The Scale Dependency of the Soil Moisture-Convection Coupling during HI-SCALE
Abstract
The impact of soil moisture gradients on convective cloud development and the scale-dependency of the existing land-atmosphere coupling metrics are studied. We use numerical simulations with a 300 m grid spacing over a large (300 km wide) domain, centered at Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains site. Simulations are run for August 30, 2016, when shallow-to-deep convection transition was observed during the Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HI-SCALE) field campaign. The Findell and Eltahir framework for soil moisture-convection feedback suggests that higher evaporative fraction is more conducive to convection, or positive feedback, based on the morning soundings on this day. However, our simulation using the observed soil moisture pattern as the initial condition indicates negative feedback by forming convective clouds preferentially over drier soils. The negative relationship is also obtained by comparing simulations with spatially uniform soil moisture and those with the observed or even stronger soil moisture gradients, but this relationship only becomes clear at spatial scales larger than ~10 km. Spectral analysis shows that these larger scales coincide with the dominant spatial scales of the observed soil moisture, which in turn may reflect the underlying topography. Similar power spectra are seen in the latent and sensible heat fluxes as well as in the kinetic energy of the boundary layer winds throughout the day, while that of the vertical wind (w) shifts its dominant scales from smaller to larger ones as the day progress. The latter indicates the transition from the boundary-layer eddies to more organized flows associated with the downdrafts from the precipitating clouds and updrafts at the cold pool boundaries as the dominant form of w. The analysis of the w-momentum budget shows that the advection terms become much smaller than the buoyancy force at the larger scales as well. Our results suggest that the one-dimensional view used in the Findell-Eltahir framework may be valid at larger spatial scales in our case study, except that the soil moisture gradient leads to a negative relationship between convective clouds and evaporative fraction.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A41L2751S
- Keywords:
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- 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3379 Turbulence;
- ATMOSPHERIC PROCESSES