NOAA/National Severe Storms Laboratory


Hypothesis (Bresch) concerning development of deep convection north of surface warm/stationary fronts

Jim Bresch on September 03, 1997 at 20:47:07:

Elevated convection initiates north of surface
warm or stationary fronts at the nose of low-level jets (Trier and
Parsons 1991; Bresch 1994). Theta-e rich boundary layer
air rises over a warm fontal surface initiating convection
at the terminus of the low-level jet where upward motion
is enhanced.

Identify potential cases by using forecast model
output, WSR-88D and profiler data to generally locate
the LLJ. Using multiple M-CLASS vehicles and possibly
aircraft, perform nearly simultaneous releases to facilitate
kinematic vertical motion calculations. Position the
soundings to search for the terminus of the LLJ and to
identify the source air for the elevated convection.

i) Convection breaks out randomly north of a warm front
despite the existence of a LLJ. ii) Enhanced vertical
velocity is not found at the terminus of the LLJ (or no terminus
is found). iii) convection forms away from the enhanced upward
motion near the LLJ. iv) Convection forms north of a warm front
when no identifiable LLJ exists.

Additional Comments: The mobile mesonet is of little use for
this scenario since surface parcels are not involved in the
convection, although stationary vehicles might be used to
detect pressure perturbations (gravity waves) north of the
warm front. Such a case should be in a region which is
well-covered by radars and profilers (such as KS and OK).

The following appear in order; discussion points may directly refer to one or more comments preceeding it.

Steve Koch on October 22, 1997 at 16:58:57:

Jim, I assume you mean that you would be using a Bellamy method or similar triangulation technique to compute vertical motion within a triangle formed by 3 balloons released by the M-CLASS. The problem as I see it is "how will you know whether you have found the MAXIMUM vertical motion, which is required in your hypothesis as it is stated? Even supposing that you could unambiguously locate a maximum (which is impossible with just a few balloons), how then would you be able to determine that its cause is definitely the low-level jet? Please explain your thinking here.

Jim Bresch on November 13, 1997 at 23:14:54:

Steve, thanks for the good comments. We know very little about the initiation of elevated convection and TIMEX provides a good opportunity to investigate it. My hypothesis is primarily based on my 3-4 June PRE-STORM work, which is also applicable to other similar synoptic situations (e.g. 6/6/93).

Yes, I figured we could use a Bellamy method using three (or more) wind profiles to compute vertical motion. Aircraft, NEXRAD, and profiler data would also be valuable. My gravity wave analysis of the 3-4 June event (based on what you did for the CCOPE case) was inconclusive - the 5-minute resolution of the PAM data and the lack of usable data south of the KS border prevented unambiguous tracking of the waves. It was clear that once started, convection caused gravity waves. There does seem to be a link between the LLJ and convective initiation in that case, so the LLJ would be something that a mobile observing crew could concentrate on.

Some of the things I'd like to know are, 1) how does the LLJ interact with the sloping frontal surface? 2) Does the LLJ have a terminus as Trier and Parsons show, (i.e. does it just end abruptly with enhanced upward motion?), 3) what is the source air for the elevated convection (this is crucial for proper modeling). Also, is there any truth to the forecasting axiom that elevated convection forms along the 850 mb front?

I don't think there are any meso-beta-scale observations in the vicinity of elevated convection. I would hope that we might find enhanced vertical motion near where the convection initiates and the most likely spot for that would be near the LLJ. Granted, that an observation of vertical motion will not tell us whether it is a maximum, but we do stand to learn about the meso-beta structure of the frontal surface. The observations can also be used in a 4-D variational analysis system to "fill in the gaps".

Attributing an observation of vertical motion to a given physical process (such as the LLJ, or gravity waves, or frontogenesis) is more difficult and will take some thought. For this task 4DVAR is ideal.

I expect that analysis of NEXRAD and profiler data (and maybe 4DVAR) prior to the experiment could refine the approach. Anyone want to fund that?

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