As noted, SRH is a partial function of the storm-motion chosen when SRH is calculated (I say "partial" because, if in the case of calm winds, storm motion has no effect on SRH since SRH = 0 m2/s2 for all storm motions in theory, assuming the storm doesn't affect the environmental or near-storm flow, etc.). For example, we can take any non-calm hodograph and calculate different values for SRH depending upon the storm motion we decide to use. As such, if we take a straight-line hodograph that we see in the Plains at times (that is, one characterized by winds being in the upper-right quad of the polar plot, and with winds that strengthen with height), SRH can be very positive , zero, and/or very negative (and all values in between) depending upon the storm motion. This is part of the problem with SRH forecasts -- the graphics you see online assume some storm-motion (using the Bunkers technique, etc.) in calculating the SRH. However, storm motion is not always well-forecast, and storm motion can change with time (and, therefore, so can SRH, even if the environmental winds remain constant). For this reason, we may see a NAM SRH forecast that shows only 100 m2/s2, but that's for a right-mover (often, assuming it's an storm-relative helicity forecast and not an environmental helicity forecast). If a supercell splits, the left-mover may experience significantly negative SRH (i.e. SRH of -300, -400, etc.), and thus take on significant anticylonic rotation. SRH tends to be maximized when the hodograph is strongly curved. In the northern hemisphere, low-level warm-air advection (often desired for destabilization) is associated with a veering vertical wind profile, which means that we typically see a clockwise-curved hodograph in moderate-to-high CAPE situations, and thus we tend to see considerably more in the way of cyclonic supercells than anticyclonic supercells. There are times, however, when we can still have a veering vertical wind profile, yet have significantly negative SRH for a left-moving supercell.
This is much easier to see if you examine an actual hodograph, though. Hodographs can help immensely when assessing the potential for updraft rotation, etc.. For example, on some hodographs, SRH increases for a slower storm motion; SRH may increase for fast storm motions on other hodographs. At other times, SRH would be greater if surface or 1km winds were to WEAKEN. Again, this is easy to see on an actual hodograph, but sometimes confusing and difficult to explain using words.
Also note that a straight-line hodograph doesn't necessarily mean (nor should it imply) a unidirectional wind profile. In other words, you can have winds that veer with height (e.g. southeasterly at the surface veering to southwesterly at 6 km) yet still yield a straight-line hodograph. Therefore, we're interested in whether we have a straight-line
shear profile, which includes, but is not limited to, straight-line
wind profiles (e.g. weak southwest at the surface increasing to strong southwest aloft).
Remember too that thermal buoyancy is only one part of the vertical acceleration equation... Supercell updrafts may, particularly in the low-levels, be negatively buoyant in terms of thermal buoyancy (e.g. air in the updraft may have a lower/"colder" virtual temperature than the surrounding "base state" air), yet vertical acceleration may still be upward as a result of the vertical perturbation pressure gradient associated with a strongly-rotating updraft.
A post I made a few years ago (
HERE) is related to this topic, so I'll just link to it instead of copy/pasting it.
I also suggest the following for those who are interested in learning more about hodographs:
Doswell -
A REVIEW FOR FORECASTERS ON THE APPLICATION OF HODOGRAPHS TO FORECASTING SEVERE THUNDERSTORMS
Sirvatka -
Vertical wind shear, hodographs, and tornadoes
