Observational cues that a storm is elevated?

[Darren Lo]

I'm curious if there is a visual characteristic or other direct observational cue that distinguishes between elevated storms and those which are surface-based or rooted in the boundary layer. I know of a few factors that suggest a storm is elevated, although they are not foolproof and are not directly based on immediate observation (i.e. require a bit of interpretation):

1. The storm base is significantly higher than would be suggested by the surface-based LCL implied by surface obs nearby in the inflow sector. (Could go wrong if surface parcels are much different from the average parcel in the effective inflow layer, e.g. if there is a very shallow moist layer at the surface.)

2. There is reason to suspect the presence of a temperature inversion (storms late at night or early in the morning, storms on the cold side of a front...)

3. Convection initiated by strong upper lift might start off elevated before eventually becoming surface-based (if it ever does).


So what I'm asking is, is there a more direct way to just look at a storm and determine if it's elevated or not?

 

[Paul Knightley]

As you've noted, some storms can be determined to be elevated simply by the cloud base height (e.g. storms from Altocumulus Castellanus), where the base is several km off the surface. However, in very dry airmasses, a storm may form from a very deep boundary layer (think portions of the SW'ern USA), and have a very high base, but still be, technically, 'surface based'.

A fairly good visual clue to an isolated supercell which is starting to become more detached from the boundary layer is when the lower levels become much more laminar...this is when the so-called 'mothership' may make an appearance. The storm is still ingesting parcels from a cooling boundary layer, but not due to convection from this layer - rather, the storm is dynamically 'sucking' air into its base, which otherwise would not convect.

 

[Jeff Duda]

I think you have a pretty good idea of what to look for. Most of the clues are going to be those which are difficult to perceive in person, but which can be easily surmised by examining environmental information.

1. The storm base is significantly higher than would be suggested by the surface-based LCL implied by surface obs nearby in the inflow sector. (Could go wrong if surface parcels are much different from the average parcel in the effective inflow layer, e.g. if there is a very shallow moist layer at the surface.)

I think this point is probably the best way to determine in person whether a storm is elevated. The visual cloud base marks the LCL of the parcels the storm is ingesting. So if a storm's cloud base seems to match the SBLCL, then it is likely the storm is at least somewhat surface based, if not entirely. This came up last year when I was chasing with @Jeff Snyder in southwest OK. It was a day with high instability, yet the storm struggled and the base appeared to be higher than what the LCL would have suggested. Jeff suggested that perhaps the storm wasn't ingesting parcels from the surface even though the SBCAPE/SBCIN combination suggested surface based storms should occur. A similar situation may have occurred on 26 March this year in C OK.

I disagree with @Paul Knightley's comment about the laminar appearance. A laminar appearance indeed means that parcels are working through a stable layer, but that doesn't mean the storm isn't still ingesting parcels from below that layer. Many times the most unstable parcel is still a surface parcel even when there is substantial SBCIN. You generally will not see this appearance until later in a storm's life after the updraft has become established. The storm can suck up parcels through the stable layer because of the extreme convergence below the updraft that has become established earlier on in the storm's life. This process can be responsible for an initially elevated storm "digging" down into the boundary layer and becoming "rooted", however.

 

[Paul Knightley]

Jeff - perhaps I worded it such that I wasn't quite so clear - I didn't mean it was elevated at that point - simply that it's getting to the stage where *might* do so. The point was that the storm *is* pulling in more stable air, hence the laminated appearance. So I'm agreeing with you!