Formation of horizontal convective rolls in urban areas

Abstract The formation of horizontal convective rolls (HCRs) in urban areas is investigated in this paper using observations and fine-scale numerical simulations. Cloud streets organized parallel to the mean boundary-layer wind (a manifestation of HCRs) are seen in the Fengyun-2C satellite imagery around local noon in Beijing. Observed vertical velocity and horizontal wind fields from an urban wind profiler suggest that the time scale for alternating updraft and downdraft in the boundary layer is about 30 min, and the length of the updraft/downdraft is about 9 km. Numerical simulations show that most HCRs occur in the urban areas with −  z i  /  L z i : the boundary-layer depth, L : the Monin–Obukhov length). Sensitivity tests reveal that HCRs are common in urban boundary layers, while rural areas are more conducive to forming cellular convection; the aspect ratio of HCRs in urban areas is smaller than the typical value over natural landscapes.

[1]  B. W. Atkinson,et al.  Mesoscale shallow convection in the atmosphere , 1996 .

[2]  B. Hicks,et al.  Land–Atmosphere Interaction Research, Early Results, and Opportunities in the Walnut River Watershed in Southeast Kansas: CASES and ABLE , 2000 .

[3]  G. Powers,et al.  A Description of the Advanced Research WRF Version 3 , 2008 .

[4]  Jason Ching,et al.  Simulation of Meteorological Fields Within and Above Urban and Rural Canopies with a Mesoscale Model , 2004 .

[5]  J. Monteith,et al.  Boundary Layer Climates. , 1979 .

[6]  Matthias Roth,et al.  Review of atmospheric turbulence over cities , 2007 .

[7]  H. Kondo,et al.  A Simple Single-Layer Urban Canopy Model For Atmospheric Models: Comparison With Multi-Layer And Slab Models , 2001 .

[8]  D. Etling,et al.  Roll vortices in the planetary boundary layer: A review , 1993 .

[9]  T. W. Horst,et al.  An Observational Study of the Evolution of Horizontal Convective Rolls , 1999 .

[10]  E. Inoue,et al.  On the Turbulent Structure of Airflow within , 1963 .

[11]  R. Kropfli,et al.  Persistent Horizontal Rolls in the Urban Mixed Layer as Revealed by Dual-Doppler Radar , 1978 .

[12]  M. Kanda,et al.  Numerical Study On Cloud Lines Over An Urban Street In Tokyo , 2001 .

[13]  Kevin W. Manning,et al.  A study of convection initiation in a mesoscale model using high-resolution land surface initial conditions , 2004 .

[14]  Fei Chen,et al.  An Observational and Modeling Study of Characteristics of Urban Heat Island and Boundary Layer Structures in Beijing , 2009 .

[15]  Margaret A. LeMone,et al.  The Structure and Dynamics of Horizontal Roll Vortices in the Planetary Boundary Layer , 1973 .

[16]  Jordan G. Powers,et al.  A Description of the Advanced Research WRF Version 2 , 2005 .