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Abstract Observed supercell updrafts consistently produce the fastest mid- to upper-tropospheric vertical velocities among all modes of convection. Two hypotheses for this feature are investigated. In the dynamic hypothesis, upward, largely rotationally driven pressure gradient accelerations enhance supercell updrafts relative to other forms of convection. In the thermodynamic hypothesis, supercell updrafts have more low-level inflow than ordinary updrafts because of the large vertical wind shear in supercell environments. This large inflow makes supercell updrafts wider than that of ordinary convection and less susceptible to the deleterious effects of entrainment-driven updraft core dilution on buoyancy. These hypotheses are tested using a large suite of idealized supercell simulations, wherein vertical shear, CAPE, and moisture are systematically varied. Consistent with the thermodynamic hypothesis, storms with the largest storm-relative flow have larger inflow, are wider, have larger buoyancy, and have faster updrafts. Analyses of the vertical momentum forcing along trajectories shows that maximum vertical velocities are often enhanced by dynamic pressure accelerations, but this enhancement is accompanied by larger downward buoyant pressure accelerations than in ordinary convection. Integrated buoyancy along parcel paths is therefore a strong constraint on maximum updraft speeds. Thus, through a combination of processes consistent with the dynamic and thermodynamic hypotheses, supercell updrafts are able to realize a larger percentage of CAPE than ordinary updrafts.
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Environmental Controls on Simulated Deep Moist Convection Initiation Occurring during RELAMPAGO-CACTI
Abstract This study synthesizes the results of 13 high-resolution simulations of deep convective updrafts forming over idealized terrain using environments observed during the RELAMPAGO and CACTI field projects. Using composite soundings from multiple observed cases, and variations upon them, we explore the sensitivity of updraft properties (e.g., size, buoyancy, and vertical pressure gradient forces) to influences of environmental relative humidity, wind shear, and mesoscale orographic forcing that support or suppress deep convection initiation (CI). Emphasis is placed on differentiating physical processes affecting the development of updrafts (e.g., entrainment-driven dilution of updrafts) in environments typifying observed successful and null (i.e., no CI despite affirmative operational forecasts) CI events. Thermally induced mesoscale orographic lift favors the production of deep updrafts originating from ∼1- to 2-km-wide boundary layer thermals. Simulations without terrain forcing required much larger (∼5-km-wide) thermals to yield precipitating convection. CI outcome was quite sensitive to environmental relative humidity; updrafts with increased buoyancy, depth, and intensity thrived in otherwise inhospitable environments by simply increasing the free-tropospheric relative humidity. This implicates the entrainment of free-tropospheric air into updrafts as a prominent governor of CI, consistent with previous studies. Sensitivity of CI to the environmental wind is manifested by 1) low-level flow affecting the strength and depth of mesoscale convergence along the terrain, and 2) clouds encountering updraft-suppressing pressure gradient forces while interacting with vertical wind shear in the free troposphere. Among the ensemble of thermals occurring in each simulation, the widest deep updrafts in each simulation were the most sensitive to environmental influences.
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- Award ID(s):
- 1661707
- PAR ID:
- 10406688
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 79
- Issue:
- 7
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- p. 1941-1964
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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