Louisa Nance

Observations of Precipitation Size and Fall Speed Characteristics within Coexisting Rain and Wet Snow

Abstract Ground-based measurements of particle size and fall speed distributions using a Particle Size and Velocity (PARSIVEL) disdrometer are compared among samples obtained in mixed precipitation (rain and wet snow) and rain in the Oregon Cascade Mountains and in dry snow in the Rocky Mountains of Colorado. Coexisting rain and snow particles are distinguished using a classification method based on their size and fall speed properties. The bimodal distribution of the particles’ joint fall speed–size characteristics at air temperatures from 0.5° to 0°C suggests that wet-snow particles quickly make a transition to rain once melting has progressed sufficiently. As air temperatures increase to 1.5°C, the reduction in the number of very large aggregates with a diameter > 10 mm coincides with the appearance of rain particles larger than 6 mm. In this setting, very large raindrops appear to be the result of aggregrates melting with minimal breakup rather than formation by coalescence. In contrast to dry snow an...

Gap flow in an Alpine valley during a shallow south föhn event: Observations, numerical simulations and hydraulic analogue

This paper examines the three-dimensional structure and dynamics of southerly hybrid gap/mountain flow through the Wipp valley (Wipptal), Austria, observed on 30 October 1999 using high-resolution observations and model simulations. The observations were obtained during a shallow south fohn event documented in the framework of the Mesoscale Alpine Programme (MAP). Three important data sources were used: the airborne differential-absorption lidar LEANDRE 2, the ground-based Doppler lidar TEACO2 and in situ measurements from the National Oceanic and Atmospheric Administration P-3 aircraft. This event was simulated down to 2 km horizontal resolution using the non-hydrostatic mesoscale model Meso-NH. The structure and dynamics of the flow were realistically simulated. The combination of high-resolution observations and numerical simulations provided a comprehensive three-dimensional picture of the flow through the Wipptal: in the gap entrance region (Brenner Pass, Austria), the low-level jet was not solely due to the channelling of the southerly synoptic flow through the elevated gap. Part of the Wipptal flow originated as a mountain wave at the valley head wall of the Brenner Pass. Downstream of the pass, the shallow fohn flow had the characteristics of a downslope windstorm as it rushed down towards the Inn valley (Inntal) and the City of Innsbruck, Austria. Downhill of the Brenner Pass, the strongest flow was observed over a small obstacle along the western side wall (the Nosslachjoch), rather than channelled in the deeper part of the valley just to the east. Further north, the low-level jet was observed in the centre of the valley. Approximately halfway between Brenner Pass and Innsbruck, where the along-axis direction of the valley changes from north to north-north-west, the low-level jet was observed to be deflected to the eastern side wall of the Wipptal. Interaction between the Stubaier Alpen (the largest and highest topographic feature to the west of the Wipptal) and the south-westerly synoptic flow was found to be the primary mechanism responsible for the deflection. The along- and cross-valley structure and dynamics of the flow were observed to be highly variable due to the influence of surrounding mountains, localized steep slopes within the valley and outflows from tributaries (the Gschnitztal and the Stubaital) to the west of the Wipptal. For that shallow fohn case, observations and simulations provided a large body of evidence that downslope flow created thinning/thickening fluid and accelerations/ decelerations reminiscent of mountain wave/hydraulic theory. Along the Wipptal, two hydraulic-jump-like transitions were observed and simulated, (i) on the lee slope of the Nosslachjoch and (ii) in the Gschnitztal exit region. A hydraulic solution of the flow was calculated in the framework of reduced-gravity shallow-water theory. The down-valley evolution of the Froude number computed using LEANDRE 2, P-3 flight level and TEACO2 measurements confirmed that these transitions were associated with super- to subcritical transitions. Copyright

A modeling study of nonstationary trapped mountain lee waves. Part I: Mean-flow variability

Abstract The impact of mean-flow variability on finite-amplitude trapped mountain lee waves is investigated by conducting two-dimensional mountain wave simulations for a set of idealized, time-dependent background flows. The lee-wave patterns generated by these time-dependent flows depend on two factors: 1) the degree to which the transition in the background flow changes the amplitude of the stationary trapped lee wave and 2) the difference between the group velocities of the trapped waves generated before and after the transition. When the transition in the background flow significantly reduces the amplitude of the stationary lee wave, the lee-wave pattern generated prior to the transition gradually drifts downstream away from the mountain or back over the mountain, depending on the sign of this wave packet’s group velocity after the transition. When the transition in the background flow changes the resonant wavelength while leaving the lee-wave amplitude relatively unchanged, the lee-wave train develop...

A Modeling Study of Nonstationary Trapped Mountain Lee Waves. Part II: Nonlinearity

The generation of nonstationary trapped mountain lee waves through nonlinear wave dynamics without any concomitant change in the background flow is investigated by conducting two-dimensional mountain wave simulations. These simulations demonstrate that finite-amplitude lee-wave patterns can exhibit temporal variations in local wavelength and amplitude, even when the background flow is perfectly steady. For moderate amplitudes, a nonlinear wave interaction involving the stationary trapped wave and a pair of nonstationary waves appears to be responsible for the development of nonstationary perturbations on the stationary trapped wave. This pair of nonstationary waves consists of a trapped wave and a vertically propagating wave, both having horizontal wavelengths approximately twice that of the stationary trapped wave. As the flow becomes more nonlinear, the nonstationary perturbations involve a wider spectrum of horizontal wavelengths and may dominate the overall wave pattern at wave amplitudes significantly below the threshold required to produce wave breaking. Sensitivity tests in which the wave propagation characteristics of the basic state are modified without changing the horizontal wavelength of the stationary trapped wave indicate these nonstationary perturbations are absent when the background flow does not support nonstationary trapped waves with horizontal wavelengths approximately twice that of the stationary trapped mode. These sensitivity tests also show that a second nonstationary trapped wave can assume the role of the nonstationary vertically propagating wave when the Scorer parameter in the upper layer is reduced below the threshold that will support the vertically propagating wave. In this case, a resonant triad composed of three trapped waves appears to be responsible for the development of nonstationary perturbations. The simulations suggest that strongly nonlinear wave dynamics can generate a wider range of nonstationary trapped modes than that produced by temporal variations in the background flow. It is suggested that the irregular variations in lee-wave wavelength and amplitude observed in real atmospheric flows and the complex fluctuations above a fixed point that are occasionally found in wind profiler observations of trapped lee waves are more likely to be generated by nonlinear wave dynamics than changes in the background flow.

Beyond the Basics: Evaluating Model-Based Precipitation Forecasts Using Traditional, Spatial, and Object-Based Methods

AbstractWhile traditional verification methods are commonly used to assess numerical model quantitative precipitation forecasts (QPFs) using a grid-to-grid approach, they generally offer little diagnostic information or reasoning behind the computed statistic. On the other hand, advanced spatial verification techniques, such as neighborhood and object-based methods, can provide more meaningful insight into differences between forecast and observed features in terms of skill with spatial scale, coverage area, displacement, orientation, and intensity. To demonstrate the utility of applying advanced verification techniques to mid- and coarse-resolution models, the Developmental Testbed Center (DTC) applied several traditional metrics and spatial verification techniques to QPFs provided by the Global Forecast System (GFS) and operational North American Mesoscale Model (NAM). Along with frequency bias and Gilbert skill score (GSS) adjusted for bias, both the fractions skill score (FSS) and Method for Object-Ba...

A comparison of the accuracy of three anelastic systems and the pseudo-incompressible system

Abstract The accuracy of three anelastic systems (Ogura and Phillips; Wilhelmson and Ogura; Lipps and Hemler) and the pseudo-incompressible system is investigated for small-amplitude and finite-amplitude disturbances. Based on analytic solutions to the linearized, hydrostatic mountain wave problem, the accuracy of the Lipps and Hemler and pseudo-incompressible systems is distinctly superior to that of the other two systems. The linear dispersion relations indicate the accuracy of the pseudo-incompressible system should improve and the accuracy of the Lipps and Hemler system should decrease as the waves become more nonhydrostatic. Since analytic solutions are not available for finite-amplitude disturbances, five nonlinear, nonhydrostatic numerical models based on these four systems and the complete compressible equations are constructed to determine the ability of each “sound proof” system to describe finite-amplitude disturbances. A comparison between the analytic solutions and numerical simulations of th...

THE DEVELOPMENTAL TESTBED CENTER AND ITS WINTER FORECASTING EXPERIMENT

Abstract The Weather Research and Forecasting (WRF) Developmental Testbed Center (DTC) was formed to promote exchanges between the development and operational communities in the field of Numerical Weather Prediction (NWP). The WRF DTC serves to accelerate the transfer of NWP technology from research to operations and to support a subset of the current WRF operational configurations to the general community. This article describes the mission and recent activities of the WRF DTC, including a detailed discussion about one of its recent projects, the WRF DTC Winter Forecasting Experiment (DWFE). DWFE was planned and executed by the WRF DTC in collaboration with forecasters and model developers. The real-time phase of the experiment took place in the winter of 2004/05, with two dynamic cores of the WRF model being run once per day out to 48 h. The models were configured with 5-km grid spacing over the entire continental United States to ascertain the value of high-resolution numerical guidance for winter weat...

Bridging Research to Operations Transitions: Status and Plans of Community GSI

AbstractWith a goal of improving operational numerical weather prediction (NWP), the Developmental Testbed Center (DTC) has been working with operational centers, including, among others, the National Centers for Environmental Prediction (NCEP), National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA), and the U.S. Air Force, to support numerical models/systems and their research, perform objective testing and evaluation of NWP methods, and facilitate research-to-operations transitions. This article introduces the first attempt of the DTC in the data assimilation area to help achieve this goal. Since 2009, the DTC, NCEP’s Environmental Modeling Center (EMC), and other developers have made significant progress in transitioning the operational Gridpoint Statistical Interpolation (GSI) data assimilation system into a community-based code management framework. Currently, GSI is provided to the public with user support and is open for contributions from inter...

On the Inclusion of Compressibility Effects in the Scorer Parameter

The vertical structure of steady, two-dimensional, linear mountain waves depends on the Scorer parameter profile of the background flow. This note briefly reviews the approximations associated with each simplified equation, discusses their relationship to standard filtered systems of equations, and evaluates the performance of each simplified vertical structure equation by determining the error introduced by each approximation in the calculation of the horizontal wavelength of trapped mountain lee waves.

Evaluating the Use of a Nonlinear Two-Dimensional Model in Downslope Windstorm Forecasts

Abstract Severe downslope windstorms are a mesoscale, primarily wintertime, phenomenon that affect regions in the lee of large mountain ranges. The resolution of current weather prediction models is too coarse to explicitly predict downslope windstorms. Hence, additional operational tools are needed for making downslope windstorm forecasts. Current windstorm forecast techniques commonly utilize a tool referred to as a “decision tree.” Although decision trees provide valuable guidance, operational forecasters have not found this type of tool to be highly reliable. With recent advances in computer technology, a new type of operational tool is available for forecasting downslope windstorms: two-dimensional, nonlinear, mesoscale numerical models. This study investigates whether this type of model, initialized with upstream profiles taken from operational Eta Model forecasts, can produce accurate downslope windstorm forecasts. Numerical simulations for high-wind events that affected seven regions in the United...