A Brief Look at Rapid Environmental Changes in a HSLC Case (January 29-30, 2013)

Note: A few figures are provided in the following PowerPoint: Figures

One of the goals of the current CSTAR project is to study the environmental evolution that occurs during HSLC events.  Dr. Parker and I have been primarily focused on the changes in the synoptic-to-mesoscale environment in the ~6 hours leading up to convection. In order to study a handful of HSLC events, we have performed simulations with the Advanced Research Weather Research and Forecasting (WRF-ARW) model using NAM analyses as initial and boundary conditions. These simulations are run at 3 km horizontal grid spacing, and data is output every 5 minutes.  This allows us to examine the evolution of the simulated synoptic and mesoscale environments on relatively small time and spatial scales.

Animations of simulated composite reflectivity and surface equivalent potential temperature (see PowerPoint link) inform us that these environments can change very rapidly, and that the associated boundaries (e.g., cold fronts, outflow) can be quite extreme. In the particular case shown from January 29-30, 2013, an intense outflow boundary races ahead of a surface cold front, triggering significant convection in Tennessee and Kentucky regardless of CAPE values less than 500 J/kg. An interesting thing to note is the increase of surface equivalent potential temperature just ahead of the outflow boundary as it approaches; this occurs from 0500 to 1000 UTC and therefore cannot be attributed to diurnal heating. Our goal is to clarify how the environment is changing in the few hours prior to severe convection.

In order to determine how and why CAPE might be changing, we took 6 hour boundary-relative time series of surface based CAPE, surface potential temperature, surface mixing ratio, and lapse rate at several points ahead of the outflow boundary and took an average (see plots in PowerPoint link). Increases in all variables are evident, though some increase more than others.  This is common among the several cases we have simulated thus far.

The question we are targeting to answer here is how changes in each of these variables individually affect CAPE.  The next step is to determine how much of the increases in surface-based CAPE can be attributed to increases in solely surface temperature and/or surface moisture.  We have made some progress in answering this question and an update will be provided soon.

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3 Responses to A Brief Look at Rapid Environmental Changes in a HSLC Case (January 29-30, 2013)

  1. Chris Rohrbach says:

    Thanks for posting the update Jessica! If you are seeing the increase in equivalent pot. temperature from 5-10z then maybe a nocturnal lower level jet is aiding in moisture transport? Then turbulence along the outflow boundary allows the parcels to mix to the surface?

    • jrking2 says:

      Indeed, Chris, strong southwesterly low-level flow is common among many of the cases we have simulated. Not only do these jet-like features provide moisture transport and warm advection, but they also intensify low-level wind shear!

  2. Jonathan Blaes @ WFO RAH says:

    Nice post Jessica.

    It is really amazing how TN just gets slammed in that event.

    We have seen in some nocturnal cases across the NC Coastal Plain, most of which seem to be several years ago, where the destabalization wasn’t continuous like a frontal passage, but almost bubbles or patches where instability bubbled down from aloft. This could be an artifact of the METARs we had available. Your simulation seems to show a more steady areal change although the time plots show jumps in the last hour or two.

    I compared your plots of various variables for the 7 points for a bit. One thing that struck me was the 0-1.5km lapse rate and the CAPE have a slope that changes, especially within 2 to 3 hours while the other variables are more gradual. Pretty subjective for sure, but I think its there.

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