On the Structure of the Lower Troposphere in the Summertime Stratocumulus Regime of the Northeast Pacific
Data collected in situ as part of the second field study of the Dynamics and Chemistry of Marine Stratocumulus field program are used to evaluate the state of the atmosphere in the region of field operations near 30°N, 120°W during July 2001, as well as its representation by a variety of routinely available data. The routine data include both the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and NCEP–NCAR reanalyses, forecasts from their respective forecast systems (the Integrated and Global Forecast Systems), the 30-km archive from the International Satellite Cloud Climatology Project (ISCCP), the Quick Scatterometer surface winds, and remotely sensed fields derived from radiances measured by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), the Advanced Microwave Sounding Unit, and the Advanced Very High Resolution Radiometer. The analysis shows that outside of the boundary layer the state of the lower troposphere is reasonably represented by the reanalysis and forecast products, with the caveat of a slight warm bias at 850 hPa in the NCEP–NCAR products. Within the planetary boundary layer (PBL) the agreement is not as good: both the boundary layer depth and cloud amount are underpredicted, and the boundary layer temperature correlates poorly with the available data, which may be related to a poor representation of SSTs in this region of persistent cloud cover. ERA-40 also suffers from persistently weak zonal winds within the PBL. Among the satellite records the ISCCP data are found to be especially valuable, evincing skill in both predicting boundary layer depth (from cloud-top temperatures and TMI surface temperatures) and cloud liquid water paths (from cloud optical depths). An analysis of interannual variability (among Julys) based on ERA-40 and the 1983–2001 ISCCP record suggests that thermodynamic quantities show similar interannual and synoptic variability, principally concentrated just above the PBL, while dynamic quantities vary much more on synoptic time scales. Furthermore, the analysis suggests that the correlation between stratocumulus cloud amount and lower-tropospheric stability exhibits considerable spatial structure and is less pronounced than previously thought.
Additional Information© 2007 American Meteorological Society. (Manuscript received 30 June 2005, in final form 26 January 2006) ECMWF ERA-40 data used in this study have been obtained from a variety of locations, including the ECMWF data server, and from the data systems section at NCAR. NNRA data were provided by the NOAA–CIRES Climate Diagnostics Center, Boulder, Colorado (from their Web site online at http://www.cdc.noaa.gov). The ISCCP DX data were obtained from the ISCCP data archives maintained by the NASA DAAC. Dave Stepaniak is thanked for his help with the ERA-40 reanalyses, discussions with Bill Rossow helped facilitate the ISCCP analysis, and Hua-Lu Pan helped provide the GFS data. The QuikSCAT Level 3 Ocean Wind Vector data are obtained from the Physical Oceanography Distributed Active Archive Center (P0.DAAC) at NASA JPL in Pasadena, California (see online at http://podaac.jpl.nasa.gov). TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science REASoN DISCOVER Project. (Data are available online at http://www.remss.com.) GOES-10 data were obtained from the Space Science and Engineering Data Center at the University of Wisconsin. This work was supported by NASA through Fellowship NGT530499 and Grant NAG512559. Comments by B. Albrecht, C. Bretherton, and S. Klein on an early draft of this manuscript greatly improved its presentation.
Published - STEmwr07.pdf