Technology / Engineering guide
Wind Shear & Hub-Height Extrapolation
A resource measured at one height is not automatically the resource at another. Height extrapolation uses a model to bridge that gap. Keep the measured values, model assumptions and estimated hub-height result separate when comparing tower options.
Keep the model behind the height adjustment.
An extrapolated speed is a conditional estimate, with a reference height and an explicit shear assumption.
01 / Height definitions
Use a consistent vertical reference.
Record measurement height and proposed hub height relative to the same ground reference.
Record measurement height and proposed hub height relative to the same ground reference. Height above sea level is a different quantity. A station elevation does not replace the instrument height needed for a wind-profile calculation.
Also retain measurement location and the period represented by the speed statistic. Moving between heights at one location is a different assumption from moving a resource between sites. The resource-data guide addresses the original records.
02 / Power-law model
State the equation and its assumption.
A common model is v2 = v1 × (z2/z1)^α, where speeds v1 and v2 refer to heights z1 and z2 and α is a shear exponent.
A common model is v2 = v1 × (z2/z1)^α, where speeds v1 and v2 refer to heights z1 and z2 and α is a shear exponent. NOAA documents this power-law form; the exponent is an assumption or assessed parameter, not a universal turbine constant.
In a hypothetical calculation, v1 = 6 m/s at z1 = 20 m, z2 = 40 m and an assumed α = 0.2 give v2 about 6.89 m/s. These numbers illustrate the calculation only; they are neither a MARSTON site measurement nor a recommended exponent.
03 / Measured profile
Distinguish observed height differences from assumed shear.
Concurrent measurements at different heights can support a profile assessment.
Concurrent measurements at different heights can support a profile assessment. Compare values with consistent timing, exposure and aggregation before interpreting their difference. A pair of unrelated annual averages does not establish the same evidence as simultaneous measurements.
The SAM wind-model documentation distinguishes the use of multi-height observations from an assumed power-law adjustment. Retain the height range and conditions supporting a fitted parameter. The same parameter need not describe every season, direction or atmospheric condition.
04 / Model limitations
Ask where the profile representation stops being credible.
A simple monotonic profile cannot describe every flow situation.
A simple monotonic profile cannot describe every flow situation. Terrain, nearby obstacles and varying atmospheric conditions can complicate the relationship. Treat those as reasons to review model applicability rather than attach a fixed adjustment to any taller tower.
Interpolation between measured heights and extrapolation beyond them also use different evidence. State which is being performed. A software result does not erase the assumptions introduced when the proposed hub lies outside the observed height range.
05 / Sensitivity
Compare estimates without promising additional output.
Keep the same measured record and target height while comparing plausible assessed model assumptions.
Keep the same measured record and target height while comparing plausible assessed model assumptions. Report how the estimate changes and why those alternatives were considered. A sensitivity range is different from a statistically supported uncertainty interval.
A higher estimated speed does not by itself establish the increase in electrical energy. That step requires applicable turbine output evidence and project assumptions. Use the power-curve guide to retain the separate performance boundary.
06 / Assessment record
Make the adjusted resource traceable.
The useful deliverable contains measured source identity, heights, represented period, chosen model, parameter origin and estimated result.
The useful deliverable contains measured source identity, heights, represented period, chosen model, parameter origin and estimated result. Label measured and modelled columns clearly so a later tower change can be reassessed.
For a MARSTON enquiry, identify the candidate turbine and proposed hub height with the resource assessment. Rotor diameter and hub height are different dimensions. No default tower height, shear exponent or height-related energy gain is established here.
Continue the review with Wind Roses, Direction Sectors & Energy Assessment.
Continue the review with Long-Term Wind Resource & MCP Assessment Review.
Continue the review with Wind Energy P50, P90 & Exceedance Probability.
General background: NOAA: wind-profile power-law equation · NLR SAM: measured heights and wind-profile modelling. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Prepare the information for this review.
Use the checklist to organise the project information. Mark unavailable items and retain document references.
| Review item | Information to provide | Purpose |
|---|---|---|
| Reference heights | Ground reference, measurement and proposed hub heights | Avoid mixing elevation and instrument height. |
| Source statistic | Location, period and averaging basis | Describe the value being adjusted. |
| Model identity | Equation, parameters and their origin | Expose the extrapolation assumption. |
| Applicable range | Observed heights and relevant flow conditions | Review the model boundary. |
| Sensitivity record | Alternatives, rationale and estimated outcomes | Separate estimates from guarantees. |
Engineering questions.
Is there one correct shear exponent for every site?
No. Its suitability depends on the resource evidence and conditions being represented.
Is extrapolated hub-height wind speed measured data?
No. Label it as a modelled estimate linked to its measured source.
Does elevation above sea level give measurement height?
No. The wind-profile heights require a consistent ground reference.
Can a tower-height change be converted directly into more kWh?
Only through a separate assessment using applicable performance evidence and project assumptions.
Is sensitivity the same as a confidence interval?
No. A confidence or uncertainty statement needs its own statistical basis.
What accompanies a height-adjusted resource value?
Reference height, target height, source period, model, parameter basis and limitations.
Technical enquiry
Bring your project information to the discussion.
A resource measured at one height is not automatically the resource at another. Share the available information and the equipment scope you want reviewed in a technical quotation.