92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 2:45 PM
Effect of Sub-Kilometer Grid-Spacing and Time Step Magnitude on the Formation of Horizontal-Vorticies in Urbanized-WRF Simulations of NYC
Room 339 (New Orleans Convention Center )
Robert D. Bornstein, San Jose State Univ., San Jose, CA; and E. Gutierrez, J. Gonzalez, A. Martilli, F. Chen, and J. K. Ching

The community Weather Research and Forecasting (WRF) model, coupled with a multi-layer urban-canopy scheme, was used to evaluate the evolution of horizontal vortices in NYC during a summer 2010 heat wave. The urban scheme (developed by Martilli) considers direct (thermal and mechanical) effects of urban environments on PBL structure, and it contains a building energy model to account for anthropogenic-heat from air conditioning systems. High resolution (250 m) urban canopy parameters from the National Urban Database were used to specify input parameters required by the urban scheme. Simulations were configured to determine impacts of horizontal grid-spacing (Δx) and numerical time-step (Δt) magnitude on horizontal-vortex formation, evolution, and dissipation. Results showed that they developed (with Δx = 1000, 333, and 111 m) after sunrise, reached maximum strength in mid-afternoon, and dissipated by evening. They were 2 km deep, with maximum vertical velocities (of 1-2 m/s) at about 1.2 km, weakened when Δt was decreased below WRF default input values, and had a horizontal vortex size of (6-9)Δx, regardless of Δx. While numerically stable (they did not “blow up”), the Madrid group is investigating the adequacy of WRF horizontal turbulence-closures at such small Δx-values. This is an important issue to resolve, because while these vortices sometimes have an observational basis as horizontal convective rolls in satellite cloud-images, their modeling veracity is suspected, especially as their horizontal size may be Δx-dependent, and as they are near-universal in such fine-scale simulations.

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