Incidence and environmental conditions of 50-80m high-shear days in NW Iowa

We examine high shear days in NW Iowa using 10-minute tall tower measurements at 50m and 80m for 2008 June through 2010 January. These levels are representative of conditions near the base and at hub height of a typical wind turbine. We develop shear categories through specification of percentiles for shear for averaging periods ranging 10 minutes (no averaging) to 24 hours. This enables us to determine the daily average shear, diurnal cycle of shear, and frequencies of shear of different averaging periods exceeding thresholds throughout the daily period. Thus, we are able to discuss wind turbine stress from the perspectives of persistent shear stress and episodes of short-duration but intense shear stress.

We further examine the wind shear statistics in light of environmental conditions to understand episodes of short-duration with intense shear stress and moderate duration with moderate shear stress. We use a synoptic type class system and human experts to classify the shear episodes. The classification system includes but is not limited organized convective storms, low-level jets, frontal passages, and quiescent conditions.