A Case Study of Radar Observations and WRF LES Simulations of the Impact of Ground-Based Glaciogenic Seeding on Orographic Clouds and Precipitation. Part I: Observations and Model Validations

Abstract

Profiling airborne radar data and accompanying large-eddy-simulation (LES) modeling are used to examine the impact of ground-based glaciogenic seeding on cloud and precipitation in a shallow stratiform orographic winter storm. This storm occurred on 18 February 2009 over a mountain in Wyoming. The numerical simulations use the Weather Research and Forecasting (WRF) Model in LES mode with horizontal grid spacings of 300 and 100 m in a domain covering the entire mountain range, and a glaciogenic seeding parameterization coupled with the Thompson microphysics scheme. A series of non-LES simulations at 900-m resolution, each with different initial/ boundary conditions, is validated against sounding, cloud, and precipitation data. The LES runs then are driven by the most representative 900-m non-LES simulation. The 100-m LES results compare reasonably well to the vertical-plane radar data. The modeled vertical-motion field reveals a turbulent boundary layer and gravity waves above this layer, as observed. The storm structure also validates well, but the model storm thins and weakens more rapidly than is observed. Radar reflectivity frequency-by-altitude diagrams suggest a positive seeding effect, but time-and space-matched model reflectivity diagrams only confirm this in a relative sense, in comparison with the trend in the control region upwind of seeding generators, and not in an absolute sense. A model sensitivity run shows that in this case natural storm weakening dwarfs the seeding effect, which does enhance snow mass and snowfall. Since the kinematic and microphysical structure of the storm is simulated well, future Part II of this study will examine how glaciogenic seeding impacts clouds and precipitation processes within the LES.

A Census of Atmospheric Variability From Seconds to Decades

“…Rapid atmospheric turbulence at the air-sea interface may have significant climatic impacts because of nonlinearities in the ocean's response to atmospheric forcing (Williams, 2012). At short spatiotemporal scales, large-eddy simulations may be used to assess the atmospheric boundary layer's impacts on clouds (Chu et al, 2014), pollution transport (Cécé et al, 2016), wildfires (Coen et al, 2012), and renewable energy (Worsnop et al, 2017). Dynamic downscaling of mesoscale simulations to large-eddy simulations incorporates mesoscale variability while introducing challenges when parameterized atmospheric boundary layer turbulence interacts with directly represented turbulence (Shin & Dudhia, 2016).…”

Snow Growth and Transport Patterns in Orographic Storms as Estimated from Airborne Vertical-Plane Dual-Doppler Radar Data

“…A shallow low-level downslope windstorm and a hydraulic jump are present not just in cases C and D, but also on 18 February 2009 and 11 January 2013 (i.e., in four of the seven STRF cases listed in Table 1). Other examples of the STRF type with plunging flow, BL separation, and a hydraulic jump in the lee of the MB range are documented in Chu et al (2014) andFrench et al (2014, manuscript submitted to J. Atmos. Sci.).…”

“…This condition applies frequently over the MB range and is not sufficient. On some flights downslope accelerations and even hydraulic jumps were observed over the MB range (e.g., Chu et al 2014;French et al 2014, manuscript submitted to J. Atmos. Sci.).…”

Toward Low‐Level Turbulence Forecasting at Eddy‐Resolving Scales

“…With the continuing growth of computing capabilities, and in particular accelerated architectures like graphics processing units (e.g., Schalkwijk et al, ), it is expected that NWP models will in the near future provide forecasts at LES scales for daytime conditions over a limited area (Δ ≈ 25–100 m, resolving up to mesobeta scales). Consequently, there is a recent trend to incrementally push the application of NWP models to dynamically downscale to LES grid resolutions (e.g., Cécé et al, ; Chu et al, ; Heinze et al, ; Joe et al, ; Liu et al, ; Muñoz‐Esparza et al, ; Rai et al, ; Talbot et al, ; Zhou & Chow, ). In anticipation of the development of LES‐type operational capabilities, a more comprehensive evaluation and understanding of the potential and added value of these eddy‐scale turbulence predictions is needed, beyond previous isolated efforts that have examined specific events spanning a few hours of duration and typically focusing on nonturbulent quantities.…”