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 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 Atmospheric dynamics drive most interannual U.S. droughts over the last millennium

2020 ◽  
Vol 6 (32) ◽  
pp. eaay7268
Author(s):  
M. P. Erb ◽  
J. Emile-Geay ◽  
G. J. Hakim ◽  
N. Steiger ◽  
E. J. Steig
Keyword(s):  
American West ◽  
Large Scale ◽  
Sea Surface ◽  
System Model ◽  
Drought Risk ◽  
Last Millennium ◽  
Atmospheric Variability ◽  
The Past ◽  
Small Influence ◽  
Community Earth System Model

The American West exemplifies drought-sensitive regions with growing populations. Paleoclimate investigations have documented severe droughts in this region before European settling, with major implications for water management and planning. Here, we leverage paleoclimate data assimilation to reconstruct past climate states, enabling a large-scale multivariate investigation of U.S. drought dynamics over the last millennium. These results confirm that La Niña conditions significantly influence southwest U.S. drought over the past millennium but only account for, by one metric, ~13% of interannual drought variability in that region. Atlantic sea surface temperatures may also contribute a small influence, but unexplained variability suggests a substantial role for internal atmospheric variability. This conclusion is buttressed by analysis of simulations from the Community Earth System Model Last Millennium Ensemble. While greenhouse gases will increase future drought risk, as shown in other work, interannual U.S. drought variations will also be widely influenced by processes internal to the atmosphere.

scholarly journals Multidecadal preconditioning of the Maud Rise polynya region

Ocean Science ◽  
2020 ◽  
Vol 16 (6) ◽  
pp. 1443-1457
Author(s):  
René M. van Westen ◽  
Henk A. Dijkstra
Keyword(s):  
High Resolution ◽  
Southern Ocean ◽  
Large Scale ◽  
Heat Content ◽  
Earth System Model ◽  
System Model ◽  
Earth System ◽  
Model Resolution ◽  
Community Earth System Model ◽  
Maud Rise

Abstract. In this paper, we consider Maud Rise polynya formation in a long (250-year) high-resolution (ocean 0.1∘, atmosphere 0.5∘ horizontal model resolution) of the Community Earth System Model. We find a dominant multidecadal timescale in the occurrence of these Maud Rise polynyas. Analysis of the results leads us to the interpretation that a preferred timescale can be induced by the variability of the Weddell Gyre, previously identified as the Southern Ocean Mode. The large-scale pattern of heat content variability associated with the Southern Ocean Mode modifies the stratification in the Maud Rise region and leads to a preferred timescale in convection through preconditioning of the subsurface density and consequently to polynya formation.

scholarly journals Tropical Cyclone Genesis Factors in a Simulation of the Last Two Millennia: Results from the Community Earth System Model

2015 ◽  
Vol 28 (18) ◽  
pp. 7182-7202 ◽  
Author(s):  
Qing Yan ◽  
Robert Korty ◽  
Zhongshi Zhang
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Earth System Model ◽  
System Model ◽  
Earth System ◽  
Entire Interval ◽  
Community Earth System Model ◽  
The Last Two Millennia

Abstract Using a coupled global climate model, Community Earth System Model (CESM), the authors investigate the response of tropical cyclone (TC) genesis factors (i.e., potential intensity, vertical wind shear, midtropospheric moisture content, and absolute vorticity) to external forcings in the last two millennia (L2M). They then examine how the large-scale conditions that favor TC activity varied using a genesis potential index (GPI). These large-scale genesis factors generally exhibit no long-term trend in the simulation of the L2M prior to the industrial revolution, and the spread in the interannual variability lies within a small window. The estimated TC activity is highly variable from region to region on multidecadal time scales. Conditions appear to be more favorable for TC genesis in the twentieth century in the Northern Hemisphere relative to earlier centuries of the L2M. Additionally, conditions in this simulation are not more favorable for TC formation during the Medieval Climate Anomaly (AD 1000–1200) relative to the Little Ice Age (AD 1500–1700) except in the eastern North Pacific and south Indian Ocean. Although a comparison of conditions simulated in the model with proxy-based reconstructions of prehistoric storm activity finds agreement during several active periods in the western North Pacific, the time series of simulated genesis factors does not match that of proxy reconstructions over the entire interval in either the western North Pacific or North Atlantic; this discrepancy likely arises from uncertainties in both the model and reconstructions.

Improving the parameterization of dust emission threshold in the Community Earth System Model (CESM)

2021 ◽  
Author(s):  
Danny Min Leung ◽  
Jasper Kok ◽  
Longlei Li ◽  
Natalie Mahowald ◽  
Laurent MENUT ◽  
...  
Keyword(s):  
Dust Emission ◽  
Earth System Model ◽  
System Model ◽  
Earth System ◽  
Community Earth System Model ◽  
Emission Threshold
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