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Thomas M. Gowan
,
W. James Steenburgh
, and
Justin R. Minder

Abstract

Landfalling lake- and sea-effect (hereafter lake-effect) systems often interact with orography, altering the distribution and intensity of precipitation, which frequently falls as snow. In this study, we examine the influence of orography on two modes of lake-effect systems: long-lake-axis-parallel (LLAP) bands and broad-coverage, open-cell convection. Specifically, we generate idealized large-eddy simulations of a LLAP band produced by an oval lake and broad-coverage, open-cell convection produced by an open lake (i.e., without flanking shorelines) with a downstream coastal plain, 500-m peak, and 2000-m ridge. Without terrain, the LLAP band intersects a coastal baroclinic zone over which ascent and hydrometeor mass growth are maximized, with transport and fallout producing an inland precipitation maximum. The 500-m peak does not significantly alter this structure, but slightly enhances precipitation due to orographic ascent, increased hydrometeor mass growth, and reduced subcloud sublimation. In contrast, a 2000-m ridge disrupts the band by blocking the continental flow that flanks the coastlines. This, combined with differential surface heating between the lake and land, leads to low-level flow reversal, shifting the coastal baroclinic zone and precipitation maximum offshore. In contrast, the flow moves over the terrain in open lake, open-cell simulations. Over the 500-m peak, this yields an increase in the frequency of weaker (<1 m s−1) updrafts and weak precipitation enhancement, although stronger updrafts decline. Over the 2000-m ridge, however, buoyancy and convective vigor increase dramatically, contributing to an eightfold increase in precipitation. Overall, these results highlight differences in the influence of orography on two common lake-effect modes.

Significance Statement

Landfalling lake- and sea-effect snowstorms frequently interact with hills, mountains, and upland regions, altering the distribution and intensity of snowfall. Using high-resolution numerical modeling with simplified lake shapes and terrain features, we illustrate how terrain features affect two common types of lake-effect storms and why long-lake-axis-parallel (LLAP) bands can feature high precipitation rates but weaker orographic enhancement than broad-coverage, open-cell convection.

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Thomas M. Gowan
,
W. James Steenburgh
, and
Justin R. Minder

Abstract

The distribution and intensity of lake- and sea-effect (hereafter lake-effect) precipitation are strongly influenced by the mode of landfalling lake-effect systems. Here, we used idealized large-eddy simulations to investigate the downstream evolution and coastal-to-inland transition of two lake-effect modes: 1) a long-lake-axis-parallel (LLAP) band generated by an oval body of water (hereafter lake; e.g., Lake Ontario) and 2) broad-coverage, open-cell convection generated by an open lake (e.g., Sea of Japan). Under identical atmospheric conditions and lake-surface temperatures, the oval lake generates a LLAP band with heavy precipitation along the midlake axis, whereas the open lake generates broad-coverage, open-cell convection with widespread, light accumulations. Over the oval lake, the LLAP band features a thermally forced and diabatically enhanced cross-band secondary circulation with convergence and ascent over the midlake axis. Downstream of the lake, flanking airstreams that avoid lake modification merge beneath the band where they experience sublimational cooling, producing a cold pool. At the upstream edge of the cold pool, a coastal baroclinic zone forms. Above this zone, ascent and hydrometeor mass growth are maximized, resulting in an inland precipitation maximum due to subsequent hydrometeor transport and fallout. Over the open lake, individual open cells grow larger and stronger with overwater extent, but a convective-to-stratiform transition begins at the coast. Here, convective vigor decays, mesoscale ascent begins, and enhanced hydrometeor growth results in an inland precipitation maximum. These results highlight variations in the coastal-to-inland transition of lake-effect systems that ultimately influence the distribution and intensity of lake-effect precipitation.

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Justin R. Minder
,
Theodore W. Letcher
,
Leah S. Campbell
,
Peter G. Veals
, and
W. James Steenburgh

Abstract

A pronounced snowfall maximum occurs about 30 km downwind of Lake Ontario over the 600-m-high Tug Hill Plateau (hereafter Tug Hill), a region where lake-effect convection is affected by mesoscale forcing associated with landfall and orographic uplift. Profiling radar data from the Ontario Winter Lake-effect Systems field campaign are used to characterize the inland evolution of lake-effect convection that produces the Tug Hill snowfall maximum. Four K-band profiling Micro Rain Radars (MRRs) were aligned in a transect from the Ontario coast onto Tug Hill. Additional observations were provided by an X-band profiling radar (XPR). Analysis is presented of a major lake-effect storm that produced 6.4-cm liquid precipitation equivalent (LPE) snowfall over Tug Hill. This event exhibited strong inland enhancement, with LPE increasing by a factor of 1.9 over 15-km horizontal distance. MRR profiles reveal that this enhancement was not due to increases in the depth or intensity of lake-effect convection. With increasing inland distance, echoes transitioned from a convective toward a stratiform morphology, becoming less intense, more uniform, more frequent, and less turbulent. An inland increase in echo frequency (possibly orographically forced) contributes somewhat to snowfall enhancement. The XPR observations reproduce the basic vertical structure seen by the MRRs while also revealing a suppression of snowfall below 600 m AGL upwind of Tug Hill, possibly associated with subcloud sublimation or hydrometeor advection. Statistics from 29 events demonstrate that the above-described inland evolution of convection is common for lake-effect storms east of Lake Ontario.

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Leah S. Campbell
,
W. James Steenburgh
,
Peter G. Veals
,
Theodore W. Letcher
, and
Justin R. Minder

Abstract

Improved understanding of the influence of orography on lake-effect storms is crucial for weather forecasting in many lake-effect regions. The Tug Hill Plateau of northern New York (hereafter Tug Hill), rising 500 m above eastern Lake Ontario, experiences some of the most intense snowstorms in the world. Herein the authors investigate the enhancement of lake-effect snowfall over Tug Hill during IOP2b of the Ontario Winter Lake-effect Systems (OWLeS) field campaign. During the 24-h study period, total liquid precipitation equivalent along the axis of maximum precipitation increased from 33.5 mm at a lowland (145 m MSL) site to 62.5 mm at an upland (385 m MSL) site, the latter yielding 101.5 cm of snow. However, the ratio of upland to lowland precipitation, or orographic ratio, varied with the mode of lake-effect precipitation. Strongly organized long-lake-axis parallel bands, some of which formed in association with the approach or passage of upper-level short-wave troughs, produced the highest precipitation rates but the smallest orographic ratios. Within these bands, radar echoes were deepest and strongest over Lake Ontario and the coastal lowlands and decreased in depth and median intensity over Tug Hill. In contrast, nonbanded broad-coverage periods exhibited the smallest precipitation rates and the largest orographic ratios, the latter reflecting an increase in the coverage and frequency of radar echoes over Tug Hill. These findings should aid operational forecasts and, given the predominance of broad-coverage lake-effect periods during the cool season, help explain the climatological snowfall maximum found over the Tug Hill Plateau.

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Geng Xia
,
Matthew C. Cervarich
,
Somnath Baidya Roy
,
Liming Zhou
,
Justin R. Minder
,
Pedro A. Jimenez
, and
Jeffrey M. Freedman

Abstract

This study simulates the impacts of real-world wind farms on land surface temperature (LST) using the Weather Research and Forecasting (WRF) Model driven by realistic initial and boundary conditions. The simulated wind farm impacts are compared with the observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the first Wind Forecast Improvement Project (WFIP) field campaign. Simulations are performed over west-central Texas for the month of July throughout 7 years (2003–04 and 2010–14). Two groups of experiments are conducted: 1) direct validations of the simulated LST changes between the preturbine period (2003–04) and postturbine period (2010–14) validated against the MODIS observations; and 2) a model sensitivity test of LST to the wind turbine parameterization by examining LST differences with and without the wind turbines for the postturbine period. Overall, the WRF Model is moderately successful at reproducing the observed spatiotemporal variations of the background LST but has difficulties in reproducing such variations for the turbine-induced LST change signals at pixel levels. However, the model is still able to reproduce coherent and consistent responses of the observed LST changes at regional scales. The simulated wind farm–induced LST warming signals agree well with the satellite observations in terms of their spatial coupling with the wind farm layout. Moreover, the simulated areal mean warming signal (0.20°–0.26°C) is about a tenth of a degree smaller than that from MODIS (0.33°C). However, these results suggest that the current wind turbine parameterization tends to induce a cooling effect behind the wind farm region at nighttime, which has not been confirmed by previous field campaigns and satellite observations.

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Dan Welsh
,
Bart Geerts
,
Xiaoqin Jing
,
Philip T. Bergmaier
,
Justin R. Minder
,
W. James Steenburgh
, and
Leah S. Campbell

Abstract

The distribution of radar-estimated precipitation from lake-effect snowbands over and downwind of Lake Ontario shows more snowfall in downwind areas than over the lake itself. Here, two nonexclusive processes contributing to this are examined: the collapse of convection that lofts hydrometeors over the lake and allows them to settle downwind; and stratiform ascent over land, due to the development of a stable boundary layer, frictional convergence, and terrain, leading to widespread precipitation there. The main data sources for this study are vertical profiles of radar reflectivity and hydrometeor vertical velocity in a well-defined, deep long-lake-axis-parallel band, observed on 11 December 2013 during the Ontario Winter Lake-effect Systems (OWLeS) project. The profiles are derived from an airborne W-band Doppler radar, as well as an array of four K-band radars, an X-band profiling radar, a scanning X-band radar, and a scanning S-band radar.

The presence of convection offshore is evident from deep, strong (up to 10 m s−1) updrafts producing bounded weak-echo regions and locally heavily rimed snow particles. The decrease of the standard deviation, skewness, and peak values of Doppler vertical velocity during the downwind shore crossing is consistent with the convection collapse hypothesis. Consistent with the stratiform ascent hypothesis are (i) an increase in mean vertical velocity over land; and (ii) an increasing abundance of large snowflakes at low levels and over land, due to depositional growth and aggregation, evident from flight-level and surface particle size distribution data, and from differences in reflectivity values from S-, X-, K-, and W-band radars at nearly the same time and location.

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David M. Schultz
,
Jeffrey Anderson
,
Tommaso Benacchio
,
Kristen L. Corbosiero
,
Matthew D. Eastin
,
Clark Evans
,
Jidong Gao
,
Joshua P. Hacker
,
Daniel Hodyss
,
Daryl Kleist
,
Matthew R. Kumjian
,
Ron McTaggart-Cowan
,
Zhiyong Meng
,
Justin R. Minder
,
Derek Posselt
,
Paul Roundy
,
Angela Rowe
,
Michael Scheuerer
,
Russ S. Schumacher
,
Stan Trier
, and
Christopher Weiss
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