Introduction

1. strength of the wind
2. nature of surface materials

Each surface has a critical wind-strength threshold at which materials will begin to move. This threshold depends on the type and size of surface particles, soil moisture content, and the presence (or absence) of protective vegetation.

• High susceptibility toward erosion: Dry barren sands and soils.

• Low susceptibility toward erosion: Surfaces with high soil moisture content, cementation, and/or layer of protective vegetation.

Objectives

Investigate how natural climate variability, land-use changes, and human-induced climate change affect wind erosion and the regional transport of dust. Critical issues:

• How does wind strength vary with natural climate cycles on decadal and century timescales? Wind strength during the spring may be sensitive to changes in the jet stream. How has this natural variability affected the wind’s ability to erode and transport surface materials?

• If climate does change as a result of human activities, to what extent will winds become stronger or weaker? How will this affect the wind’s ability to erode and transport surface materials?

• How have climate-related soil moisture and vegetation changes affected wind erosion in the past? What can we expect under future climatic and land-use scenarios?

Modeling Studies

We are using numerical models of the atmosphere to help investigate some of these questions.

• ABL Model

  Surface materials move in response to the frictional force (also known as the shear stress on the surface) between the moving atmosphere and the ground. Our sophisticated atmospheric boundary layer (ABL) model will calculate the frictional force for all atmospheric conditions in the southwestern US. The frictional force depends principally on:

  1. wind speed
  2. degree of atmospheric stability
  3. surface roughness

  Figure 1. Sensitivity of the frictional force (shear stress) imposed by the atmosphere on the land surface to windspeed (100 m above the surface) and atmospheric stability. Also shown are the critical shear stresses needed to initiate wind erosion and particle transport for various barren surfaces found in the Southwest.

  For any given surface, this model can be used to determine:

  1. Whether the frictional force exceeds that needed to set surface materials in motion.

  2. The rate at which materials are being transported by the wind.

  Figure 2. The shear stress on the surface depends on the rate at which the wind speed increases with height above the surface. This figure shows the dependence of the wind speed increase with height to atmospheric stability.

• Model Validation

  • Specific Test Sites: Meteorological data (e.g., wind speed, temperature, atmospheric stability, soil moisture, particle fluxes) are being collected at CLIM-MET and GEOMET stations to help validate/calibrate the ABL model at specific sites.

  • Extrapolating to the Rest of the Southwest: Between the test sites where we have both detailed surface property and meteorological data, we will:

    • Use a combination of published maps (e.g. geologic, soil, vegetation maps) and new remote sensing data to characterize present-day surface properties to identify areas currently vulnerable to wind erosion.

    • Use a regional climate model (e.g., NCAR’s RegCM2) to predict upper-level wind speeds between the MET stations for specific dates. These wind speeds will then be fed into the ABL model to calculate frictional forces and particle fluxes and thereby predict those areas where wind erosion and particle transport should have occurred on those dates.

    • Compare model predictions with remote sensing data. Does the model correctly predict surface-material movement and emission into the atmosphere?

• Forecasting and Hindcasting

  • Once validated, the model will be linked with a regional climate model and an interactive vegetation package to forecast how the critical wind-erosion wind speeds and particle fluxes change with various climate and land-use scenarios.

  • Climate scenarios to be investigated include: decade-long or multi-decadal droughts, periods of known dune reactivation, periods of enhanced or diminished El Niños, and 2xCO₂.

  • 2xCO₂ projections for the Southwest suggest a decrease in the diurnal temperature range (which will affect the range of atmospheric stability values) and a decrease in soil moisture (particularly during the winter). The latter effect may lead to enhanced wind erosion in the future.