Effective-layer storm-relative helicity (ESRH) is SRH integrated through the effective inflow layer, which is the depth of the profile where lifted parcels have enough buoyancy and tolerable inhibition to reach the updraft. It replaces the fixed 0-1 or 0-3 kilometer layers with one that adapts to the actual thermodynamics of the sounding.
Defining the layer
Thompson, Edwards, and Hart proposed the effective inflow layer in the 2007 Weather and Forecasting paper. Starting at the ground, the algorithm lifts each parcel and records the first level where convective available potential energy (CAPE) is at least 100 J/kg and convective inhibition (CIN) is weaker than about 250 J/kg with the sign convention used at SPC. That level becomes the base of the inflow layer.
The algorithm keeps stepping upward until a parcel fails either the CAPE or the CIN threshold. That level becomes the top. The distance between the two is the effective inflow depth, which typically ranges from about 1 to 3 kilometers above ground and can be much shallower or much higher above the surface in elevated situations.
Why this beats a fixed layer
A fixed 0-3 kilometer SRH counts wind shear whether or not the storm can actually ingest air from that column. On an elevated storm sitting above a cool boundary layer, the lowest kilometer of the sounding is irrelevant to the updraft, but its shear still shows up in fixed-layer SRH. Thompson et al. showed cases where 0-3 kilometer SRH on elevated supercells was several times larger than the effective value, because the fixed layer was counting shear the storm never touched.
The effective layer strips that noise out. On surface-based cases with no cap, the effective layer often starts at the ground and reduces to something close to the traditional 0-1 or 0-3 kilometer numbers. On elevated cases, it lifts the base off the ground and only counts shear inside the actual inflow depth.
How forecasters use it
ESRH is the shear input to the SPC Supercell Composite Parameter and to the Effective Layer Significant Tornado Parameter. Values are compared with the local climatology on a given day and against the surface-based versus effective versions of the same parameter to check whether the storm has surface-based inflow or is drawing from aloft.
The 2007 climatology across 916 proximity soundings showed a clear ordering. ESRH decreased from significantly tornadic supercells through weakly tornadic and marginal supercells to nontornadic supercells. That ordering, together with the layer's ability to describe elevated storms, made it a mainstay of the operational parameter suite.
Important limits
When no effective inflow layer exists, ESRH is undefined and often plotted as zero. This can be legitimate, such as on the cold side of a boundary where no parcel can reach the LFC. It can also be misleading. A very strong cap that keeps parcels from reaching the LFC will zero out the value even though a supercell may fire nearby once the cap breaks.
The parameter also depends on the assumed storm motion, on the objective analysis of temperature and dewpoint that feeds parcel selection, and on the small CAPE and CIN thresholds that define the layer edges. Two nearby soundings can produce very different ESRH values if one just barely satisfies the base criteria and the other does not. Treat ESRH as a probabilistic indicator that a supercell environment supports low-level rotation, not as a switch.