Sensitivity of Moist Convection Forced by Boundary Layer Processes to Low-Level Thermodynamic Fields

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

The sensitivity of moist convection to a number of low-level thermodynamic parameters is examined with a high-resolution, nonhydrostatic numerical model. The parameters examined are the surface temperature dropoff (defined as the difference between the potential temperature measured at the surface and that in the well-mixed boundary layer), the surface moisture dropoff (defined similarly for moisture), the boundary layer moisture dropoff (defined as the vertical decrease in moisture within the boundary layer), and the depth of the moisture. The typical variability in these parameters is estimated from two field experiments in northeastern Colorado. Sensitivity is then defined relative to this typical observational variability.

Two convection initiation cases from northeastern Colorado are examined. In both cases, convection initiation is found to be most sensitive to the surface temperature dropoff and the surface moisture dropoff. It is found that variations in boundary layer temperature and moisture that are within typical observational variability (1°C and 1 g kg−1, respectively) can make the difference between no initiation and intense convection. For cases in which convection is well developed, the storm's strength is more sensitive to the typical observational variability in moisture than in temperature. However. at the convection/no convection boundary, the storm's strength is more sensitive to the surface temperature dropoff than to the surface moisture dropoff (both in terms of equivalent moist static energy and, for the cases studied, in terms of typical observational variability). It is shown that this is due to the greater sensitivity of the negative area (or convective inhibition) to temperature variations than to moisture variations. The implications of these results for the predictability of convection initiation are then briefly discussed.

Abstract

The sensitivity of moist convection to a number of low-level thermodynamic parameters is examined with a high-resolution, nonhydrostatic numerical model. The parameters examined are the surface temperature dropoff (defined as the difference between the potential temperature measured at the surface and that in the well-mixed boundary layer), the surface moisture dropoff (defined similarly for moisture), the boundary layer moisture dropoff (defined as the vertical decrease in moisture within the boundary layer), and the depth of the moisture. The typical variability in these parameters is estimated from two field experiments in northeastern Colorado. Sensitivity is then defined relative to this typical observational variability.

Two convection initiation cases from northeastern Colorado are examined. In both cases, convection initiation is found to be most sensitive to the surface temperature dropoff and the surface moisture dropoff. It is found that variations in boundary layer temperature and moisture that are within typical observational variability (1°C and 1 g kg−1, respectively) can make the difference between no initiation and intense convection. For cases in which convection is well developed, the storm's strength is more sensitive to the typical observational variability in moisture than in temperature. However. at the convection/no convection boundary, the storm's strength is more sensitive to the surface temperature dropoff than to the surface moisture dropoff (both in terms of equivalent moist static energy and, for the cases studied, in terms of typical observational variability). It is shown that this is due to the greater sensitivity of the negative area (or convective inhibition) to temperature variations than to moisture variations. The implications of these results for the predictability of convection initiation are then briefly discussed.

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