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The Effect of Soil Moisture on Slope Flows

The Effect of Soil Moisture on Slope Flows. Jeff Massey Sept. 18, 2012. Theory. For a given slope, the temperature gradient between the surface and the free atmosphere ultimately controls the strength and structure of the slope flows.

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The Effect of Soil Moisture on Slope Flows

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  1. The Effect of Soil Moisture on Slope Flows Jeff Massey Sept. 18, 2012

  2. Theory For a given slope, the temperature gradient between the surface and the free atmosphere ultimately controls the strength and structure of the slope flows The skin temperature iscontrolled by the surface energy budget

  3. Daytime Surface Energy Budget Diagram Net radiation at surface after effects of albedo and LW emission Latent Heat Flux. Energy used to evaporate water from soil or plants Sensible Heat Flux Radiation heats surrounding air Ground Heat Flux. Energy heats soil below the surface. Ground acts as heat sink How does soil moisture affect the partitioning of the different heat flux terms?

  4. High Soil moisture Net radiation Latent Heat Flux Sensible Heat Flux Ground Heat Flux Latent Heat Flux increases – more water for evaporation Ground Heat Flux increases – soil thermal conductivity, k, increases with increasing water content Sensible heat flux decreases – less energy available -> temperature gradient decreases so upslope flows decrease in magnitude

  5. At night, higher ground heat storage will lead to less surface cooling and thus weaker downslope flow. This overpowers the contribution of cooling from latent heat Thermal Conductivity • Thermal conductivity, k, increases with increasing SMC regardless of parameterization • GHF increases with increasing k

  6. Previous modeling work • Ookouchi et al. (1984) – moist slope produces a 33% reduction in peak upslope flow compared to a dry slope • Used Pielke(1974) incompressible primitive equation model • 5km horizontal resolution and 14 vertical levels (5m,15m, 50m, 100m, …, 7000m) • Tested idealized slopes and flat terrain • Banta and Gannon (1995) – downslope flow over a moist slope is weaker than over a dry slope • RAMS non-hydrostatic, quasi-boussinesq • 100m horizontal resolution and ~44m vertical resolution • Tested idealized 2-D 750m hill

  7. Can the modern WRF model replicate these results? Can it offer any new insights? Model Configuration Slope shading and slope radiation modification turned on 6th order monotonic diffusion turned on 33 updated landuse categories Vertical velocity damping turned on

  8. Innermost domain Issues: Slope not uniform Close proximity to GSL and playa Soil and vegetation types vary

  9. Experiment Design

  10. control • early morning 4 m/sdownslope flow on west side, weak on east side • easterly flow to 700m AGL • 1 – 2 m/s upslope flow starts around 16:20Z • Light westerly flow until 2Z when lake breeze starts • 4 m/sdownslope flow redevelops around 3Z

  11. No smc • Much Stronger westerly flow around midday • Lake breeze starts earlier at 0Z now • Stronger 6 m/sdownslope flow • Stronger nighttime inversions

  12. maxsmc • Extremely weak flow in the afternoon • Much weaker, if any, lake breeze • Downslope flow only on west slope

  13. Wind direction Upslope Flow - Not captured in moist run Downslope flow -captured by all runs Wind Speed • 1-3 m/s upslope, strongest late morning/ early afternoon • 4 -8 m/sdownslope, strongest just after sunset

  14. 2-m temperature verification

  15. Control Run

  16. Control Run No Soil Moisture

  17. No Soil Moisture Max Soil Moisture

  18. Conclusions • Downslope and upslope flows are generally weaker over moist slopes compared to dry slopes • Soil moisture also affects other thermally driven flows such as playa and lake breezes • My coarse resolution WRF runs simulated slope flows, mainly on the west facing slope

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