The GMD Isentropic Transport Model was first used in a study of flow patterns for Barrow Observatory, Alaska (Harris & Kahl, 1994). Isentropic trajectories account for adiabatic vertical motions that air parcels may experience en route to their destinations. However, in the near-surface layer an air parcel cannot always be traced isentropically because the isentropic surface on which it is travelling may either intersect the ground or be ill-defined in an unstable boundary layer. This transport model, therefore, calculates trajectories on isentropic surfaces until the specified surface descends to within 100 m of the ground. At this point, the model switches to a layer-averaged mode, where an air parcel is advected by winds averaged through the layer 100-600 m above the surface topography. These heights were chosen to diminish the effects of surface friction and to represent winds in the lower boundary layer.
Input to the trajectory model is in the form of 2.5 degree latitude-longitude gridded meteorological parameters and topography furnished by the European Centre for Medium Range Weather Forecasts or the U.S. National Centers for Environmental Prediction. Most of the techniques employed in this model, such as the transformation from isobaric to isentropic coordinates, horizontal interpolation procedures, and the predictor-correction method for advection, derive from the earlier isobaric (Harris, 1982) and isentropic (Harris and Bodhaine, 1983) trajectory models. All trajectory models are subject to uncertainty arising from interpolation of sparse meteorological data, assumptions regarding vertical transport, observational errors, sub-grid-scale phenomenon, turbulence, convection, evaporation, and condensation. Five studies (Kuo et al., 1985; Kahl and Samson, 1986, 1988; Haagenson et al., 1987; Draxler, 1987) estimated average horizontal trajectory errors to be 140-290 km in 24 hrs. Trajectory uncertainties are also detailed in Merrill et al. (1985) and Harris (1992) and references therein.
Any given trajectory produced by this model should be reasonably representative of the large scale circulation, and as such, may be used to suggest potential source regions. However, this does not imply that a particular air parcel sampled at the trajectory destination followed this path.
Caveat
Trajectories have been shown to be sensitive to the different types of gridded data from which they are calculated; for example, those from the European Centre for Medium Range Weather Forecasts (ECMWF) vs those from the National Centers for Environmental Prediction (NCEP). In other words, different winds as input will produce different trajectories as output.
The standard source of input data for GMD trajectories is ECMWF, however the "realtime" trajectories are produced from NCEP data. These trajectories are labeled "Preliminary - based on NCEP met data. "
Description of isentropic trajectory plots.
- The first line of the title gives a 3-letter ID and the latitude and longitude
of the destination.
- The second line gives the date (UT) of arrival.
- The third line of the title gives the potential temperature* of the transport
surface for the 00 UT and 12 UT trajectories appearing on this page.
- The fourth line of the title, if present, indicates that the trajectories
were created from NCEP MET data. If this line is absent, the the trajectories
were created from ECMWF Operational Analyses Level III-B Basic Consolidated
Dataset.
- The upper (large) plot shows the horizontal extent of the 00 UT (blue)
and 12 UT (red) trajectories. Symbols along the trajectories mark the number
of days back from the destination.
- The lower plot shows the height in kilometers along the 00 UT (blue) and
12 UT (red) trajectories. The X-axis gives the number of days back from
the destination.
- The tables to the right give the height, pressure, and temperature along
the trajectory for each day back. Day 0 indicates conditions at the destination.
* Potential temperature is the temperature a dry parcel of air would have if brought to 1000 hPa adiabatically. This quantity should be conserved during adiabatic transport.
Atmospheric Transport Data Formats
Isentropic Trajectories
The format for isentropic trajectories is one trajectory per record. Each record may be read with a Fortran formatted read:
read(IUN,1500)ITIM, IHR, THETA,
OID, IYR, IMON
* IDAY, IFLAG, (TLAT(I),
TLON(I), PR(I),
* TMP(I), HGT(I), I=1,81)
1500 format(I7,I2,F5.1,A3,I4,2I2,A1,5(81(F8.2,1X)))
Definition of variables in the read:
- ITIM - YYYYDDD (year * 1000 plus day of year of trajectory)
- IHR - 0 or 12 UT (arrival hour)
- THETA - potential temperature of isentropic surface
- OID - 3-letter identifier for the destination
- IYR - year (YYYY)
- IMO - month
- IDAY - day
- IFLAG - blank for backward; "F" for forward trajectory
- TLAT(I), TLON(I) - arrays containing trajectory latitudes and longitudes from
the destination (TLAT(1), TLON(1)) and every 3 hours back in time and space
to (TLAT(81), TLON(81)) which is the location 10 days back.
- PR(I) - pressure on the isentropic surface for TLAT(I), TLON(I)
- TMP(I) - temperature on the isentropic surface for TLAT(I), TLON(I)
- HGT(I) - height of the isentropic surface for TLAT(I), TLON(I)
Isobaric Trajectories
The format for isobaric trajectories is one trajectory per record. Each record may be read with a Fortran formatted read:
read(IUN,1500)ITIM, IHR, LVL,
OID, IYR, IMON, IDAY,
* IFLAG, (TLAT(I), TLON(I),
I=1,81)
1500 format(I7,I2,I4,A3,I4,2I2,A1,162(F7.2))
Definition of variables in the read:
- ITIM - YYYYDDD (year * 1000 plus day of year of trajectory)
- IHR - 0 or 12 UT (arrival hour)
- LVL - pressure level (1000,850,700,500)
- OID - 3-letter identifier for the destination
- IYR - year (YYYY)
- IMO - month
- IDAY - day
- IFLAG - blank for backward; "F" for forward trajectory
Atmospheric Transport References
Draxler, R. R., Sensitivity of a trajectory model to the spatial and temporal resolution of the meteorological data during CAPTEX, J. Climate Appl. Meteor. 26, 1577-1588, 1987.
Haagenson, P. L., Kuo, Y.-H., Skumanich, M. and Seaman, N. L., Tracer verification of trajectory models, J. Climate Appl. Meteor. 26, 410-426, 1987.
Harris, J.M., The GMCC atmospheric trajectory program, NOAA Tech. Memo, ERL-ARL-116, 30 pp., NOAA Environmental Research Laboratories, Boulder, CO, 1982.
Harris, J.M., An analysis of 5-day midtropospheric flow patterns for the South Pole: 1985-1989, Tellus, 44B, 409-421, 1992.
Harris, J.M. and B.A. Bodhaine (eds.), Geophysical Monitoring for Climatic Change, No. 11: Summary Report 1982, pp. 67-75, NOAA Environmental Research Laboratories, Boulder, CO, 1983.
Harris, J.M. and J.D.W. Kahl, An analysis of 10-day Isentropic Flow Patterns for Barrow, Alaska: 1985-1992, J. Geophys. Res., vol 99 (D12), 25845 - 25855, 1994.
Kahl, J. D. and Samson, P. J., Uncertainty in trajectory calculations due to low resolution meteorological data, J. Climate Appl. Meteor. 25 , 1816-1831, 1986.
Kahl, J. D. and Samson, P. J., Uncertainty in estimating boundary-level transport during highly convective conditions, J. Appl. Meteor. 27, 1024-1035, 1988.
Kuo, Y.-H., Skumanich, M., Haagenson, P. L. and Chang, J. S., The accuracy of trajectory models as revealed by the observing system simulation experiments, Mon. Wea. Rev. 113, 1852-1867, 1985.
Merrill, J.T., R. Bleck, and L. Avila, Modeling atmospheric transport to the Marshall Islands, J. Geophys. Res.,90,12927-12936, 1985.
A list of papers dealing with transport to the GMD observatories.
Barrow:
Halter, B. C. and Harris, J. M., On the variablity of atmospheric carbon dioxide concentrations at Barrow, Alaska during winter, J. Geophys. Res., 88(C11), 6858-6864, 1983.
Harris J. M., and Kahl, J. D. V., An analysis of 10-day isentropic flow patterns for the Barrow, Alaska: 1985-1992, J. Geophys. Res. 99(D12), 25845-25855, 1994.
Harris J.M., E. J. Dlugokencky, S.J. Oltmans, P.P. Tans, T.J. Conway, P.C. Novelli, and K. W. Thoning, An interpretation of trace gas correlations during Barrow, Alaska, winter dark periods, 1986-1997, J. Geophys. Res., 105, 17267-17278, 2000.
Mauna Loa:
Harris, J. M., Oltmans, S. J., Dlugokencky, E. J., Novelli, P. C., Johnson, B. J. and Mefford, T., An investigation into the source of the springtime troposperic ozone maximum at Mauna Loa Observatory, Geophys. Res. Letts, 25, 1895-1898, 1998.
Harris, J. M., Tans, P. P., Dlugokencky, E. J., Masarie, K. A., Lang, P. M., Whittlestone, S. and Steele, L. P., Variations in atmospheric methane at Mauna Loa-Obervatory related to long-range transport, J. Geophys. Res., 97(D5), 6003-6010, 1992.
Harris, J. M. and Kahl, J. D., A descriptive atmospheric transport climatology for the Mauna Loa Observatory using clustered trajectories, J. Geophys. Res., 95(D9), 13,651-13,667, 1990.
Samoa:
Halter, B. C., Harris, J. M. and Conway, T. J., Component signals in the record of atmospheric carbon dioxide concentration at American Samoa, J. Geophys. Res., 93, 15914-15918, 1988.
Harris, J. M. and Oltmans, S. J., Variations in troposperic ozone related to transport at American Samoa, J. Geophys. Res., 102, 8781-8791, 1997.
South Pole:
Bodhaine, B. A., Deluisi, J. J., Harris, J. M., Houmere, P. and Bauman, S., Aerosol measurements at the South Pole, Tellus, 38B, 223-235, 1986.
Harris, J. M., An analysis of 5-day midtropospheric flow patterns for the South Pole: 1985-1989, Tellus, 44B, 409-421, 1992.