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

Thursday, 26 January 2012: 11:00 AM
NCEP Regional Models Cannot Resolve Marine-PBL Bases: Implications for Regional Air Quality Simulations
Room 339 (New Orleans Convention Center )
Bereket Lebassi-Habtezion, Stanford Univ., Stanford, CA; and J. Gonzalez, W. T. Thompson, J. M. Baldasano, and R. D. Bornstein

This paper focuses on three studies in which too low (or missing) west-coast General Circulation (GC) marine boundary layers (MBLs) imported from large-scale models into mesoscale met-models adversely impacted the mesocale simulations. GC forcing of mesoscale models enters through its initial (ICs) and lateral boundary (BCs) conditions. In the first study, i.e., RAMS simulations of summer sea breeze flows in California with domains of 16 and 4 km, NCEP 2.5 deg spatial-resolution sea level pressures were interpolated from the input geopotential heights that initialize RAMS and that provide updated lateral BCs at 6 h intervals. Monthly-averaged ICOADS SSTs were also used as BCs (at 2- or 1-deg spatial resolution), while input NCEP 1000-hPa temperatures had a 2.5 deg spatial resolution. RAMS simulated temperatures showed an over-ocean Pacific High subsidence inversion-tops at reasonable heights (900 m), but bases much below (i.e., at the height of the analytical SBL grid point) reasonable MBL heights (i.e., up to several 100 m). These too low values may have resulted as the NCEP model coarse vertical-resolution is unable to resolve MBLs, and as the RAMS surface roughness scheme is only valid for smooth swell conditions and its TKE scheme does not generate sufficient mechanical-turbulence in near-neutral over ocean MBLs. Too-low MBL heights also occurred in COAMPSŪ meso-met model output until recent changes were made to its Mellor and Yamada level 2.5 TKE scheme. COAMPS also has a sophisticated data assimilation scheme, in which a previous forecast is combined with available observations to formulate ICs. The MBL is thus maintained though the initialization procedure. Its MBL, however, was too shallow, particularly with stratocumulus clouds at its top, because its maximum TKE was occurring in the subcloud layer, rather than within the clouds. This was addressed though re-formulation of its virtual buoyancy flux, part of buoyant TKE production. Impacts from too-low MBL-heights also result in over-estimated pollutant concentrations from coastal ship-emissions, as they are mixed vertically into too-shallow layers, producing unrealistically high concentrations in a narrow band around the Iberian peninsula. It is believed these issues occur in many other such simulations. ŪCOAMPS is a registered trademark of the Naval Research Laboratory

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