Mesoscale Boundaries on Doppler Radar

Mesoscale Boundaries on Doppler Radar

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Weather forecasters
use base velocities to detect mesoscale
boundaries, which separate contrasting air masses. Mesoscale boundaries
are important, because convergence along
them has the potential to initiate deep
moist convection. Now, a couple of common
types of mesoscale boundaries are outflow boundaries and
sea and lake breeze fronts. Sea and lake breeze fronts
mark the leading edge of cooler maritime
air pressing inland from the coast, while outflow
boundaries mark the leading edge of rain-cooled
air spreading out from the area where
a thunderstorm’s down draft splashes down. So to start by examining
mesoscale boundaries on radar, we’ll look at this loop
of base reflectivity from the radar near Chicago on
the afternoon of June 24, 2009. And we’ll watch the
animation here for a moment. We can get the sense
that there were certainly mesoscale boundaries
present on this afternoon. You can see, actually,
some thunderstorms developed along a
boundary, and then they threw off their own
outflow boundaries as rain-cooled air spread
out from those thunderstorms. And you can pick out
the boundary here. Now, we’ll go back here
shortly and take a closer look at some parts of this loop to
really dissect what’s going on. So we watch the
loop finish here. You can see there’s a complex
pattern of outflow boundaries. And what really happened
is the outflow boundaries from those thunderstorms merged
with the original lake breeze front coming inland. So we’ll go back to
the very beginning, and we’ll watch this
loop progresses. And you can see just a few
frames in that we certainly have a lake breeze front
that has developed. There’s this enhanced zone of
reflectivity or reflectivities a little bit higher. That’s because of things
like bugs and dirt that was drawn together by the
low-level conversions occurring along the boundary. And you can certainly see that
boundary highlighted here. Here’s the lake breeze front
with this linear feature of enhanced reflectivity. As the loop goes
on, you can start to see that that we get
some high reflectivity, some thunderstorms developed
along that boundary. And actually, the radar operator
switched from clear air mode to precipitation mode, which is
why the appearance of the radar looks much different
halfway through. You can see the
thunderstorms, and then off to the west of
those thunderstorms, you have the hybrid
boundary, which is the outflow boundary combined
with that original lake breeze front, and that continues
to press farther inland from the thunderstorms
that continue to develop near Chicago on this date. Now, in this case,
the boundaries showed up pretty nicely on this
image of base reflectivity. You can follow these arcs
of reflectivity marking those boundaries, these
hybrid mesoscale boundaries. And I have those
labeled for you here. You can see the
boundaries there. But it really
seals the deal when you look at base velocities. So here’s the base
velocities at this time, and you can see that
we have this area of incoming velocities, inbound
velocities, along the radials. This green shading
meets up with an area of outgoing velocities,
these positive velocities, along the radial– certainly
a zone of convergence here sealing the deal that
this was a mesoscale boundary.

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