Regional Simulations of the North American Monsoon System

Regional simulations over the eastern Pacific Ocean and adjacent land areas during Northern Hemisphere summer

Pennsylania State University/National Center for Atmospheric Research Mesoscale Model Version 5.2 (MM5)

92-day simulations beginning on June 1 and ending on August 31 for each summer from 1990-1997

Forced at surface by evolving SST and on lateral boundaries by NCEP reanalyses

Model characteristics:

• non-hydrostatic with 50 km horizontal resolution and 25 vertical levels
• Kain-Fritsch cumulus parameterization and Reisner mixed phase microphysics
• prescribed land use and soil moisture

Further description of model and results:

• Regional-Scale Model Climatology of the North American Monsoon System by R. Swanson, Jr. Ph.D. Thesis. University of Utah. 159pp.
• Thesis available on-line: www.met.utah.edu/jhorel/pacs_new/dissertation.html

Questions

Can the regional model develop realistic atmospheric circulations of the North American Monsoon System on a variety of temporal scales (diurnal, intraseasonal, interannual)?

Since the model simulations are constrained by the observed state of the ocean surface and the large-scale atmospheric circulation, what value is gained by running a regional model?

Are complex mesoscale phenomena such as tropical cyclones and aspects of the cold tongue-ITCZ system simulated realistically by the regional model?

What are the impacts of these mesoscale phenomena on the regional-scale climate?

Summary

A stronger meridional circulation developed in the MM5 compared to the NCEP reanalyses

• heavier convection along ITCZ that is displaced to the north
• stronger southerly flow across the cold-tongue in the PBL with stronger northerly flow in upper troposphere

Time-averaged PBL circulation in MM5 consistent with conceptual model across cold tongue ITCZ complex:

• low-level jet with peak winds near 950 mb
• strong subsidence over the cold tongue with lowest inversion height slightly to north of equator; breakdown of inversion near &N during summer
• high liquid-water content in PBL associated with simulated stratiform clouds

Planetary-scale changes associated with the 1997 ENSO led to most unusual conditions during the 8-year period

• deeper, warmer boundary layer over the cold tongue with less stratiform clouds and greater convection
• southward displacement of the ITCZ with weaker southerly cross-equatorial flow

Simulation of tropical cyclones is a challenge in the context of climate variations

• Cyclones affect the energetics of the regional circulation as well as the distribution of precipitation over ocean and along coasts
• MM5 develops too many weak cyclones as a result of:
  • 50 km resolution
  • prescribed SST
  • enhanced cyclonic absolute vorticity in basic state in genesis region