NOAA/National Severe Storms Laboratory


Hypothesis (Ziegler-3) concerning the role of thermal solenoids in developing vertical circulations across and localizing convergence along mesoscale boundaries

Conrad Ziegler on August 27, 1997 at 22:15:29:

Vertical wind shear normal to and within the cool side of the mesoscale boundary increases during the afternoon from the action of a thermal solenoid which induces a thermally direct, frontogenetic secondary circulation. This evolution leads to increased moisture convergence to assist storm initiation and increased hodograph curvature to increase probability of updraft rotation.

Evaluate component of thermal solenoid and wind shear normal to the mesoscale boundary with aircraft stepped traverses and mobile soundings, and evaluate wind shear with clear air Doppler radar data. Correlate changes of the magnitudes of wind shear and solenoid based on averages over periods of 0.5-1 hour duration.
Either thermal solenoids are absent or locally increasing wind shear cannot be correlated via the horizontal vorticity equation in a sufficient number of cases.

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

Bob Rabin on September 02, 1997 at 13:57:02:

Conrad: I am interested in learning why a narrow dry band is sometimes observed to develop aloft (300-500 mb) in satellite water vapor imagery along dry lines during the late afternoon. I've observed such 'dry lines aloft' in a couple of cases. I don't know how common they are. They suggest that significant sinking and drying aloft may be linked to the PBL circulations (thermal solenoids you have proposed. It would be interesting to measure 2-d cross sections in their vicinity and to investigate their possible role in convection initiation/evolution. While it would be possible to create a separate hypothesis concerning the formation of 'dry lines aloft', it might be better to leave this as a topic related to your hypothesis. What do you think?

Click here to comment on this hypothesis. Please reference: ZIEGLER-3.