A validation of local aerodynamic roughness length estimates at a coastal residence via a multi-instrument approach.

The determination of winds within the canopy layer, such as near roof height, is important with respect to estimating the loading in design (forensic) applications. This is challenging in the presence of a heterogeneous upstream fetch where there are large changes in roughness elements as well as abrupt transitions within the complex geography of the coastal zone. In an attempt to extend the log layer flow into the canopy we deployed both a LiDAR and roof mounted anemometer in a suburban neighborhood – at a residence located approximately 400 m from the coast. The instruments collected data over two separate periods from 27 April to 10 June and 1 October to 7 November 2022. The LiDAR samples up to 300 m and has 11 configurable range gates. During these two windows, there were approximately 400,000 vertical profiles (about 1 every 20-s). These profiles are filtered over 10-mintue windows to ensure neutral stability, stationarity, and to minimize directional variability in both time and height. Using morphometric-derived displacement heights, direction-dependent local aerodynamic roughness z₀ estimates (15° bins) are calculated by applying a least-squares fit to each of the filtered 10 min mean profiles. The impact of both blending and displacement height on z₀ is examined. Preliminary results indicate that filtering reduces the spread in the z₀ distributions. The direction sorted wind profiles indicate the presence of a shallow internal boundary layer (IBL) for short fetch (onshore) flow – which decreases the mean roughness if the blending height is assumed to be deeper than the IBL. The profile-derived z₀ distributions are compared to the land-use based estimates in the Florida Public Hurricane Loss Model, ASCE 7-22 Exposure B, as well as those obtained from a morphometric approach. The roughness estimates are also evaluated by calculating the potential wind whereby the observed 10 m wind at a nearby ASOS (for which the roughness is known) is extrapolated up to the blending height via the log law, and then brought back down using the different roughness values at the residence. This latter exercise is applied to data collected during Hurricanes Ian and Nicole.