Wind-terrain effects on firebrand dynamics

Abstract The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. This study evaluates an apparent tornado activity maximum across the Sand Mountain and Lookout Mountain plateaus of northeastern Alabama. These plateaus, separated by the narrow Wills Valley, span ~5000 km2 and were impacted by 79 tornadoes from 1992 to 2016. This area represents a relative regional statistical maximum in tornadogenesis, with a particular tendency for tornadogenesis on the northwestern side of Sand Mountain. This exploratory paper investigates storm behavior and possible physical explanations for this density of tornadogenesis events and tornadoes. Long-term surface observation datasets indicate that surface winds tend to be stronger and more backed atop Sand Mountain than over the adjacent Tennessee Valley, potentially indicative of changes in the low-level wind profile supportive to storm rotation. The surface data additionally indicate potentially lower lifting condensation levels over the plateaus versus the adjacent valleys, an attribute previously shown to be favorable for tornadogenesis. Rapid Update Cycle and Rapid Refresh model output indicate that Froude numbers for the plateaus in tornadic environments are likely supportive of enhanced low-level flow over the plateaus, which further indicates the potential for favorable wind profile changes for tornado production. Examples of tornadic storms rapidly acquiring increased low-level rotation while reaching the plateaus of northeast Alabama are presented. The use of this background to inform the VORTEX-SE 2017 field campaign is discussed.

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A Study of Two Propagating Heavy-Rainfall Episodes near Taiwan during SoWMEX/TiMREX IOP-8 in June 2008. Part II: Sensitivity Tests on the Roles of Synoptic Conditions and Topographic Effects

Monthly Weather Review ◽

10.1175/mwr-d-13-00330.1 ◽

2014 ◽

Vol 142 (8) ◽

pp. 2644-2664 ◽

Cited By ~ 13

Author(s):

Chung-Chieh Wang ◽

Jason Chieh-Sheng Hsu ◽

George Tai-Jen Chen ◽

Dong-In Lee

Keyword(s):

Heavy Rainfall ◽

Warm Season ◽

Southwest Monsoon ◽

First Episode ◽

Topographic Effects ◽

Terrain Effects ◽

Sensitivity Tests ◽

Synoptic Conditions ◽

Synoptic Forcing ◽

Central United States

Abstract This study is the second of a two-part series to investigate two rainfall episodes in the Hovmöller space near Taiwan during the eighth intensive observing period (IOP-8, 12–17 June 2008) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). The first episode moved eastward and the second westward, and both caused heavy rainfall in Taiwan. The goal of Part I was to better understand the mechanism and controlling factors for the organization and propagation of the episodes. Here in Part II, the detailed roles played by synoptic conditions and terrain effects are further examined. Three sensitivity tests (at 2.5-km grid spacing) are designed to include only the effects of synoptic evolution (SNP), and those from land–sea distribution–diurnal variations on top of a mean background with/without topography (DIU/DNT). As the benchmark, the control (CTL) experiment captures the 6-day event successfully and is validated in Part I. In SNP, the two episodes are reproduced with overall similarity to CTL and the observation, and this confirms that the general location/time of rainfall are mainly controlled by synoptic forcing in this case, in contrast to typical warm-season conditions in the central United States. Even so, diurnal effects can still exert discernible impacts and modulate local convective development in many instances, particularly an afternoon enhancement over terrain, and the averaged diurnal cycle in CTL over southeastern China resembles those in DIU/DNT rather than that in SNP (with no land). The steep topography of Taiwan is especially important for its rainfall distribution, including the heavy rainfall on 16 June through processes as postulated by Xu et al.

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