Goal:

Investigate longer time-scales in the model. What is the exchange time between the northern and southern hemispheres? How are these time scales defined? What is the influence of the tracer lifetime and convection?

Exercise 3: Interhemispheric transport

The intertropical convergence zone (ITCZ) forms a duct between the Northern Hemisphere (NH) and the Southern Hemisphere (SH). Due to convergence near the equator, deep convection occurs. Air from the NH encounters air from the SH and airmasses are lifted (this process is enhanced by condensation).

Chemically speaking, differences exist in NH air and SH air. Because most emissions occur in the NH, concentrations of (longer lived) pollutants are normally higher in the NH.

Mixing of NH and SH air masses occurs relatively slowly because of the presence of the ITCZ. In the exercises described here, several methods will be investigated to determine the mixing time scale between the NH and SH.

First, we will release a tracer over land during 3 months. This will create an interhemispheric gradient. Then, the decaying interhemispheric concentration gradient will be used to determine a characteristic time scale.

• Run MOGUNTIA with the input file inter1.in
• Run MOGUNTIA with the input file inter2.in

Analyze the output

First, have a look at the zonally averaged concentration plots.

• Where does the interhemispheric transport takes place in the model, in the upper or lower troposphere?

Next, analyze the station output. Remember that the output is requested at 500 hPa.

• Can you understand the strange peak at the 25S station?

From the decaying gradient, a time scale can be derived, using:

d N / dt = - d S / dt = -(Na-Sa)/tau

d (N-S) / dt = d N/ dt - d S/ dt = -2 (Na-Sa)/tau

tau = -2 (Na-Sa)/ (d (N-S) /dt)

N : mean concentration NH

S: mean concentration SH

(N-S): concentration gradient

tau: time scale for interhemispheric transport

(Na-Sa): mean concentration gradient, averaged over the simulation period (e.g. one year).

• Use the 45N and 45S output to determine tau.
• Repeat the same for 65N / 65S and 25N / 25S
• Is the result consistent?

Exercise 4: The effect of convection on tau

A much more accurate approximation of the interhemispheric transport time (tau) can be made by using the mean tracer concentrations at the NH and SH. These hemispheric mean concentrations are automatically calculated using the NH_MEAN or SH_MEAN station (see STATION in the MOGUNTIA input description). Precooked input files are available:

• Run MOGUNTIA with inter3.in to generate again an interhemispheric gradient
• Run MOGUNTIA with inter4.in to watch (and print) the decay of the gradient
• Rerun with inter3.in and inter4.in, but change in inter4.in the STATION_OUTPUT value to 4320 to calculate the yearly averages Na and Sa
• Deduce tau from these experiments.
• How does this correspond to the values derived in Exercise 5

The role of convection

• Do you expect faster or slower interhemispheric transport with convection turned off?
• Run MOGUNTIA, first with inter_nokonv1.in, and then with inter_nokonv2.in
• Analyze (print) the output.
• Repeat with the STATION_OUTPUT value set to 4320 in order to calculate the yearly averages Na and Sa
• Calculate the interhemispheric transport time scale without convection.
• Try to explain (analyze zonal averages, and lat-lon plots).

Other exercises (pick one or more)

• Perform an experiment to calculate the interhemispheric exchange time using a decaying tracer.
• Perform an experiment with a reversed North/South gradient.
• Perform an experiment with a different geographical distribution of the sources.

Advanced

• Deduce a formula for tau in which the emissions are not switched off.
• Set up an experiment from which the emission strength in the NH and SH can be deduced (in mixing ratio change per year). Define for this purpose your own point emissions in molecules/cm²/s.
• Use the deduced formula to calculate tau with the emissions still turned on.