Abstract
A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transport and scavenging), chemistry, and advection in the vertical redistribution of HO X and their precursors in the upper tropical troposphere. The study is focused on marine deep convection over the South Pacific Convergence Zone (SPCZ) during the PEM-Tropics B Flight 10 aircraft mission. The model reproduces well the HO X mixing ratios. Vertical variations and the contrast between north and south of the SPCZ for O 3 are captured. Convection uplifted 0 3 -poor air at higher altitude, creating a minimum in the 9-12 km region, in both modeled and observed profiles. The model captured 60% of the observed HCHO variance but fails to reproduce a peak of HCHO mixing ratio at 300 hPa sampled during the northern spirals. Simulated HCHO mixing ratios underestimate observations in the marine boundary layer. In the model, convection is not an efficient process to increase upper tropospheric HCHO, and HCHO is unlikely to serve as a primary source of HO x . Convection plays an important role in the vertical distribution of CH 3 OOH with efficient vertical transport from the boundary layer to the 10-15 km region where it can act as a primary source of HO X . The SPCZ region acts as a barrier to mixing of tropical and subtropical air at the surface and at high altitudes (above 250 hPa). The 400-270 hPa region over the convergence zone was more permeable, allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ and to be entrained in the SPCZ-related convection. In this altitude range, exchange of subtropical and tropical air also occurs via airflow, bypassing the convective region SW and proceeding toward the north of the SPCZ.
Highlights
Background Meteorology DuringFlight 10[6] Flight 10 was dedicated to the study of South Pacific Convergence Zone (SPCZ) convection [Mari et al, 2002; Pickering et al, 2001]
[3] In this study, we present an analysis of the South Pacific Convergence Zone (SPCZ) during the PEMTropics B Flight 10 in March 1999 [Raper et al, 2001]
The SPCZ region is characterized by a diagonal band of convective clouds oriented NW-SE as illustrated by the satellite image (Figure 1)
Summary
[2] Deep convection plays a critical role in determining the vertical structure of the atmosphere. From a large-scale point of view, convection is an essential dynamical process to uplift air from the boundary layer to higher altitudes, seeding the upper troposphere with pollutants. At these high altitudes, air more likely experiences long-range transport. SPCZ is well suited for a study of the vertical transport and scavenging of chemical species by convection with marked gradients of chemical concentrations both sides of the cloud band. Mari et al [2002] used a zero-dimensional model to contrast the relative contributions of different sources of upper tropospheric HOx during that flight. The three-dimensional mesoscale model provides an integrative tool to study the interplay between the dynamical and physicochemical processes at regional scale. We imposed initial and boundary conditions for these species on the basis of the aircraft observations
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