Abstract

Two topographically generated cirrus plume events have been examined through satellite observations and real-data simulations. On 30 October 2002, an approximately 70-km-wide cirrus plume, revealed by a high-resolution Moderate Resolution Imaging Spectroradiometer (MODIS) image and a series of Geostationary Operational Environmental Satellite (GOES) images, originated from the Sierra Nevada ridge and extended northeastward for more than 400 km. On 5 December 2000, an approximately 400-km-wide cloud plume originated from the Southern Rocky Mountain massif and extended eastward for more than 500 km, the development of which was captured by a series of GOES images. The real-data simulations of the two cirrus plume events successfully capture the presence of these plumes and show reasonable agreement with the MODIS and GOES images in terms of the timing, location, orientation, length, and altitude of these cloud plumes. The synoptic and mesoscale aspects of the plume events, and the dynamics and microphysics relevant to the plume formation, have been discussed. Two common ingredients relevant to the cirrus plume formation have been identified, namely, a relatively deep moist layer aloft with high relative humidity and low temperature (≤−40°C near the cloud top), and strong updrafts over high terrain and slow descent downstream in the upper troposphere associated with terrain-induced inertia–gravity waves. The rapid increase of the relative humidity associated with strong updrafts creates a high number concentration of small ice crystals through homogeneous nucleation. The overpopulated ice crystals decrease the relative humidity, which, in return, inhibits small crystals from growing into large crystals. The small crystals with slow terminal velocities (<0.2 m s⁻¹) can be advected hundreds of kilometers before falling out of the moist layer.

Abstract

The impact of diurnal forcing on a downslope wind event that occurred in Owens Valley in California during the Sierra Rotors Project (SRP) in the spring of 2004 has been examined based on observational analysis and diagnosis of numerical simulations. The observations indicate that while the upstream flow was characterized by persistent westerlies at and above the mountaintop level the cross-valley winds in Owens Valley exhibited strong diurnal variation. The numerical simulations using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) capture many of the observed salient features and indicate that the in-valley flow evolved among three states during a diurnal cycle. Before sunrise, moderate downslope winds were confined to the western slope of Owens Valley (shallow penetration state). Surface heating after sunrise weakened the downslope winds and mountain waves and eventually led to the decoupling of the well-mixed valley air from the westerlies aloft around local noon (decoupled state). The westerlies plunged into the valley in the afternoon and propagated across the valley floor (in-valley westerly state). After sunset, the westerlies within the valley retreated toward the western slope, where the downslope winds persisted throughout the night.

Abstract

The impact of moist processes on mountain waves over Sierra Nevada Mountain Range is investigated in this study. Aircraft measurements over Owens Valley obtained during the Terrain-induced Rotor Experiment (T-REX) indicate that mountain waves were generally weaker when the relative humidity maximum near the mountaintop level was above 70%. Four moist cases with a RH maximum near the mountaintop level greater than 90% have been further examined using a mesoscale model and a linear wave model. Two competing mechanisms governing the influence of moisture on mountain waves have been identified. The first mechanism involves low-level moisture that enhances flow–terrain interaction by reducing windward flow blocking. In the second mechanism, the moist airflow tends to damp mountain waves through destratifying the airflow and reducing the buoyancy frequency. The second mechanism dominates in the presence of a deep moist layer in the lower to middle troposphere, and the wave amplitude is significantly reduced associated with a smaller moist buoyancy frequency. With a shallow moist layer and strong low-level flow, the two mechanisms can become comparable in magnitude and largely offset each other.

Abstract

The characteristics and dynamics of the Chilean low-level coastal jet (CLLCJ) are examined here through diagnosing real-time mesoscale model forecasts in support of the Variability of the American Monsoon System (VAMOS) Ocean–Cloud–Atmosphere Land Study (VOCALS) and additional sensitivity simulations. The forecasted surface winds over the southeast Pacific compare favorably with available observations. According to the forecasts and sensitivity simulations, the Southeast Pacific high pressure system (SEPH) plays a primary role in driving the CLLCJ. The Andes significantly intensify the CLLCJ mainly through interacting with the SEPH and anchoring a baroclinic zone along the Chilean coast. The land–sea differential heating also enhances the CLLCJ by strengthening the coastal baroclinic zone. Based on the location of the SEPH center, the CLLCJ can be separated into two types: a strong-forcing jet, with the SEPH close to the central Chilean coastline; and a weak-forcing jet, with the SEPH centered far away from the coastline. The former is much more intense and associated with stronger interaction between the SEPH and the Andes.

The CLLCJ is slightly supergeostrophic within the marine boundary layer top inversion, where weak easterlies develop, and subgeostrophic in the turbulent boundary layer below, where westerlies are present. The inversion easterlies induce strong subsidence along the coast, which contributes to the formation of the coastal low and the coastal baroclinic zone.

Abstract

Measurements from the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) Gulfstream V (G-V) obtained during the recent Terrain-Induced Rotor Experiment (T-REX) indicate marked differences in the character of the wave response between repeated flight tracks across the Sierra Nevada, which were separated by a distance of approximately 50 km. Observations from several of the G-V research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of 2 between the southern and northern portions of the racetrack flight segments in the lower stratosphere, with the largest amplitude waves most often occurring over the southern flight leg, which has a terrain maximum that is 800 m lower than the northern leg. Multiple racetracks at 11.7- and 13.1-km altitudes indicate that these differences were repeatable, which is suggestive that the deviations were likely due to vertically propagating mountain waves that varied systematically in amplitude rather than associated with transients. The cross-mountain horizontal velocity perturbations are also a maximum above the southern portion of the Sierra Nevada ridge.

Real data and idealized nonhydrostatic numerical model simulations are used to test the hypothesis that the observed variability in the wave amplitude and characteristics in the along-barrier direction is a consequence of blocking by the three-dimensional Sierra Nevada and the Coriolis effect. The numerical simulation results suggest that wave launching is sensitive to the overall three-dimensional characteristics of the Sierra Nevada barrier, which has an important impact on the wave amplitude and characteristics in the lower stratosphere. Real-time high-resolution Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) forecasts successfully capture the along-barrier variations in the wave amplitude (using vertical velocity as a proxy) as well as skillfully distinguishing between large- and small-amplitude stratospheric wave events during T-REX.

Abstract

The utility of static and adaptive mesh refinement (SMR and AMR, respectively) are examined for idealized tropical cyclone (TC) simulations in a two-dimensional spectral element f-plane shallow-water model. The SMR simulations have varying sizes of the statically refined meshes (geometry based) while the AMR simulations use a potential vorticity (PV) threshold to adaptively refine the mesh to the evolving TC. Numerical simulations are conducted for four cases: (i) TC-like vortex advecting in a uniform flow, (ii) binary vortex interaction, (iii) barotropic instability of a PV ring, and (iv) barotropic instability of a thin strip of PV. For each case, a uniform grid high-resolution “truth” simulation is compared to two different SMR simulations and three different AMR simulations for accuracy and efficiency. The multiple SMR and AMR simulations have variations in the number of fully refined elements in the vicinity of the TC. For these idealized cases, it is found that the SMR and AMR simulations are able to resolve the vortex dynamical processes (e.g., barotropic instability, Rossby wave breaking, and filamentation) as well as the truth simulations, with no significant loss in accuracy in the refined region in the vortex vicinity and with significant speedups (factors of 4–15, depending on the total number of refined elements). The overall accuracy is enhanced by a greater area of fully refined mesh in both the SMR and AMR simulations.

Abstract

Numerical 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 velocity standard deviation is twice as large in the free-slip experiments relative to the no-slip simulations. Nevertheless, the no-slip simulations also exhibit considerable variations in the wave characteristics. The results imply relatively low predictability of key characteristics of topographically forced flows such as the strength of downslope winds and stratospheric wave breaking. The vertical flux of horizontal momentum, which is a domain-integrated quantity, exhibits considerable spread among the models, particularly for the experiments with the 2500-m ridge and Sierra terrain. The differences among the various model simulations, all initialized with identical initial states, suggest that model dynamical cores may be an important component of diversity for the design of mesoscale ensemble systems for topographically forced flows. The intermodel differences are significantly larger than sensitivity experiments within a single modeling system.