MATTHEW F. GARVERT, BRADLEY SMULL, and CLIFF MASS Multiscale Mountain Waves Influencing a Major Orographic Precipitation Event MATTHEW F. GARVERT, BRADLEY SMULL, and CLIFF MASS 2007, J. Atmos. Sci., 64, 711-737

Outline Introduction Methodology and data sources Observed and modeled flow kinematics over the Oregon Cascades Observed reflectivity patterns and modeled precipitation fields Summary and conclusions

Introduction Improvements of the model’s bulk microphysical parameterizations (BMPs) are needed to correct shortcomings in model QPF over mountainous terrain (Mass 2000). Observations are necessary to isolate errors in model parameterizations of microphysical processes from problems with basic-state kinematic fields. Field studies have highlighted the importance of the flow kinematics in determining the intensity and distribution of orographic precipitation. (e.g. Marwitz 1987a,b; Cox et al. 2005) IMPROVE-2 provides a unique opportunity to examine kinematic, dynamic, and micro-physical processes and associated flow and precipitation structure over topography, and hence to assess the validity of BMPs.

Methodology and data sources a) Airborne dual-Doppler measurements 2300UTC 13 Dec — 0100UTC 14 Dec 2001 3D reflectivity and radial velocity data Due to the slow evolution of flow and precipitation structure, the composite analysis may be viewed as a steady state. 140 km 40 km profiler sounding S-band S-Pol radar

b) MM5 model setup and description 32 full-sigma levels Explicit moisure scheme of Reisner 2 Grell cumulus parameterization (for D_36km and D_12km only) MRF planetary boudary layer scheme FDDA during first 24h on D_36km and D_12km 0000UTC 13 DEC 1200UTC 14 DEC D_12km D_36km D_1.33km D_4km

c) Case overview 1.0 km 3.0 km 13 Dec forward-tilted cold front; no blocking; warm air advection to east of the front mountain wave with θ contures descent and rebound to the lee; Enhanced vertical gradients in θ and θe in the shadow area; wind directional shear; 14 Dec

Winds from the NACR ISS profiler c) Case overview Winds from the NACR ISS profiler relatively unchanged with time low-level veering relatively uniform above 2.5km

Observed and modeled flow kinematics a) Upstream conditions Sounding at 0000UTC 14 neutral to stable flow enhanced stablity between 2 and 3 km a linear (i.e., “flow over”) mountain-wave regime underprediction of the vertical shear magnitude the mean height of the crest h=1.68km

b) Windward slopes and leeside southwestly flow at 1.5km enhanced reflectivity over the windward slope and a sharp reduction leeward a maximum in wind speeds to the lee veering of winds betwwen 1.5 and 4.5km 1.5 km 4.5 km

average east-west cross section b) Windward slopes and leeside dual-Doppler Shear layer Jet of cross-barrier flow Vertically propagating mountain wave, flow-over regime, error increase spillover, underpredicted the strength and depth of the shear layer. average east-west cross section

c) Sensitivity tests of PBL parameterization schemes The vertical extent of the low θe and depth of the shear layer was further reduced. U was stronger. Stronger U was even closer to the surface and over a wider area to the lee. W was of higher amplitude and downward displaced in the lee. Depth of shear layer further reduced in Eta, U is stronger than control run. Too closer to surface and a wider area. Higher amplitude and downward displaced vertical motion.

Observed reflectivity patterns and modeled precipitation fields Cloud liquid water Snow a) Larger-scale (>20km) precipitation feathers The slope of the bright band is consistent with the presence of cooled air on the windward side. Radar reflectivities decreased rapidly downstream of the creast. The perturbations were stationary and persistent during 2 hours interval. The highest snow mixing ratios were advected over the crest before being shunted earthward.

a) Larger-scale (>20km) precipitation feathers Lee-wave response downwind of the coastal mountains effectively enhanced precipitation over WV . Short-wavelength features response to the local steepness and details of the underlying terrain.

b) Smaller-scale (<20km) precipitation feathers critical horizontal wavelength of ~18 km (2πV/N=2π*22/0.0077) vertically propagating mountain wave Strongly sheared southerly wind would tend to favor limited upward penetration of these waves.

b) Smaller-scale (<20km) precipitation feathers dual-Doppler Model b) Smaller-scale (<20km) precipitation feathers Enhancement of reflectivities repeatedly occurred downstream of strong positive W. Pockets of high CLW and some enhancement in the snow mixing ratios are over the individual ridges. Riming of the snow and graupel formation was diagnosed. Precipitation particles encountered strong downdrafts in the lee and rapidly fell. The precipitation distribution at the surface was very sensitive to the amount of riming, the fall speed of the particles, and the phase and amplitude of these smaller-sacle mountain waves.

b) Smaller-scale (<20km) precipitation feathers The U and V varied by 5-7m/s over horizontal distances of 10- 15 km by observation. The model represented the variations of U and V ,except for overpredicting their amplitude. The model also represented the magnitudes and positions of these W oscillations but overpredict the resulting enhancement of CLW. 10-s running mean H=2.5km

3h cumulative QPF (2200-0100UTC) c) Surface precipitation patterns 3h cumulative QPF (2200-0100UTC) The smoothed-terrain simulation showsFar more uniform distribution over the windward slopes. A slight reduction is 10-20 km upwind of the crest in both simulations. A sharper gradient to the lee appears in the control run. control For waves of horizontal scales of 10-30km, condensate advection slightly dominates any upstream wave tilt that may occur, with the resulting maximum precipitation being located over the highest terrain smooth

c) Surface precipitation patterns The differences in topography did not significantly alter the total amount of precipitation over the domain as a whole, but instead horizontally redistributed this precipitation.

Summary and conclusions Two distinct scales of mesoscale wave–like air motions are identified. The mountain wave’s amplitude and placement are extremely sensitive to the planetary boundary layer (PBL) parameterization. Smaller-scale waves (20-km horizontal wavelength) over the windward slopes significantly impact the horizontal pattern of precipitation and hence quantitative precipitation forecast (QPF) accuracy.

Thank you!

Two distinct scales of mesoscale wave–like air motions are identified.

to identify kinematic structures influencing the production and mesoscale distribution of precipitation and microphysical processes during a period of heavy prefrontal orographic rainfall

Flow across Cascade crest subsiding branch of the mountain-wave circulation on the lee of Cascade

Model sensitivity studies assist in analyzing the complex interaction between the upstream flow field and the mountain wave anchored to the Cascade crest

smaller-scale gravity waves (horizontal wavelengths of 20 km) generated by topography influenced cloud structure and modified precipitation distributions. and produced high CLW independent of the large-scale baroclinic storm system

(IMPROVE-2) provides a unique opportunity to examine kinematic, dynamic, and microphysical processes and associated flow and precipitation structures over topography, and hence to assess the validity of present-day numerical model BMPs Isolate potential problems in the model BMP from model errors in the dynamic representation of such storm systems

How utilizes the unique airborne dual-Doppler dataset to qualitatively assess the amplitude and strength of mountain waves on several distinct scales over both the leeside and windward slopes Output from MM5 is evaluated alongside the observations, and then employed to examine the evolution of these standing waves

Model sensitivity studies assist in analyzing the complex interaction between the upstream flow field and the mountain wave anchored to the Cascade crest

details of the microphysical structures aloft and surface precipitation patterns are examined as they relate to these diverse mountainwave structures.