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Summer Report

Summer Report. Estatio Gutierrez and Sacha Bardy -Bowman. Formation: They are present in moderate flows with modest unstable stratification due to dynamic and thermal instabilities.

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Summer Report

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  1. Summer Report Estatio Gutierrez and SachaBardy-Bowman

  2. Formation: • They are present in moderate flows with modest unstable stratification due to dynamic and thermal instabilities. • Well defined HCRs only occurred while –zi/L < 25 where zi is the convective boundary layer depth and L is the Monin-Obukhov length. • The HCRs orientation is governed by the mean convective boundary layer wind direction. • Kanda et al. (2001) used the Regional Atmospheric Modeling System (RAMS) to simulate ‘Kanpachi Street Cloud (KSC)’ which is visible during calm summer conditions, and show that the formation of the KSC requires two processes at the same time: (1) the convergence of two sea breezes and (2) forcing due to an urban heat island.

  3. WRF Simulations: • Five Domains (40.5 km,13.5 km, 4.5 km,1.5 km and 0.5 km). • Beijing: 0000UTC 18 Aug 2005-0000UTC 20 Aug with the initial and boundary conditions from the operational Global Final (FNL) Analyses on 1.0×1.0 degree. • WRF coupled to the Noah land surface model and to a single-layer urban canopy model (UCM). • Mellor–Yamada–Janjic (MYJ) PBL parameterization scheme is used in our simulations. • Sensitivity test: the whole Beijing urban area was replaced by croplands in WRF.

  4. Results: • The general features (e.g., vertical extent, intensity, and alternating time) of simulated updraft/downdraft and the variation of the mixing layer height at Baolian are similar to the observations. • The length of HCRs is ~9 km, the wavelength (spacing) λ is ~3 km, and the vertical extent H is ~2 km, so the aspect ratio λ/H is ~1.5. • From the sensitivity case: the land-use change from buildings to cropland leads to smaller surface roughness length, increasing surface wind speed, and decreasing surface friction velocity. The combination of these factors increases −zi / L and −zi / L is larger than 25 in most areas, and results in a more unstable boundary layer and decreasing the chance for forming HCRs. Consequently, there is much more cellular convection in the domain than HCRs.

  5. Vertical velocity and wind (Left), zi/L (right)

  6. Temperature (12m) uWRF WRF

  7. Temperature (58m) uWRF WRF

  8. Temperature (245m) uWRF WRF

  9. Temperature (580m) uWRF WRF

  10. Wind (12m) uWRF WRF

  11. Wind (58m) uWRF WRF

  12. Wind (245m) uWRF WRF

  13. Wind (580m) uWRF WRF

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