Qingfang Jiang

Large-Amplitude Mountain Wave Breaking over Greenland

Abstract A large-amplitude mountain wave generated by strong southwesterly flow over southern Greenland was observed during the Fronts and Atlantic Storm-Track Experiment (FASTEX) on 29 January 1997 by the NOAA G-IV research aircraft. Dropwindsondes deployed every 50 km and flight level data depict a vertically propagating large-amplitude wave with deep convectively unstable layers, potential temperature perturbations of 25 K that deformed the tropopause and lower stratosphere, and a vertical velocity maximum of nearly 10 m s−1 in the stratosphere. The wave breaking was associated with a large vertical flux of horizontal momentum and dominated by quasi-isotropic turbulence. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) nonhydrostatic model with four-nested grid meshes with a minimum resolution of 1.7 km accurately simulates the amplitude, location, and timing of the mountain wave and turbulent breakdown. Finescale low-velocity plumes that resemble wakelike structures emanate from highl...

Cloud Timescales and Orographic Precipitation

Abstract Orographic precipitation is studied by analyzing the sensitivity of numerical simulations to variations in mountain height, width, and wind speed. The emphasis is on upslope lifting over isolated mountains in cold climates. An attempt is made to capture the essential steady-state volume-averaged cloud physics in a pair of coupled nonlinear algebraic equations. To do this, single-pathway snow formation models are analyzed with both linear and nonlinear accretion formulations. The linear model suggests that the precipitation efficiency is determined by three timescales—the advection timescale (τa), fallout timescale (τf), and a constant timescale for snow generation (τcs). Snow generation is controlled by the ratio of τcs/τa and the fraction of the snow that falls to the ground is controlled by the ratio of τf/τa. Nonlinear terms, representing accretion, reduce the utility of the timescale concept by introducing a threshold or “bifurcation” point, that is, a critical condensation rate that separate...

Mountain Waves Entering the Stratosphere

Using the National Science Foundation (NSF)‐NCAR Gulfstream V and the NSF‐Wyoming King Air research aircraft during the Terrain-Induced Rotor Experiment (T-REX) in March‐April 2006, six cases of Sierra Nevada mountain waves were surveyed with 126 cross-mountain legs. The goal was to identify the influence of the tropopause on waves entering the stratosphere. During each flight leg, part of the variation in observed parameters was due to parameter layering, heaving up and down in the waves. Diagnosis of the combined wave-layering signal was aided with innovative use of new GPS altitude measurements. The ozone and water vapor layering correlated with layered Bernoulli function and cross-flow speed. GPS-corrected static pressure was used to compute the vertical energy flux, confirming, for the first time,

Interaction between Trapped Waves and Boundary Layers

Abstract The absorption of trapped lee waves by the atmospheric boundary layer (BL) is investigated based on numerical simulations and theoretical formulations. It is demonstrated that the amplitude of trapped waves decays exponentially with downstream distance due to BL absorption. The decay coefficient, α, defined as the inverse of the e-folding decay distance, is found to be sensitive to both surface momentum and heat fluxes. Specifically, α is larger over a rougher surface, associated with a more turbulent BL. On the other hand, the value of α decreases with increasing surface heating and increases with increasing surface cooling, implying that a stable nocturnal BL is more efficient in absorbing trapped waves than a typically deeper and more turbulent convective BL. A stagnant layer could effectively absorb trapped waves and increase α. Over the range of parameters examined, the absorption coefficient shows little sensitivity to wave amplitude. A relationship is derived to relate the surface reflecti...

An Intercomparison of T-REX Mountain Wave Simulations and Implications for Mesoscale Predictability

AbstractNumerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed. A basic benchmark and five other test cases are simulated in a two-dimensional framework using the same initial state, which is based on conditions during Intensive Observation Period (IOP) 6 of the Terrain-Induced Rotor Experiment (T-REX), in which intense mountain-wave activity was observed. All of the models use an identical horizontal resolution of 1 km and the same vertical resolution. The six simulated test cases use various terrain heights: a 100-m bell-shaped hill, a 1000-m idealized ridge that is steeper on the lee slope, a 2500-m ridge with the same terrain shape, and a cross-Sierra terrain profile. The models are tested with both free-slip and no-slip lower boundary conditions.The results indicate a surprisingly diverse spectrum of simulated mountain-wave characteristics including lee waves, hydraulic-like jump features, and gravity wave breaking. The vertical v...

Observations and Numerical Simulations of Subrotor Vortices during T-REX

High-resolution observations from scanning Doppler and aerosol lidars, wind profiler radars, as well as surface and aircraft measurements during the Terrain-induced Rotor Experiment (T-REX) provide the first comprehensive documentation of small-scale intense vortices associated with atmospheric rotors that form in the lee of mountainous terrain. Although rotors are already recognized as potential hazards for aircraft, it is proposed that these small-scale vortices, or subrotors, are the most dangerous features because of strong wind shear and the transient nature of the vortices. A life cycle of a subrotor event is captured by scanning Doppler and aerosol lidars over a 5-min period. The lidars depict an amplifying vortex, with a characteristic length scale of 500–1000 m, that overturns and intensifies to a maximum spanwise vorticity greater than 0.2 s−1. Radar wind profiler observations document a series of vortices, characterized by updraft/downdraft couplets and regions of enhanced reversed flow, that are generated in a layer of strong vertical wind shear and subcritical Richardson number. The observations and numerical simulations reveal that turbulent subrotors occur most frequently along the leading edge of an elevated sheet of horizontal vorticity that is a manifestation of boundary layer shear and separation along the lee slopes. As the subrotors break from the vortex sheet, intensification occurs through vortex stretching and in some cases tilting processes related to three-dimensional turbulent mixing. The subrotors and ambient vortex sheet are shown to intensify through a modest increase in the upstream inversion strength, which illustrates the predictability challenges for the turbulent characterization of rotors.

A Theory of Gravity Wave Absorption by a Boundary Layer

Abstract A one-layer model of the atmospheric boundary layer (BL) is proposed to explain the nature of lee-wave attenuation and gravity wave absorption seen in numerical simulations. Two complex coefficients are defined: the compliance coefficient and the wave reflection coefficient. A real-valued ratio of reflected to incident wave energy is also useful. The key result is that, due to horizontal friction, the wind response in the BL is shifted upstream compared to the phase of disturbances in the free atmosphere. The associated flow divergence modulates the thickness of the BL so that it partially absorbs incident gravity waves. A simple expression is derived relating the reflection coefficient to the attenuation and wavelength shift of trapped lee waves. Results agree qualitatively with the numerical simulations, including the effects of increased surface roughness and heat flux.

Gravity Wave Breaking over the Central Alps: Role of Complex Terrain

Abstract The characteristics of gravity waves excited by the complex terrain of the central Alps during the intensive observational period (IOP) 8 of the Mesoscale Alpine Programme (MAP) is studied through the analysis of aircraft in situ measurements, GPS dropsondes, radiosondes, airborne lidar data, and numerical simulations. Mountain wave breaking occurred over the central Alps on 21 October 1999, associated with wind shear, wind turning, and a critical level with Richardson number less than unity just above the flight level (∼5.7 km) of the research aircraft NCAR Electra. The Electra flew two repeated transverses across the Otztaler Alpen, during which localized turbulence was sampled. The observed maximum vertical motion was 9 m s−1, corresponding to a turbulent kinetic energy (TKE) maximum of 10.5 m2 s−2. Spectrum analysis indicates an inertia subrange up to 5-km wavelength and multiple energy-containing spikes corresponding to a wide range of wavelengths. Manual analysis of GPS dropsonde data indic...

Review of wave‐turbulence interactions in the stable atmospheric boundary layer

Flow in a stably stratified environment is characterized by anisotropic and intermittent turbulence and wavelike motions of varying amplitudes and periods. Understanding turbulence intermittency and wave-turbulence interactions in a stably stratified flow remains a challenging issue in geosciences including planetary atmospheres and oceans. The stable atmospheric boundary layer (SABL) commonly occurs when the ground surface is cooled by longwave radiation emission such as at night over land surfaces, or even daytime over snow and ice surfaces, and when warm air is advected over cold surfaces. Intermittent turbulence intensification in the SABL impacts human activities and weather variability, yet it cannot be generated in state-of-the-art numerical forecast models. This failure is mainly due to a lack of understanding of the physical mechanisms for seemingly random turbulence generation in a stably stratified flow, in which wave-turbulence interaction is a potential mechanism for turbulence intermittency. A workshop on wave-turbulence interactions in the SABL addressed the current understanding and challenges of wave-turbulence interactions and the role of wavelike motions in contributing to anisotropic and intermittent turbulence from the perspectives of theory, observations, and numerical parameterization. There have been a number of reviews on waves, and a few on turbulence in stably stratified flows, but not much on wave-turbulence interactions. This review focuses on the nocturnal SABL; however, the discussions here on intermittent turbulence and wave-turbulence interactions in stably stratified flows underscore important issues in stably stratified geophysical dynamics in general.

What Is the Source of the Stratospheric Gravity Wave Belt in Austral Winter

AbstractDuring austral winter, and away from orographic maxima or “hot spots,” stratospheric gravity waves in both satellite observations and Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data reveal enhanced amplitudes in a broad midlatitude belt extending across the Southern Ocean from east of the Andes to south of New Zealand. The peak latitude of this feature slowly migrates poleward from 50° to 60°S. Wave amplitudes are much weaker across the midlatitude Pacific Ocean. These features of the wave field are in striking agreement with diagnostics of baroclinic growth rates in the troposphere associated with midlatitude winter storm tracks and the climatology of the midlatitude jet. This correlation suggests that these features of the stratospheric gravity wave field are controlled by geographical variations of tropospheric nonorographic gravity wave sources in winter storm tracks: spontaneous adjustment emission from the midlatitude winter jet, frontogenesi...