Vanda Grubišić

High resolution coupled climate-runoff simulations of seasonal snowfall over Colorado: A process study of current and warmer climate

AbstractClimate change is expected to accelerate the hydrologic cycle, increase the fraction of precipitation that is rain, and enhance snowpack melting. The enhanced hydrological cycle is also expected to increase snowfall amounts due to increased moisture availability. These processes are examined in this paper in the Colorado Headwaters region through the use of a coupled high-resolution climate–runoff model. Four high-resolution simulations of annual snowfall over Colorado are conducted. The simulations are verified using Snowpack Telemetry (SNOTEL) data. Results are then presented regarding the grid spacing needed for appropriate simulation of snowfall. Finally, climate sensitivity is explored using a pseudo–global warming approach. The results show that the proper spatial and temporal depiction of snowfall adequate for water resource and climate change purposes can be achieved with the appropriate choice of model grid spacing and parameterizations. The pseudo–global warming simulations indicate enha...

HYMEX , a 10-year Multidisciplinary Program on the mediterranean water cycle.

The Mediterranean countries are experiencing important challenges related to the water cycle, including water shortages and floods, extreme winds, and ice/snow storms, that impact critically the socioeconomic vitality in the area (causing damage to property, threatening lives, affecting the energy and transportation sectors, etc.). There are gaps in our understanding of the Mediterranean water cycle and its dynamics that include the variability of the Mediterranean Sea water budget and its feedback on the variability of the continental precipitation through air–sea interactions, the impact of precipitation variability on aquifer recharge, river discharge, and soil water content and vegetation characteristics specific to the Mediterranean basin and the mechanisms that control the location and intensity of heavy precipitating systems that often produce floods. The Hydrological Cycle in Mediterranean Experiment (HyMeX) program is a 10-yr concerted experimental effort at the international level that aims to advance the scientific knowledge of the water cycle variability in all compartments (land, sea, and atmosphere) and at various time and spatial scales. It also aims to improve the processes-based models needed for forecasting hydrometeorological extremes and the models of the regional climate system for predicting regional climate variability and evolution. Finally, it aims to assess the social and economic vulnerability to hydrometeorological natural hazards in the Mediterranean and the adaptation capacity of the territories and populations therein to provide support to policy makers to cope with water-related problems under the influence of climate change, by linking scientific outcomes with related policy requirements.

THE TERRAIN-INDUCED ROTOR EXPERIMENT : A Field Campaign Overview Including Observational Highlights

Abstract The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotem-poral characteristics ...

Aerial Observations of Hawaii's Wake

Abstract Under the influence of the east-northeasterly trade winds, the island of Hawaii generates a wake that extends about 200 km to the west-southwest. During the HaRP project in July and August 1990, five wake surveys were carried out by the NCAR Electra. The patterns of wind and aerosol concentration revealed by these flights suggest that Hawaiis wake consists of two large quasi-steady counterrotating eddies. The southern clockwise-rotating eddy carries a heavy aerosol load due to input from the Kī volcano. At the eastern end of the wake, the eddies are potentially warmer and more humid than the surrounding trade wind air. Several other features are discussed: sharp shear lines near the northern and southern tips of the island, dry and warm air bands along the shear lines, a small embedded wake behind the Kohala peninsula, wake centerline clouds, hydraulic jumps to the north and south of the island, a descending inversion connected with accelerating trade winds, and evidence for side-to-side wake mo...

An intercomparison of model predicted wave breaking for the 11 January 1972 Boulder windstorm

Abstract Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave bre...

The wake of St. Vincent

Abstract The island of St. Vincent and the other Windward Islands in the southeastern Caribbean were chosen as a field site for the study of weak mountain wakes. By the authors’ definition, a “weak wake” forms when the potential vorticity generated by a mountain is not strong enough to advect itself into eddies; rather, it is simply advected downstream by the ambient flow. GOES-8 and Landsat sunglint images unambiguously revealed that the mountainous Windward Islands have remarkably long straight wakes. The length of St. Vincent’s wake exceeds 300 km although its width is only 20 km. Near the islands, the wake structures reflect the details of the island topography. These wakes do not exhibit any obvious diurnal effect. Boat surveys in the lee of St. Vincent confirmed the existence of features seen in the images: the sharp wake boundary, the small valley-induced jet embedded in the near wake, and the absence of any reverse flow. Aircraft surveys gave evidence of descent over the island and showed that the...

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,

The Effect of Bottom Friction on Shallow-Water Flow past an Isolated Obstacle

Abstract The effect of bottom friction on fluid flow past an isolated obstacle is investigated in the shallow-water framework. The controlling parameter for this effect is the nondimensional bottom friction number, defined as a ratio of friction to inertia. With the bottom stress related to the horizontal wind via standard bulk aerodynamic formula, the friction number is proportional to the surface roughness, the horizontal scale of the obstacle, and the inverse of the upstream fluid depth. Thus, under otherwise identical conditions, the flow past larger obstacles will be more “viscous.” Bottom friction modifies the vorticity generation in several ways, but under normal conditions, the wake formation remains dominated by a pseudoinviscid process related to the presence of hydraulic jumps. However, friction strongly controls the velocity-deficit region of the wake and thus influences the stability of the steady-state wakes. Predictions of the linear stability analysis are compared with numerical simulation...

Quantitative Precipitation Forecasting of Wintertime Storms in the Sierra Nevada: Sensitivity to the Microphysical Parameterization and Horizontal Resolution

Abstract The skill of a mesoscale model in predicting orographic precipitation during high-impact precipitation events in the Sierra Nevada, and the sensitivity of that skill to the choice of the microphysical parameterization and horizontal resolution, are examined. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and four bulk microphysical parameterization schemes examined are the Dudhia ice scheme, and the Schultz, GSFC, and Reisner2 mixed-phase schemes. The verification dataset consists of ground precipitation measurements from a selected number of wintertime heavy precipitation events documented during the Sierra Cooperative Pilot Project in the 1980s. At high horizontal resolutions, the predicted spatial precipitation patterns on the upwind Sierra Nevada slopes were found to have filamentary structure, with precipitation amounts over the transverse upwind ridges exceeding severalfold those over the nearby deep river valleys...

Vorticity and potential vorticity in mountain wakes

A wake is traditionally defined as the region of nearly stagnant flow downstream of a body in a uniform stream. In a stratified fluid, the motions and density surfaces downstream of an obstacle become primarily horizontal; the vertical component of the vorticity associated with the wake, coexisting with the stable vertical density stratification, implies that there is potential vorticity (PV) in the wake. Recent work has demonstrated that dissipation aloft, associated with a breaking mountain wave over an isolated peak, produces a dipole in PV downstream; the dipolar vertical vorticity of the wake is associated with the PV dipole. Although one may infer the existence of vorticity downstream, the PV argument is silent on the question, Where does the wake vorticity come from? To answer this question, a weakly nonlinear model for PV production and wake formation in the case of a small-amplitude mountain has been analyzed, and numerical simulations pertaining to the strongly nonlinear large-amplitude case have been carried out. The simple model indicates that even with dissipation in the system, the vertical vorticity of the wake arises through the tilting of baroclinically generated horizontal vorticity by the dissipating mountain wave. This analysis shows that there need not be any direct effect of friction in the vorticity equation to produce the vorticity of the wake; dissipation (due to friction and/or heating) enters indirectly through its effect on the tilting term. Analysis of numerical simulations of the large-amplitude case shows that the conclusions from the weakly nonlinear model regarding the source of wake vorticity continue to hold in the strongly nonlinear regime.