10A.2 Lower-Tropospheric Conditions Influencing the Initiation of Severe Convection near Terrain Features in a Convection-Allowing Ensemble

Wednesday, 24 October 2018: 2:15 PM
Pinnacle C (Stoweflake Mountain Resort )
Stanley B. Trier, NCAR, Boulder, CO; and G. Romine, D. Ahijevych, and R. A. Sobash

The initiation of isolated severe convection away from synoptic or antecedent
outflow boundaries is an important and challenging forecast problem. Such
storms are initiated more regularly near terrain features. However, their
initiation is often quite sensitive to lower-tropospheric flow conditions.
In the current study we use a 50 member convection-allowing ensemble to examine
effects on convection initiation (CI) owing to the interaction of
lower-tropospheric flow with relatively modest terrain in the immediate lee of
the Rocky Mountains. The case selected occurred on 4 June 2015, when several
isolated supercell storms initiated during mid-to-late afternoon near the
northern portion of the Palmer Ridge, which is a ~500-m deep zonally
oriented terrain feature in east-central Colorado. To diagnose conditions most
favorable for storm development, the 50-member ensemble is divided into
two 10 member subensembles. One consists of 10 members with late-afternoon
mature storm intensities, initiation times and locations closest to those
observed (storm ensemble). The second consists of members where no storms form
within 2 hours or 100 km of the observed storms (null ensemble). In a composite
of the 10 members from the storm ensemble, 2-4 m/s stonger surface southerly
flow impinging on the Palmer Ridge results in stronger horizontal convergence
and vertical vorticity with a corresponding enhanced horizontal moisture
gradient on the downwind (north) side of this terrain feature. This, in turn,
contributes to a deeper PBL with less CIN within the moisture gradient (where
storms initiate) and stronger advections of high theta-e air on the north and
east sides of the vorticity zone (where the developing storms intensify). The
evolution of these two different environmental composites, together with
examples from individual simulations will be presented at the conference to
illustrate physical processes important to initiation of severe CI in the
vicinity of modest terrain features.
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