NOAA/National Severe Storms Laboratory


Hypothesis (Crook) concerning the depth of lifting at a convergence line.

Andrew Crook on October 03, 1997 at 11:10:18:

The depth that boundary layer air is lifted at a convergence line depends, among other things, on 3 parameters; the strength of convergence, the stability above the boundary layer and the flow above the convergence line. Analytical relations between the depth of lifting and these three parameters have been developed for an idealized convergence line. It is hypothesized that the depth of lifting for an observed convergence line will be approximately related to these parameters.

First, identify a boundary layer convergence line before moist convection has developed along it. Stability and flow aloft can be measured with M-CLASS and/or aircraft. Convergence can be measured with Doppler radar and/or mobile mesonet. The depth that boundary layer air is lifted will need to be measured by aircraft flights across and along the convergence zone.

In order to verify the hypothesis, it will be necessary to find variations in the three parameters for the same convergence line. An individual line should exhibit variations in low-level convergence along it. Second, the stability aloft should decrease as the boundary layer heats up. Finally, the flow across the convergence line will vary if the orientation of the line varies. Hence an individual line should exhibit variations in the three parameters, which we hypothesize will be correlated with changes in the depth of lifting.

If the depth of lifting shows no dependence to the three parameters, or the opposite dependence to that predicted.

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

Steve Koch on October 22, 1997 at 17:03:22:

I generally like the nature of this hypothesis, and it looks like it is refutable with the measurement systems suggested. However, I would add an additional consideration which follows from my research with remote sensing systems studying low-level convergence phenomena over the last few years: the DEPTH of the convergence is probably an additional important parameter to consider (Koch et al. 1991; Koch et al. 1997, etc. ... I can provide the references later). Also - could you clarify what you mean when you say that "the flow above the convergence line" is an important variable in this problem?

Andrew Crook on October 24, 1997 at 12:01:38:

Yes, I agree that the depth of lifting will depend on the depth of convergence. In the analytical model, the lifting depth scales with the convergence depth, in other words they are linearly proportional. A conference paper (in postscript format) describing the model is online at:

In the hypothesis I tried to limit the number of control variables to three (by the phrase ``among other things''). The hypothesis could then be tested by finding a convergence line for which two of the variables were relatively constant while the third varied. The convergence depth could be added as a fourth control variable but the other 3 variables would need to be held fixed to test the hypothesis. Of course, this is assuming that we only verify by examining trends in the lifting depth. The model also gives a quantitative estimate of this depth. Hence, by using the observed lifting depth and not just the trends, the hypothesis could be tested in cases in which all four parameters varied, (however, I am less confident in this verification procedure).

``The flow above the convergence line'' is depicted in Fig. 4 of the conference paper. In the model, the flow above the boundary layer is assumed to be constant with height upstream of the convergence line. In a real situation, the flow is likely to be sheared. It probably wouldn't be too difficult to include shear into the analytical model. Alternatively, a mean flow could be calculated from the observations by averaging the velocity above the boundary layer over a depth with scale similar to the boundary layer depth.

Click here to comment on this hypothesis. Please reference: CROOK.