scholarly journals Warm-air entrainment and advection during alpine blowing snow events

2020 ◽  
Vol 14 (9) ◽  
pp. 2795-2807
Author(s):  
Nikolas O. Aksamit ◽  
John W. Pomeroy
Keyword(s):  
Time Series ◽  
Hydrological Cycle ◽  
Surface Wind ◽  
Variable Interval ◽  
Air Entrainment ◽  
Heat Fluxes ◽  
Time Averaging ◽  
Blowing Snow ◽  
Near Surface ◽  
Scaling Relationship

Abstract. Blowing snow transport has considerable impact on the hydrological cycle in alpine regions both through the redistribution of the seasonal snowpack and through sublimation back into the atmosphere. Alpine energy and mass balances are typically modeled with time-averaged approximations of sensible and latent heat fluxes. This oversimplifies nonstationary turbulent mixing in complex terrain and may overlook important exchange processes for hydrometeorological prediction. To determine if specific turbulent motions are responsible for warm- and dry-air advection during blowing snow events, quadrant analysis and variable interval time averaging was used to investigate turbulent time series from the Fortress Mountain Snow Laboratory alpine study site in the Canadian Rockies, Alberta, Canada, during the winter of 2015–2016. By analyzing wind velocity and sonic temperature time series with concurrent blowing snow, such turbulent motions were found to supply substantial sensible heat to near-surface wind flows. These motions were responsible for temperature fluctuations of up to 1 ∘C, a considerable change for energy balance estimation. A simple scaling relationship was derived that related the frequency of dominant downdraft and updraft events to their duration and local variance. This allows for the first parameterization of entrained or advected energy for time-averaged representations of blowing snow sublimation and suggests that advection can strongly reduce thermodynamic feedbacks between blowing snow sublimation and the near-surface atmosphere. The downdraft and updraft scaling relationship described herein provides a significant step towards a more physically based blowing snow sublimation model with more realistic mixing of atmospheric heat. Additionally, calculations of return frequencies and event durations provide a field-measurement context for recent findings of nonstationarity impacts on sublimation rates.

scholarly journals Dry-Air Entrainment and Advection during Alpine Blowing Snow Events

2020 ◽  
Author(s):  
Nikolas Olson Aksamit ◽  
John Pomeroy
Keyword(s):  
Hydrological Cycle ◽  
Air Entrainment ◽  
Considerable Change ◽  
Heat Fluxes ◽  
Blowing Snow ◽  
Near Surface ◽  
Dry Air ◽  
Alpine Regions ◽  
Physically Based ◽  
Simple Scaling

Abstract. Blowing snow transport has considerable impact on the hydrological cycle in alpine regions both through the redistribution of the seasonal snowpack and through sublimation back into the atmosphere. Alpine energy and mass balances are typically modelled with time-averaged approximations of sensible and latent heat fluxes. This oversimplifies non-stationary turbulent mixing in complex terrain and may overlook important exchange processes for hydrometeorological prediction. To determine if warm and dry air advection during blowing snow events from atmospheric sweep and ejection motions can provide such exchange mechanisms, they were investigated at an alpine site in the Canadian Rockies and found to supply substantial sensible heat to blowing snow flows. These motions were responsible for temperature fluctuations of up to 1 °C, a considerable change for energy balance estimation. A simple scaling relation was derived that related the frequency of turbulent sweeps and ejections to the event magnitude. This allows the first parameterization of entrained or advected energy for time-averaged representations of blowing snow sublimation and suggests that advection can strongly reduce thermodynamic feedbacks between blowing snow sublimation and the near-surface atmosphere. The recurrence model modeled described provides a significant step towards a more physically-based blowing snow sublimation model. Additionally, calculations of return frequencies and event durations provide a field-measurement context for recent findings of non-stationarity impacts on sublimation rates.

scholarly journals Blowing Snow Sublimation and Transport over Antarctica from 11 Years of CALIPSO Observations

2017 ◽  
Author(s):  
Stephen P. Palm ◽  
Vinay Kayetha ◽  
Yuekui Yang ◽  
Rebecca Pauly
Keyword(s):  
Hydrological Cycle ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Detection Algorithm ◽  
Blowing Snow ◽  
Sublimation Rate ◽  
Near Surface ◽  
Surface Mass ◽  
Mass Budget ◽  
Snow Transport

Abstract. Blowing snow processes commonly occur over the earth's ice sheets when near surface wind speed exceeds a threshold value. These processes play a key role in the sublimation and re-distribution of snow thereby influencing the surface mass balance. Prior field studies and modeling results have shown the importance of blowing snow sublimation and transport on the surface mass budget and hydrological cycle of high latitude regions. For the first time, we present continent-wide estimates of blowing snow sublimation and transport over Antarctica based on direct observation of blowing snow events. We use an improved version of the blowing snow detection algorithm developed for previous work that uses atmospheric backscatter measurements obtained from the CALIOP lidar aboard the CALIPSO satellite. The blowing snow events identified by CALIPSO and meteorological fields from MERRA-2 are used to compute the sublimation and transport rates. Our results show that maximum sublimation occurs along and slightly inland of the coastline. This is contrary to the observed maximum blowing snow frequency which occurs over the interior. The associated temperature and moisture re-analysis fields likely contribute to the spatial distribution of the maximum sublimation values. However, the spatial pattern of the sublimation rate over Antarctica is consistent with modeling studies and precipitation estimates. Overall, our results show that Antarctica average integrated blowing snow sublimation is about 393.4 ± 138 Gt yr−1 which is considerably larger than previous model-derived estimates. We find maximum blowing snow transport amount of 5 Megatons km−1 yr−1 over parts of East Antarctica and estimate that the average snow transport from continent to ocean is about 3.68 Gt yr−1. These continent-wide estimates are the first of their kind and can be used to help model and constrain the surface-mass budget over Antarctica.

scholarly journals Dynamical Attribution of Recent Variability in Atlantic Overturning

2016 ◽  
Vol 29 (9) ◽  
pp. 3339-3352 ◽  
Author(s):  
Helen R. Pillar ◽  
Patrick Heimbach ◽  
Helen L. Johnson ◽  
David P. Marshall
Keyword(s):  
Time Series ◽  
Time Scales ◽  
Surface Wind ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Buoyancy Forcing ◽  
Freshwater Forcing ◽  
Decadal Trend ◽  
Improved Model ◽  
Short Time

Abstract Attributing observed variability of the Atlantic meridional overturning circulation (AMOC) to past changes in surface forcing is challenging but essential for detecting any influence of anthropogenic forcing and reducing uncertainty in future climate predictions. Here, quantitative estimates of separate contributions from wind and buoyancy forcing to AMOC variations at 25°N are obtained. These estimates are achieved by projecting observed atmospheric anomalies onto model-based dynamical patterns of AMOC sensitivity to surface wind, thermal, and freshwater forcing over the preceding 15 years. Local wind forcing is shown to dominate AMOC variability on short time scales, whereas subpolar heat fluxes dominate on decadal time scales. The reconstructed transport time series successfully reproduces most of the interannual variability observed by RAPID–MOCHA. However, the apparent decadal trend in the RAPID–MOCHA time series is not captured, requiring improved model representation of ocean adjustment to subpolar heat fluxes over at least the past two decades and highlighting the importance of sustained monitoring of the high-latitude North Atlantic.

scholarly journals A spectral analysis of near-surface wind speed and possible sources of predictability in the Iberian Peninsula

2021 ◽  
Author(s):  
Eduardo Utrabo-Carazo ◽  
Cesar Azorin-Molina ◽  
Enric Aguilar ◽  
Manola Brunet
Keyword(s):  
Time Series ◽  
Spectral Analysis ◽  
Wind Speed ◽  
Iberian Peninsula ◽  
Southern Oscillation ◽  
Statistical Significance ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Near Surface ◽  
Mean Values

<p>Conventional time series analysis of observed near-surface wind speed (SWS) have focused both on mean values and on the sign, magnitude and statistical significance of trends. Specifically, a decrease in the SWS has been detected in continental surfaces of the planet's mid-latitudes from 1979 to 2010 approximately, the so-called <em>stilling</em> phenomenon; and an increase from 2010 until now, the <em>reversal</em> phenomenon. However, although various hypotheses have been proposed in the scientific literature, the mechanisms behind these phenomena and what evolution this parameter will follow in the future are still understudied, mainly because the response of a variable dependent on atmospheric circulation, such as wind speed, to a warming climate is uncertain. This study aims to use spectral analysis (Fourier and wavelet) to determine the most significant frequency modes associated with the SWS time series in the Iberian Peninsula (IP), for both mean wind speed and daily peak wind gusts, as well as its temporal evolution for 1961-2019. Subsequently, this study will also attempt to relate these modes to those corresponding to various modes of ocean-atmosphere variability such as the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) or, due to the influence of sudden stratospheric warming (SSW) in the European troposphere, the polar vortex. The ultimate goal will be to explore possible sources of predictability in the medium-long term for SWS across the IP, which would have direct applications in areas such as: wind-power generation, agriculture, air quality, insurance and fishing industries, among many others socioeconomic and environmental issues. </p>

scholarly journals Evaluation of dynamically downscaled near-surface mass and energy fluxes for three mountain glaciers, British Columbia, Canada

2018 ◽  
Author(s):  
Mekdes Ayalew Tessema ◽  
Valentina Radić ◽  
Brian Menounos ◽  
Noel Fitzpatrick
Keyword(s):  
British Columbia ◽  
Spatial Resolution ◽  
Dynamical Downscaling ◽  
Surface Wind ◽  
Wrf Model ◽  
Heat Fluxes ◽  
Shortwave Radiation ◽  
Positive Bias ◽  
Near Surface ◽  
Mountain Glaciers

Abstract. Most models of glacier melt are forced by observed meteorological data in the vicinity of the glacier in question. In the absence of these observations, the forcing is commonly derived from statistical or dynamical downscaling of low resolution reanalysis datasets. Here we perform dynamical downscaling via the Weather Research and Forecasting (WRF) model in order to evaluate its performance against the observations from automatic weather stations (AWSs ) for three mountain glaciers in the interior of British Columbia, Canada over several summer seasons. The WRF model, nested within the ERA-Interim global reanalysis, produced output fields at 7.5 km and 2.5 km spatial resolution for all glaciers, as well as 1 km resolution for one of the glaciers. We find that the surface energy balance (SEB) model, forced by WRF at 2.5 km, adequately simulates the AWS-derived seasonal melt rates despite large biases in the individual SEB components. Overestimation of the number of clear sky days in WRF at 2.5 km explains the positive bias in the seasonal incoming shortwave radiation. This positive bias, however, is compensated by a negative bias in the seasonal incoming longwave radiation, and by an underestimation of sensible and latent heat fluxes. The underestimation of sensible heat fluxes down to −80 % of AWS-derived fluxes, as calculated by the bulk aerodynamic method, is due to the underestimated near-surface wind speeds. An increase of WRF spatial resolution from 7.5 to 1 km does not improve the simulation of downscaled variables other than near-surface air temperature. For relatively small glaciers (

scholarly journals Comparison of Surface Wind Speed and Wind Speed Profiles in the Taklimakan Desert

2021 ◽  
Author(s):  
Liu Xinchun ◽  
kang yongde ◽  
Chen Hongna ◽  
Lu Hui
Keyword(s):  
Wind Speed ◽  
Hydrological Cycle ◽  
Similarity Theory ◽  
Surface Wind ◽  
Taklimakan Desert ◽  
Diurnal Changes ◽  
Wind Speed Profile ◽  
Near Surface ◽  
The Taklimakan Desert ◽  
Neutral Stratification

Abstract Near-surface (10 m) wind speed (NWS) plays a crucial role in many areas, including the hydrological cycle, wind energy production, and the dispersion of air pollution. Based on wind speed data from Tazhong and the northern margins of the Taklimakan Desert in Xiaotang in spring, summer, autumn, and winter of 2014 and 2015, statistical methods were applied to determine the characteristics of the diurnal changes in wind speed near the ground and the differences in the wind speed profiles between the two sites. The average wind speed on a sunny day increased slowly with height during the day and rapidly at night. At heights below 4 m the wind speed during the day was higher than at night, whereas at 10 m the wind speed was lower during the day than at night. The semi-empirical theory and Monin-Obukhov (M-O) similarity theory were used to fit the NWS profile in the hinterland of the Tazhong Desert. A logarithmic law was applied to the neutral stratification wind speed profile, and an exponential fitting correlation was used for non-neutral stratification. The more unstable the stratification, the smaller the n. Using M-O similarity theory, the “linear to tens of” law was applied to the near-neutral stratification. According to the measured data, the distribution of φM with stability was obtained. The γm was obtained when the near-surface stratum was stable in the hinterland of Tazhong Desert and the βm was obtained when it was unstable. In summer, γm and βm were 5.84 and 15.1, respectively, while in winter, γm and βm were 1.9 and 27.1, respectively.

scholarly journals Evaluation of Near-Surface Parameters in the Two Versions of the Atmospheric Model in CESM1 using Flux Station Observations

2013 ◽  
Vol 26 (1) ◽  
pp. 26-44 ◽  
Author(s):  
Jenny Lindvall ◽  
Gunilla Svensson ◽  
Cecile Hannay
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Diurnal Cycle ◽  
Surface Wind ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Seasonal Temperature ◽  
Near Surface ◽  
Surface Parameters ◽  
Low Level

Abstract This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid- and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

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