500 Convective Cold Pool Responses to Soil Moisture

Tuesday, 9 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Aryeh J. Drager, Colorado State Univ., Fort Collins, CO; and L. D. Grant and S. C. van den Heever

Convective cold pools over tropical continental regions are formed when downdrafts carry evaporatively cooled air from aloft down to the surface. This air then spreads laterally, displacing the surrounding environmental air and potentially triggering new convection while simultaneously boosting surface sensible and latent flux magnitudes through the enhancement of surface winds. In modeling studies, cold pools have been observed to generate “rings” of enhanced near-surface water vapor. These rings can combine with the mechanical triggering at cold pool boundaries to make the gust front region particularly favorable for the development of new convection. While much research has focused on cold pools’ effects on their environments, fewer studies have investigated the effects of environmental factors on the cold pools that form.

The present work seeks to elucidate the role of soil moisture in determining cold pool properties. How does soil moisture affect cold pool size, cold pool strength, and the structure of the water vapor field that accompanies cold pools, and what are the implications for convective organization?

These questions are explored using an idealized modeling framework. The Regional Atmospheric Modeling System (RAMS), coupled to the LEAF-3 interactive land surface model, is used to conduct several single-day simulations, each initialized with different soil moisture content (25%, 50%, and 75% of soil saturation, hereafter DRY-SOIL, MID-SOIL, and WET-SOIL, respectively). The dimensions of the model domain are 150 km × 150 km × 26 km, the horizontal grid spacing is 500 m, and the vertical grid is stretched from 50 m near the surface to 500 m aloft. These idealized simulations are performed with a rainforest-type surface, periodic lateral boundaries, and no Coriolis acceleration.

A cold pool identification and tracking algorithm is used to evaluate cold pool properties, and a composite cold pool is generated for each simulation. Differences between the composite cold pools are then calculated in order to reveal trends. From these trends, the cold pool responses to soil moisture—and the physical mechanisms thereof—are assessed, and the implications of these responses are explored.

It is found that the DRY-SOIL simulation produces cold pools that are 10-15% larger and ~20% stronger than those in the WET-SOIL simulation. Examination of the near-surface water vapor field reveals that water vapor rings form in all simulations but are less pronounced in the DRY-SOIL simulation than in the other two. Furthermore, the water vapor rings in the DRY-SOIL simulation are not collocated with the gust front but are rather displaced inward toward cold pool center. These results have mixed implications for the formation of subsequent convection in the gust front region in the DRY-SOIL simulation: although the mechanical triggering is stronger, the resulting updrafts are drier. The trends are highly non-linear, with the results from the MID-SOIL simulation closely mirroring those of the WET-SOIL simulation. This non-linearity is attributed to differences in stomatal resistance, which increases sharply from the MID-SOIL to the DRY-SOIL simulation.

The results of this study demonstrate the need to better understand and represent soil moisture impacts on cold pools and hence on subsequent convective initiation.

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