It was found that forty percent of ramps in central Iowa occurred within 6 hours of ramps at the meteorological tower near Pomeroy, IA. Twenty percent occurred simultaneously. These results suggest that in general ramps are very localized in nature, and knowledge of a ramp event at an upstream site may not be helpful in predicting ramp occurrence. With 100 miles separating the two sites, however, it is interesting that in 20% of events, ramps occurred simultaneously at the two sites. In addition, in some cases, ramps were not observed at all six turbines in the central Iowa farm. Due to their east-west orientation and the B line being directly south of the A line of turbines, it is likely that wind direction determines what all turbines experience a ramp. Analysis of wind directions for these cases is ongoing. In addition, we will examine 10 m wind observations from nearby AWOS sites to determine if ramp behavior at 10 m is similar to what occurs at 80 m elevation.
Atmospheric phenomena have also been examined to determine the cause of ramp events at both locations. In Pomeroy, IA, thunderstorms or a strong pressure gradient are often present during ramps. Analysis is ongoing for the central Iowa ramps. We will also study the pressure gradients present during ramp events.
Finally, the skill of one and two-day ramp persistence forecasts was evaluated and compared to skill from 6 different WRF simulations using different planetary boundary layer schemes. Threat scores were typically between 15% and 25% for both day 1 and day 2 persistence forecasts, values a bit less than the WRF simulations for day 1 but comparable for day 2.