Sunday, May 6, 2012

Convection Initiation (lack thereof)

Yesterday everyone was talking about the cap. As if that is the only player in a complex tapestry of processes that drives whether we see any convective clouds. The SPC issued a slight risk for a situation last evening in Nebraska along a boundary where CI and thus tornadoes were possible.  A TOR watch was issued with the caveat that the best thermodynamics were out in the warm sector just slightly removed from the zone of better vertical shear located in a narrow corridor behind the front.

The soundings from LBF, OAX, and TOP at 00 UTC, launched around 23 UTC or slightly thereafter usually, showed CINH ranging from -50 to -7 for surface parcels. Looking at the soundings however it seemed minimal. Minimal in the sense that low level CAPE was positive at the LCL and parcels had an additional 500-1000 m before they hit the LFC. But the so called capping layer was shallow! It wasnt a deep layer inversion. The cap is always relevant but not the most important factor.

So, on a day where the warm sector south of the warm front produced barely clouds, what could have possibly impeded dry convection from turning into even shallow moist convection?
1. The boundary layer was warming, deepening, and drying.
2. The capping layer was warming (around 800 mb).
3. There was no effective lifting mechanism that could overcome 1 and 2.

Usually there needs to be some type of trigger (organizing mechanism) to initiate clouds. In the presence of entrainment, you need a steady supply of moisture. Sometimes this may come in the form of horizontal convective rolls. These rolls scale with the PBL depth. Deeper PBLs mean deeper roll circulations. Sometimes shallower rolls means  there is less deep layer mixing and entrainment is relatively reduced. Deeper rolls may mean more entrainment as they tap progressively higher but drier air. Despite having vertical velocities in roll updrafts of a few m/s, even that may not be enough to force parcels to their LCL let alone the LFC.

Yesterdays OAX sounding indicated that the LFC-LCL depth was near 700 m with a PBL top near 1500 m and an LCL near 1660 m (surface parcel) and 1900 m (mixed layer parcel). Even roll circulations may not overcome that setup. In this case even the cold front was not sufficient.