Peer review of: A wind-albedo-wind feedback driven by landscape evolution (2020)

Reviewers' comments:  

Reviewer #1 (Remarks to the Author):  

What are the major claims of the paper?  
The manuscript proposes and investigates the hypothesis that wind changes the surface albedo in 
arid regions as fine sediments are eroded by wind, which in turn will reduce the albedo as the 
remaining coarse sediment (e.g. stones) may have a darker color, which in turn will change the 
thermal properties of the surface. In topographic basins (bright surface; higher albedo) 
surrounded by (darker) mountains (lower albedo), the reduced albedo of the basin's bottom 
sediments may reduce the difference in albedo between mountain range and basin. As the albedo 
relates to the amount of energy a surface absorbs, a spatial gradient in albedo can be referred to a 
thermal gradient, which in turn results into a local pressure gradient consequently modulating the 
regional wind fields. A reduced albedo gradient may lead to lower surface wind speeds for cases 
where the albedo gradient enforces the local wind.  

To test the hypothesis, the authors used the atmosphere model WRF over part of the western Gobi 
Desert. As the Gobi Desert is characterized by stony surfaces, this land surface type may have 
undergone an evolution fostered by erosion ultimately lowering the albedo at millennial time scale. 
To test for the albedo impact solely, the albedo is the only parameter that was changed.  

The manuscript is clearly structured and the study is well motivated. The data description is 
technically sound and the authors provide sufficient information on model setup. The model data 
itself supports the hypothesis of the wind-albedo-wind feedback and proofs the concept - which 
can be expected as the WRF model is designed to do so. However, as any model validation (e.g. 
against observational data sets) is missing, it remains open how well the reference model setup 
represents the actual conditions.  

Are the claims novel?  
The individual components of the hypothesis are known and the acknowledged by the authors. The 
authors rather suggest, that these known albedo feedbacks are not accurately included in climate 
models and therefore contribute to uncertainties in wind speed reconstructions for ancient 
climates.  

In order to underline the importance of accounting for landscape evolution by (wind) erosion, I 
wish the authors could provide more information on the significance of the change in wind speed 
due to change in albedo. As the results are shown for a rather small area and, and no interactions 
on e.g. synoptic scale circulation are discussed, the significance remains somewhat limited to a 
proof a concept that is known. Furthermore, having some information on the performance of the 
model respective how realistic the wind speed distribution over the Tarim Basin is represented may 
help to illustrate the significance.  

Will the paper be of interest to others in the field? Will the paper influence thinking in the field?  
The paper underlines the importance of an up-to-date surface albedo implemented in atmosphere 
models. Whereas model simulations covering the satellite era can be updated by satellite retrieved 
albedo maps, albedo maps used for the past are calculated from e.g. vegetation models 
accounting for a change in vegetation type and cover. Change in soil conditions e.g. due to erosion 
are less regarded. Hence, the authors point towards an important issue. However, it feels the main 
arguments are only briefly brought up and not as thoroughly discussed as could be. Maybe the 
final paragraph may be used to provide a focused conclusion rather than a general statement.  

I therefore wish the authors to strengthen their arguments by e.g. illustrating why the differences 
in wind speed matter. To someone not into paelo-wind reconstructions, what are the (quantitative) 
consequences of a monthly averaged change in wind speed of 0.75 m/s (Fig. 3) e.g. for the total 
dust emission flux? Maybe you can find a clever way to illustrate hwo much it matters? You may 
want consider including variables other than wind speed too. E.g. (potential) temperature, 
turbulent kinetic energy (TKE), and the Bowen ratio reflecting the characteristics of the planetary 
boundary layer determining the gustiness. Furthermore, how strongly are synoptic scale features 
affected by local scale albedo changes? This may contribute to the general discussion on the 
impact as well as regional atmospheric circulation and local wind systems are superimposed.  



Reviewer #2 (Remarks to the Author):  

The paper presents a feedback mechanism between thermal-driven winds and surface sediment 
conditions, whereby the development of surface armouring from wind-driven soil-erosion leads to 
more intense thermal-driven winds. The paper presents weather model simulations using the 
different surface conditions to show the impact on wind patterns and strengths.  
The wind simulations are very thorough and insightful, and the novelty claim lies in the bedrock-
atmosphere feedback (as per line 152).  
The crux of the hypothesis lies in the process of the surface armouring to develop. The armouring 
process is described in general in line 77-78, and the site-specific history in lines 89-93. On the 
general process the paper cites four sources, three of which are severely outdated (from 1904, 
1968, and 1970) and the fourth reflects a local study. Current understanding of desert pavements 
suggests that they are actively maintained and indeed accumulate underneath them aeolian silt 
and dust deposits. This contemporary understanding started with the work by McFadden et al. 
(1987) and is reflected in later work by, for example, Pelletier et al. (2007). The previous site-
specific study by the authors cited here (Pullen et al., 2018) presents a clearer picture of the 
development of the surface armouring, suggesting that wind erosion has removed layers of playa 
lake (silt and clay) sediments until interlaced conglomerate layers are exposed and no further 
(aeolian) erosion is possible.  

On the whole this hypothesis is reasonable and the consequences of the changes in surface 
conditions over time in terms of thermally-driven wind strength appears logical.  
The paper presents credible evidence that the bedrock-atmosphere feedback mechanism has 
significant impacts on paleo-climate investigations and dust source and sink identification, both in 
the specific site studied here as well as elsewhere around the globe.  
I recommend that the paper is published, provided that the authors update the description and 
literature on surface armouring and desert pavements to the contemporary understanding, and 
that the authors reflect in the text on the possible role of the local surface armouring acting as a 
potential dust trap.  

McFadden LD, Wells SG, Jercinovich MJ. 1987. Influences of eolian and pedogenic processes on the 
origin and evolution of desert pavements. Geology 15: 504–508.  
Pelletier, J.D., Cline, M. and DeLong, S.B., 2007. Desert pavement dynamics: numerical modeling 
and field‐based calibration. Earth Surface Processes and Landforms: The Journal of the British 
Geomorphological Research Group, 32(13), pp.1913-1927.  



Reviewer #3 (Remarks to the Author):  

This research presents a compelling hypothesis that the formation of gravel-mantled surfaces 
reduces the albedo of desert surfaces, thus resulting in an increase in wind speeds. To my 
knowledge, this hypothesis has not been previously considered in the literature and represents a 
novel advancement in our knowledge of landscape evolution. This hypothesis is supported by 
fieldwork and observations that deflation results in the formation of extensive, densely-packed 
gravel surfaces (Pullen et al., 2018) and modeling that isolates one variable – albedo – to 
demonstrate how wind speed increases with a decrease in albedo that resulted from the formation 
of gravel-covered surfaces (results presented in this paper). This result is not surprising; however, 
this is a unique finding with broad implications for modeling wind regimes past and present. As the 
authors note, this wind-albedo feedback extends beyond the area of gravel cover, potentially 
impacting large desert areas. While the gravel surfaces themselves become less important dust 
sources over time, the other landforms and soils adjacent to these gravel surfaces may be 
susceptible to wind erosion as a result of this feedback. I find this to be a significant advancement 
in how we think about landscape dynamics, dust emissions, and feedbacks in deserts. The article is 
well-written. I would like to pose two questions for the authors to consider:  

1. As gravel surfaces coalesce into pavements, their surface roughness can decrease (for example, 
Frankel and Dolan, 2007, Characterizing arid region alluvial fan surface roughness with airborne 
laser swath mapping digital topographic data, JGR-ES v. 112), thus also potentially resulting in 
increased wind speeds at the surface. Surface roughness plays a considerable role in surface wind 
speed (and dust emission), and is only mentioned briefly in this study.  

2. As gravel surfaces mature, they darken by accumulating desert varnish, significantly reducing 
albedo. Are the gravel surfaces in the Hami varnished, or natural rock color? The oldest gravel 
surfaces with well-developed varnish may have the lowest albedo, whereas younger gravel 
surfaces with weak varnish may have slightly higher albedo.  
How might these two variables also contribute to increasing wind speeds over time? It seems there 
may be multiple feedbacks that may combine to result in increased wind speeds over time. The 
basic concept as presented (albedo-wind feedback) is significant, but I think the overall story could 
be improved by acknowledging these two other important and related variables.