A numerical investigation of the effects of dry air aloft on deep convection
Open Access
- Author:
- James, Richard
- Graduate Program:
- Meteorology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 18, 2008
- Committee Members:
- Paul Markowski, Dissertation Advisor/Co-Advisor
Paul Markowski, Committee Chair/Co-Chair
Yvette Pamela Richardson, Committee Member
James Michael Fritsch, Committee Member
Charles Luther Hosler Jr., Committee Member
Jonathan P Mathews, Committee Member - Keywords:
- convection
severe storms
dry air aloft - Abstract:
- A high-resolution numerical model was used to investigate the direct effects of dry air above cloud base in the environment of convective storms. Simulations of both quasi-linear convective systems and supercells were performed in which the relative humidity in a midlevel dry layer was varied while preserving the buoyancy profile and CAPE. The presence of dry air caused a reduction in overall storm intensity, as measured by updraft mass flux, total condensation and total rainfall; the reduction was more dramatic at lower values of CAPE. In high-CAPE line-type simulations, the downdraft mass flux and cold pool strength were enhanced at the rear of the trailing stratiform region in a drier environment. However, the downdraft and cold pool strengths were unchanged in the convective region, and were also unchanged or reduced in simulations of supercells and of line-type systems at lower CAPE. The effects of dry air aloft are interpreted in terms of the reduction in the rate of updraft entrainment when dry air is present, leading to decreased mass flux and a lower rate of condensate production. Smaller hydrometeor mixing ratios then exert a negative influence on the latent cooling rates associated with phase changes in the downdraft formation regions. This effect offsets the enhancement of rain evaporation that is expected to occur in a drier environment and thereby prevents the strengthening of downdrafts and low-level outflow. The modeling results reported here are inconsistent with the widespread notion that dry air aloft is favorable for stronger downdrafts and greater low-level outflow intensity. A review of the literature is presented in which it is shown that observational evidence does not unambiguously support the necessity of dry air aloft for strong downdrafts and outflow. The relative importance of environmental humidity, temperature and stability are examined, and it is shown that some observational studies may have overemphasized the role of environmental humidity. Thus a re-interpretation of previous work, along with the numerical results presented here, suggest that dry air aloft is unlikely to be a critical ingredient for intense convectively-generated straight-line winds.