scholarly journals An improved dust emission model – Part 1: Model description and comparison against measurements

2014 ◽  
Vol 14 (23) ◽  
pp. 13023-13041 ◽  
Author(s):  
J. F. Kok ◽  
N. M. Mahowald ◽  
G. Fratini ◽  
J. A. Gillies ◽  
M. Ishizuka ◽  
...  
Keyword(s):  
Large Scale ◽  
Friction Velocity ◽  
Climate Models ◽  
Dust Emission ◽  
Model Description ◽  
Emission Model ◽  
Threshold Friction Velocity ◽  
Dust Flux ◽  
Physically Based ◽  
Flux Measurements

Abstract. Simulations of the dust cycle and its interactions with the changing Earth system are hindered by the empirical nature of dust emission parameterizations in weather and climate models. Here we take a step towards improving dust cycle simulations by using a combination of theory and numerical simulations to derive a physically based dust emission parameterization. Our parameterization is straightforward to implement into large-scale models, as it depends only on the wind friction velocity and the soil's threshold friction velocity. Moreover, it accounts for two processes missing from most existing parameterizations: a soil's increased ability to produce dust under saltation bombardment as it becomes more erodible, and the increased scaling of the dust flux with wind speed as a soil becomes less erodible. Our treatment of both these processes is supported by a compilation of quality-controlled vertical dust flux measurements. Furthermore, our scheme reproduces this measurement compilation with substantially less error than the existing dust flux parameterizations we were able to compare against. A critical insight from both our theory and the measurement compilation is that dust fluxes are substantially more sensitive to the soil's threshold friction velocity than most current schemes account for.

scholarly journals An improved dust emission model with insights into the global dust cycle's climate sensitivity

2014 ◽  
Vol 14 (5) ◽  
pp. 6361-6425 ◽  
Author(s):  
J. F. Kok ◽  
N. M. Mahowald ◽  
S. Albani ◽  
G. Fratini ◽  
J. A. Gillies ◽  
...  
Keyword(s):  
Climate Sensitivity ◽  
Source Function ◽  
Climate Models ◽  
Climate Model ◽  
Dust Emission ◽  
Soil Erodibility ◽  
Emission Model ◽  
Dust Flux ◽  
Physically Based ◽  
Flux Parameterization

Abstract. Simulations of the global dust cycle and its interactions with a changing Earth system are hindered by the empirical nature of dust emission parameterizations in climate models. Here we take a step towards improving global dust cycle simulations by presenting a physically-based dust emission model. The resulting dust flux parameterization depends only on the wind friction speed and the soil's threshold friction speed, and can therefore be readily implemented into climate models. We show that our parameterization's functional form is supported by a compilation of quality-controlled vertical dust flux measurements, and that it better reproduces these measurements than existing parameterizations. Both our theory and measurements indicate that many climate models underestimate the dust flux's sensitivity to soil erodibility. This finding can explain why dust cycle simulations in many models are improved by using an empirical preferential sources function that shifts dust emissions towards the most erodible regions. In fact, implementing our parameterization in a climate model produces even better agreement against aerosol optical depth measurements than simulations that use such a source function. These results indicate that the need to use a source function is at least partially eliminated by the additional physics accounted for by our parameterization. Since soil erodibility is affected by climate changes, our results further suggest that many models have underestimated the climate sensitivity of the global dust cycle.

scholarly journals An improved dust emission model – Part 2: Evaluation in the Community Earth System Model, with implications for the use of dust source functions

2014 ◽  
Vol 14 (23) ◽  
pp. 13043-13061 ◽  
Author(s):  
J. F. Kok ◽  
S. Albani ◽  
N. M. Mahowald ◽  
D. S. Ward
Keyword(s):  
Large Scale ◽  
Friction Velocity ◽  
Source Function ◽  
Dust Emission ◽  
Companion Paper ◽  
System Model ◽  
Dust Source ◽  
Threshold Friction Velocity ◽  
Scale Models ◽  
Community Earth System Model

<p><strong>Abstract.</strong> The complex nature of mineral dust aerosol emission makes it a difficult process to represent accurately in weather and climate models. Indeed, results in the companion paper indicate that many large-scale models underestimate the dust flux's sensitivity to the soil's threshold friction velocity for erosion. We hypothesize that this finding explains why many dust cycle simulations are improved by using an empirical dust source function that shifts emissions towards the world's most erodible regions. Here, we both test this hypothesis and evaluate the performance of the new dust emission parameterization presented in the companion paper. We do so by implementing the new emission scheme into the Community Earth System Model (CESM) and comparing the resulting dust cycle simulations against an array of measurements. We find that the new scheme shifts emissions towards the world's most erodible regions in a manner that is strikingly similar to the effect of implementing a widely used source function based on satellite observations of dust source regions. Furthermore, model comparisons against aerosol optical depth measurements show that the new physically based scheme produces a statistically significant improvement in CESM's representation of dust emission, which exceeds the improvement produced by implementing a source function. These results indicate that the need to use an empirical source function is eliminated, at least in CESM, by the additional physics in the new scheme, and in particular by its increased sensitivity to the soil's threshold friction velocity. Since the threshold friction velocity is affected by climate changes, our results further suggest that many large-scale models underestimate the global dust cycle's climate sensitivity.</p>

scholarly journals Numerical simulations of Greenland snowpack and comparison with passive microwave spectral signatures

2001 ◽  
Vol 32 ◽  
pp. 109-115 ◽  
Author(s):  
Christophe Genthon ◽  
Michel Fily ◽  
Eric Martin
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Structural Parameters ◽  
Microwave Radiometer ◽  
Surface Wind ◽  
Radiative Properties ◽  
Physically Based ◽  
Snow Model ◽  
Gradient Ratio ◽  
Microwave Signatures

AbstractIn this paper, we show that a detailed snow model (here, the Crocus model) may help to validate large-scale inferred meteorological datasets (e.g. from climate models or analyses) over the data-sparse ice sheets. Two series of snow simulations are carried out with two different meteorological datasets in input to the snow model. Both datasets are extracted from the European Center for Medium-range Weather Forecasts meteorological analyses and forecast archives. First, the microwave signatures of the surface of central Greenland from the Scanning Multichannel Microwave Radiometer (SMMR) are compared with the simulated density, grain-size and stratigraphy. The annual mean gradient ratio and polarization ratio, which are related to the emissivity of snow, are found to correlate spatially with these snow structural parameters. The sensitivity of the snow structure to differences in the two meteorological sets is then examined. It is found to be high for temperature and infrared radiation, precipitation and surface wind. The quantitative value of this result is limited by a still limited snow model validation over Greenland. Also, an optimal use of satellite data and a snow model for meteorological validation would require physically based translation of the simulated snow parameters into radiative properties, i. e radiation transfer modeling.

scholarly journals ESTIMATION OF VERTICAL DUST EMISSION FLUX AT A SITE IN THE MONGOLIAN GOBI DURING A DUST STORM PERIOD

2018 ◽  
pp. 25-40
Author(s):  
Jugder D
Keyword(s):  
Soil Moisture ◽  
Wind Speed ◽  
Dust Storm ◽  
Friction Velocity ◽  
Sonic Anemometer ◽  
Dust Emission ◽  
Ground Level ◽  
Gobi Desert ◽  
Moisture Sensor ◽  
Dust Flux

A meteorological and dust monitoring tower with 20 m height set up at a Nomgon site in Umnugobi Aimag in the Mongolian Gobi in 2010. The Nomgon monitoring tower equipped with wind speed sensors at 2, 4, 10 and 20 m height above the ground level (AGL), a wind direction sensor at 10 m height, a sonic anemometer to measure turbulent momentum flux at 8 m height and a soil moisture sensor at 5 cm depth. We had a purpose to measure dust concentration of PM10 at two levels using Dust-Trak instruments during an intensive observation period (IOP) of a dust event in spring. A dust storm was expected in the Mongolian Gobi from 30 April to 1 May 2016 and two Dust-Traks were set at 0.9 and 2.95 m heights in the tower during this IOP for measuring PM10. Wind data at 2 and 10 m height, three wind components at 8 m height by a sonic anemometer, soil moisture (volumetric water content) data in 5 cm depth and dust concentrations of PM10 at two levels are used in this study. These data from the sensors and instruments in the tower were used for estimation friction velocity and vertical dust flux at the Nomgon site. In association with a surface cyclone, its frontal system and a trough aloft, the expected dust storm occurred in the Mongolian Gobi during the IOP period. Dust concentrations of PM10 increased during the dust storm period due to raised wind speed in the dry conditions of air and soil. The present study aimed to estimate friction velocity (u*) and vertical dust flux (F) around Nomgon site in the Mongolian Gobi desert during the dust storm period. The estimation results were presented in this paper.

scholarly journals Using eddy covariance to measure the dependence of air–sea CO<sub>2</sub> exchange rate on friction velocity

2018 ◽  
Vol 18 (6) ◽  
pp. 4297-4315 ◽  
Author(s):  
Sebastian Landwehr ◽  
Scott D. Miller ◽  
Murray J. Smith ◽  
Thomas G. Bell ◽  
Eric S. Saltzman ◽  
...  
Keyword(s):  
Eddy Covariance ◽  
Friction Velocity ◽  
Gas Transfer ◽  
Transfer Velocity ◽  
Wind Speeds ◽  
Physically Based ◽  
Flux Measurements ◽  
Physically Based Models ◽  
Eddy Covariance Measurements ◽  
Ocean Carbon

Abstract. Parameterisation of the air–sea gas transfer velocity of CO2 and other trace gases under open-ocean conditions has been a focus of air–sea interaction research and is required for accurately determining ocean carbon uptake. Ships are the most widely used platform for air–sea flux measurements but the quality of the data can be compromised by airflow distortion and sensor cross-sensitivity effects. Recent improvements in the understanding of these effects have led to enhanced corrections to the shipboard eddy covariance (EC) measurements.Here, we present a revised analysis of eddy covariance measurements of air–sea CO2 and momentum fluxes from the Southern Ocean Surface Ocean Aerosol Production (SOAP) study. We show that it is possible to significantly reduce the scatter in the EC data and achieve consistency between measurements taken on station and with the ship underway. The gas transfer velocities from the EC measurements correlate better with the EC friction velocity (u*) than with mean wind speeds derived from shipboard measurements corrected with an airflow distortion model. For the observed range of wind speeds (u10 N = 3–23 m s−1), the transfer velocities can be parameterised with a linear fit to u*. The SOAP data are compared to previous gas transfer parameterisations using u10 N computed from the EC friction velocity with the drag coefficient from the Coupled Ocean–Atmosphere Response Experiment (COARE) model version 3.5. The SOAP results are consistent with previous gas transfer studies, but at high wind speeds they do not support the sharp increase in gas transfer associated with bubble-mediated transfer predicted by physically based models.

scholarly journals A large source of dust missing in Particulate Matter emission inventories? Wind erosion of post-fire landscapes

Elem Sci Anth ◽  
2017 ◽  
Vol 5 ◽  
Author(s):  
N. S. Wagenbrenner ◽  
S. H. Chung ◽  
B. K. Lamb
Keyword(s):  
Particulate Matter ◽  
Wind Erosion ◽  
Dust Emission ◽  
Emission Model ◽  
Modeling Framework ◽  
Dust Emissions ◽  
Fire Scar ◽  
Large Source ◽  
Precipitation Regimes ◽  
Flux Measurements

Wind erosion of soils burned by wildfire contributes substantial particulate matter (PM) in the form of dust to the atmosphere, but the magnitude of this dust source is largely unknown. It is important to accurately quantify dust emissions because they can impact human health, degrade visibility, exacerbate dust-on-snow issues (including snowmelt timing, snow chemistry, and avalanche danger), and affect ecological and biogeochemical cycles, precipitation regimes, and the Earth’s radiation budget. We used a novel modeling approach in which local-scale winds were used to drive a high-resolution dust emission model parameterized for burned soils to provide a first estimate of post-fire PM emissions. The dust emission model was parameterized with dust flux measurements from a 2010 fire scar. Here we present a case study to demonstrate the ability of the modeling framework to capture the onset and dynamics of a post-fire dust event and then use the modeling framework to estimate PM emissions from burn scars left by wildfires in U.S. western sagebrush landscapes during 2012. Modeled emissions from 1.2 million ha of burned soil totaled 32.1 Tg (11.7–352 Tg) of dust as PM10 and 12.8 Tg (4.68–141 Tg) as PM2.5. Despite the relatively large uncertainties in these estimates and a number of underlying assumptions, these first estimates of annual post-fire dust emissions suggest that post-fire PM emissions could substantially increase current annual PM estimates in the U.S. National Emissions Inventory during high fire activity years. Given the potential for post-fire scars to be a large source of PM, further on-site PM flux measurements are needed to improve emission parameterizations and constrain these first estimates.

scholarly journals Regional Model Simulations of the Bodélé Low-Level Jet of Northern Chad during the Bodélé Dust Experiment (BoDEx 2005)

2008 ◽  
Vol 21 (5) ◽  
pp. 995-1012 ◽  
Author(s):  
Martin C. Todd ◽  
Richard Washington ◽  
Srivatsan Raghavan ◽  
Gil Lizcano ◽  
Peter Knippertz
Keyword(s):  
Diurnal Cycle ◽  
Climate Models ◽  
Mesoscale Model ◽  
Regional Climate ◽  
Dust Emission ◽  
Horizontal Resolution ◽  
Emission Model ◽  
Low Level ◽  
Low Level Jet ◽  
Key Features

Abstract The low-level jet (LLJ) over the Bodélé depression in northern Chad is a newly identified feature. Strong LLJ events are responsible for the emission of large quantities of mineral dust from the depression, the world’s largest single dust source, and its subsequent transport to West Africa, the tropical Atlantic, and beyond. Accurate simulation of this key dust-generating atmospheric feature is, therefore, an important requirement for dust models. The objectives of the present study are (i) to evaluate the ability of regional climate models (RCMs) and global analyses/reanalyses to represent this feature, and (ii) to determine the driving mechanisms of the LLJ and its strong diurnal cycle. Observational data obtained during the Bodélé Dust Experiment (BoDEx 2005) are utilized for comparison. When suitably configured, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) RCM can represent very accurately many of the key features of the jet including the structure, diurnal cycle, and day-to-day variability. Surface winds are also well reproduced, including the peak winds, which activate dust emission. Model fidelity is, however, strongly dependent on the boundary layer parameterization scheme, surface roughness, and vertical resolution in the lowest layers. A model horizontal resolution of a few tens of kilometers is sufficient to resolve most of the key features of the LLJ, while in global analyses/reanalyses many features of the LLJ are not adequately represented. Idealized RCM simulations indicate that under strong synoptic forcing the surrounding orography of the Tibesti and Ennedi Mountains acts to focus the LLJ onto the Bodélé and to accelerate the jet by ∼40%. From the RCM experiments it is diagnosed that the pronounced diurnal cycle of the Bodélé LLJ is largely a result of varying eddy viscosity, with elevated heating/cooling over the Tibesti Mountains to the north as a second-order contribution.

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