Chemical Transition and Mixing in the Extratropical UTLS
Laura
Pan
National Center for Atmospheric Research
(email: liwen@ucar.edu, phone: +1 303 497 1467)
1.
Introduction
Three
related diagnostics are designed to evaluate the representation of chemical
transport processes in the extratropical upper troposphere and lower
stratosphere (UTLS) by chemistry-transport and chemistry-climate models. The diagnostics are based on in situ observations of ozone (O3), carbon monoxide
(CO), water vapor (H2O) profiles (obtained on board the NASA ER-2
research aircraft, near 65ˇ N and during 1997), and their inter-relationships
in the UTLS. The first diagnostic compares the observed and modeled UTLS trace
gas profiles in a relative altitude coordinate. The second one compares the observed and modeled UTLS tracer
relationships. The third one compares the observed and modeled thickness of the
tropopause transition layer. Together, they characterize the modelŐs ability to
reproduce the observed chemical gradient in the extratropical UTLS region and
chemical transition across the extratropical tropopause.
These
diagnostics were used to evaluate NCAR chemistry-transport model MOZART-3 and
chemistry-climate model WACCM3 [Pan et al., 2007].
2.
Data
The data used in the examples are from the ER-2 measurements during POLARIS campaign, April-October 1997. The examples shown below are based on measurements near 65ˇN. Additional data will be compiled in the future to represent individual seasons and all latitudes. Note the ER-2 water vapor measurements are more stratospheric focused. The middle to upper tropospheric values tend to be too dry. Mean and s of the data used in each diagnostic are provided in tables 1-3.
3.
Model output
required
Daily profiles of
Ozone, Water vapor and CO, up to 70 hPa, and corresponding thermal tropopause
heights near 65ˇ N are required for the calculation. The examples given below
used sub-sampled daily profiles, 3 day (5th, 15th, 25th) each month,
for similar seasons to the observations (Spring to Fall).
4.
Diagnostics
a.
Tracer profiles
in the Extratropical UTLS
This diagnostic examines the distribution of ozone, CO and water vapor below and above the extratropical tropopause. Calculations are done using daily profiles in the latitude band of 65ˇN. The vertical coordinate is the altitude relative to the thermal tropopause (RALT, zr=z-ztp). In the example plot below show results from three models, NCAR CTM MOZART-3 driven by ECMWF operational analyses, MOZART-3 driven by ECMWF reanalyses Experiment 471, and WACCM3 1.9x2.5 latxlon resolution run are compared to the ER-2 data. The mean profiles are binned averages at 1 km vertical intervals, and s is represented by the error bar.
Example plot 1:
ER-2
data mean and s used in
the plot are given in the table below:
b. Mixing and tracer-tracer correlations
This diagnostic examines the tracer-tracer correlation between O3-H2O and O3-CO. Comparisons with observations can be made for the stratospheric branch, tropospheric branch, and the mixing lines. The observations and the derived branches are given in the figure below. The solid lines are results of fit. The Dash lines mark the 3 s of the distribution. The light blue lines are the representation of the mixing lines, determined by onset of a significant fraction (more than 10%) of points outside the 3 s (dash) of the stratospheric or tropospheric branches. The colors indicate whether the point is above (red, zr>0) or below (green, zr<0). the tropopause. In this data set, the end points of the mixing lines are 400 ppbv of ozone for the stratospheric end point, and 120 ppbv of CO or 100 ppmv of water vapor for the tropospheric end point. The equations and coefficients for the fit are given in the table.
ER-2 tracer-tracer relation plot:
Parameters
for identifying the stratospheric branch of tracer-tracer correlations.
The coefficients given
in this table are defined as,
x = a0
+ a1y + a2 y2 , (1)
where y is O3, and x is CO or H2O. The standard deviation, sx, is also given in the table, which was used to define the width of the stratospheric branch and to select the transitional points in the diagnostic 3.
Parameters for identifying the tropospheric branch of tracer-tracer correlations.
The coefficients are defined as,
y = b0 + b1 x . (2)
Same as in the previous table, y is O3, and x is CO or H2O. The standard deviation, sy, is used to define the width of the tropospheric branch.
Example plot 2: The following plot is an example from the models with the branches and mixing lines derived from the observations. The results show qualitative agreement and quantitative differences. The likely causes are discussed in Pan et al., [2007].
c. Sharpness of the transition
This diagnostic quantifies the vertical distribution of the transitional points between stratosphere and tropospehre, which may be viewed as the thickness of the transition layer. It provides a measure for the model resolution used if realistic sharpness of transition is reproduced. The example plot below shows the transition points (those outside of 3s of the fit) in blue and the distribution of the transition points in RALT (zr). Here cases 3 and 4 refer to WACCM3 1.9x2.5 Lat x Lon run and 4x5 Lat x Lon run, respectively.
Example plot 3:
References:
Pan, L. L., W. J.
Randel, B. L. Gary, M. J. Mahoney, and E. J. Hintsa (2004), Definitions and
sharpness of the extratropical tropopause: A trace gas perspective, J. Geophys.
Res., 109, D23103, doi:10.1029/2004JD004982.
Pan, L. L., J. C. Wei, D. E. Kinnison, R. R. Garcia, D. J. Wuebbles, and G. P. Brasseur (2007), A set of diagnostics for evaluating chemistry-climate models in the extratropical tropopause region, J. Geophys. Res., 112, D09316, doi:10.1029/2006JD007792.