4.1.7. Atmospheric Transport
CMDL supports research to identify sources and sinks of trace gases and aerosols. These efforts contribute to a better understanding of short-term variations, seasonal cycles, and long-term trends. The CMDL isentropic transport model [Harris and Kahl, 1994] calculates trajectories at desired elevations, including those in the stratosphere. Trajectories may be compared to data collected at the surface or data collected at elevation (sonde, aircraft, and lidar data).
One objective of this program has been to describe transport to the CMDL observatories in a detailed climatological way so that potential sources and sinks impacting the measurements are clearly defined. Seasonal changes in transport can radically alter the influence of a given source or sink throughout the year. Isobaric trajectories were first used to describe seasonal flow patterns to MLO [Harris and Kahl, 1990]. A more accurate picture of these flow patterns is now available using isentropic trajectories, taking into account adiabatic vertical motions encountered by air parcels. These updated seasonal transport clusters for MLO are presented in section 4.2.3 along with other transport characteristics that help determine sources and sinks affecting measurements made at MLO. SPO transport patterns were also described with isobaric trajectories [Harris, 1992], although an analysis of the effects of adiabatic vertical motions was included in this study. More recently, transport patterns were determined for BRW [Harris and Kahl, 1994] and SMO [Harris and Oltmans, 1997] using isentropic trajectories. Knowledge of transport characteristics has led to a better understanding of seasonal patterns in MLO methane measurements [Harris et al., 1992], SMO carbon dioxide measurements [Halter et al., 1988], and SMO surface ozone measurements [Harris and Oltmans, 1997]. An investigation into MLOs springtime tropospheric ozone maximum used transport characteristics and relationships among trace gases to link this excess ozone to a possible stratospheric source [Harris et al., 1998].
Information about the isentropic transport model (methodology, description of plots, and formats of data files) is available on the Internet at http://www.cmdl.noaa.gov/traj. This home page also serves as the distribution site for realtime trajectories. These are trajectories calculated from data downloaded twice daily from the National Centers for Environmental Prediction (NCEP). For various CMDL baseline and regional observing sites, trajectories are thus provided within a day of when measurements are actually made. This capability is also available for any site on the globe as requested by the user with the use of a password. The transport home page (at the address above) also includes pointers to several archives of trajectory data.
A recently completed project made all trajectory programs, data files, and plots Year-2000 compliant. Data fields referring to the year have all been changed from two digits to four digits. The changes in data format are documented on the transport home page.