Uniformly stratified moist flow over a Gaussian-shaped circular mountain is investigated using a nonhydrostatic mesoscale model. The focus is the interaction between flow stagnation and orographic precipitation. Two closely related issues are addressed: the effect of condensation and precipitation on mountain flow stagnation, and the influence of flow blocking and latent heat on upslope precipitation. It is demonstrated that latent heat release and precipitation can significantly delay the onset of mountain flow stagnation. The dynamical and thermodynamical nature of this modification can be qualitatively understood using the moist stability concept. However, due to the vertical variation of the moist stability, it is not possible to define a single nondimensional mountain height to describe the general nonlinearity of moist orographic flow. The effect of flow blocking and splitting on the intensity and distribution of orographic precipitation is found to be significant. For low mountains, the upslope ascent dominates and the precipitation intensity is roughly proportional to the mountain height and windspeed as predicted by both a slab model and the mesoscale model. For high mountains, this relationship breaks down because the mountain lift effect is reduced as the low level moist flow passes around the peak. An arc-shaped precipitation band forms further upstream of the peak where the terrain slope is gentle, associated with the secondary circulation forced by the upstream flow blocking/reversal. The removal of latent heat processes leads to reduced upslope lift and enhanced windward blocking, thereby reducing the maximum precipitation rates and increasing the precipitation area.
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, the Eliassen-Palm relation between momentum and energy flux (EF ϭ ϪU • MF). Kinetic (KE) and potential (PE) wave energy densities were also computed. The equipartition ratio (EQR ϭ PE/KE) changed abruptly across the tropopause, indicating partial wave reflection. In one case (16 April 2006) systematically reversed momentum and energy fluxes were found in the stratosphere above 12 km. On a "wave property diagram," three families of waves were identified: up-and downgoing long waves (30 km) and shorter (14 km) trapped waves. For the latter two types, an explanation is proposed related to secondary generation near the tropopause and reflection or secondary generation in the lower stratosphere.
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.
During 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 508 to 608S. 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, frontogenesis, and convection.
SUMMARYA case of orographic precipitation in the Alps on 20 September 1999 was studied using several models, along with rain-gauge and radar data. The objective of the study is to describe the orographic transformation of an air mass, including multi-scale aspects. Several new and some conventional diagnostic quantities are estimated, including drying ratio, precipitation ef ciency, buoyancy work, condensed-water residence time, parcel changes in heat, moisture and altitude, and dominant space-and time-scales.For the case considered, the drying ratio was about 35%. Precipitation ef ciency values are ambiguous due to repeated ascent and descent over small-scale terrain. The sign of buoyancy work changed during the event, indicating a shift from stratiform orographic to weak convective clouds. Cloud-water residence times are different for the two mesoscale models (400 compared to 1000 s) due to different cloud-physical formulations. The two mesoscale models agree that the dominant spatial-scale of lifting and precipitation is about 10 km; smaller than the scale of the main Alpine massif. Trajectory analysis of air crossing the Alps casts doubt on the classic model of föhn. Few parcels exhibit classic pattern of moist ascent followed by dry descent. Parcels that gain latent heat descend only brie y, before rising into the middle troposphere. Parcels that descend along the lee slope, originate in the middle troposphere and gain little, or even lose, latent heat during the transit. As parcels seek their proper buoyancy level downstream, a surprising scrambling of the air mass occurs.Radar data con rm the model prediction that the rainfall eld is tightly controlled by local terrain on scales as small as 10 km, rather than the full 100 km cross-Alpine scale. A curious pulsing of the precipitation is seen, indicating either drifting moisture anomalies or weak convection.
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