波分析和诊断方法揭示了:该低涡系统不是由登陆台风变性而成,而是在台风外围的东风急流影响下新发展起来的,并取代了原来的台风环流。它的涡旋结构在对流层中层最为清楚,低层的暖湿平流突出,具有较强的斜压性。在对流层高层有明显的流出,形成一个中尺度的辐散中心。因此它的发展过程和结构与Maddox所总结的MCC系统相类似。卫星云图上云团的演变特点证实了上述对发展过程的描述。     

Influence of the Andes Mountains on South American moisture transport, convection, and precipitation

Mountain ranges are known to have a first-order control on mid-latitude climate, but previous studies have shown that the Andes have little effect on the large-scale circulation over South America. We use a limited-domain general circulation model (RegCM3) to evaluate the effect of the Andes on regional-scale atmospheric dynamics and precipitation. We present experiments in which Andean heights are specified at 250 m, and 25, 50, 75, and 100% of their modern values. Our experiments indicate that the Andes have a significant influence on moisture transport between the Amazon Basin and the central Andes, deep convective processes, and precipitation over much of South America through mechanical forcing of the South American low-level jet (LLJ) and topographic blocking of westerly flow from the Pacific Ocean. When the Andes are absent, the LLJ is absent and moisture transport over the central Andes is mainly northeastward. As a result, deep convection is suppressed and precipitation is low along the Andes. Above 50% of the modern elevation, a southward flowing LLJ develops along the eastern Andean flanks and transports moisture from the tropics to the subtropics. Moisture drawn from the Amazon Basin provides the latent energy required to drive convection and precipitation along the Andean front. Large northerly moisture flux and reduced low-level convergence over the Amazon Basin leads to a reduction in precipitation over much of the basin. Our model results are largely consistent with proxy evidence of Andean climate change, and have implications for the timing and rate of Andean surface uplift.     

Numerical experiments on internal gravity waves in an accelerating shear flow

The influence of an accelerating shear flow on the propagation of an internal gravity wave in a continuously stratified fluid is studied by means of two-dimensional numerical simulations. These are motivated by earlier laboratory experiments [Thorpe, S.A. 1978b. On internal gravity waves in an accelerating shear flow, Vol. 88. J. Fluid Mech. pp. 623–639]. In these experiments the mean flow is an accelerated Couette flow and the mean density profile is linear. The laboratory experiments revealed the striking effect of the unsteady shear flow in the evolution of an internal gravity wave leading to the wave focusing in a region where the flow is extremum. This phenomenon is associated with the growth of small scale density fluctuations. As a result density overturns are sometimes observed. This behaviour is well reproduced by the numerical simulations. We provide insights on the flow dynamics in particular on the possible occurrence of wavebreaking. We show that the dynamics is characterized by two competitive mechanisms that is a damping of the wave and a local enhancement of its steepness leading sometimes to density overturns. The budget for the energy of the wave reveals that the initial damping of the wave results from wave-mean flow interactions. These interactions lead to the development of a fine scale vertical density structure which is associated with high vertical shear. We find that in some cases wavebreaking occurs as a result of shear instability. The value of the acceleration of the mean flow is very likely to influence the onset of the instability. The scaling laws of the wave evolution, in particular the rate of decrease of its energy, are determined. From these laws the lifetime of the wave is found as a function of the acceleration of the shear. It may be expected that, in the ocean, this development will result in the largest fluctuations derived from wave-flow interactions occurring where the mean flow in the wave direction is greatest. Waves travelling normal to a two-dimensional shear flow will be unchanged. Waves travelling parallel will be damped. This may have particular application at the continental shelf where flow, mainly parallel to the isobaths, will damp waves travelling along-slope, but allows waves travelling normal to the isobaths (e.g., directly across the shelf-break) to be transmitted without attenuation. Similar effects are expected for the evolution of a high frequency wave interacting with a lower frequency (e.g., near inertial) motion.     

层结大气中重力惯性波的发展

应用WKB方法讨论了非均匀层结大气条件下重力惯性波的发展。分析表明,在水平方向上,当波动由层结不稳定度小的地方向不稳定度大的地方传播时,波将发展,从而修正了文献[2]的结论。     

低空急流对内重力波不稳定发展的作用

本文研究了在环境风场中存在风的垂直切变时,对内重力波发展不稳定性的影响,指出风速在垂直方向出现急流状廓线时,有利于波动的发展。在一定条件下,稳定层结仍可引起重力波的不稳定。用数值积分的方法研究了不同急流强度和急流轴置于不同高度时,对不稳定发展的影响。对同样强度的急流来说,轴的高度越低、越接近边界层内,扰动振幅的增长率就越大。最后把理论结果与观测事实作了比较。     

Internal atmospheric gravity waves near the coast of Antarctica

Two gravity wave events as observed at Georg von Neumayer Station in Antarctica are described and analyzed. Wind and temperature are recorded at a meteorological tower. Surface pressure time series are available from four sites so that rather exact evaluations of phase speed and wavelength are possible. Radiosonde ascents provide information on the structure of the atmosphere above the boundary layer.The pressure traces of both events are dominated by sinusoidal oscillations with a well defined frequency. Related variations of wind and temperature are small during the first event (16 July 1986) as are those of temperature on 29 September 1986. However, wind oscillations are quite large during this second event. An attempt is made to interpret the data in the light of linear gravity theory. It is found that linear gravity waves of frequency and phase speed as observed were able to propagate throughout the troposphere on 16 July. We conjecture on the basis of linear theory that the wave of 29 September was propagating on the surface inversion.