Simple view of atmospheric chemistry
The time evolution of q depends of A and B:
dq
= AqBq
dt
q: quantity of interest (e.g. concentration)
A, B: operators (e.g. transport and chemistry)
Operator adding
q
n1
AB
n
n
n
=q  t Aq  t Bq 
t A(qn)
qn
t B(qn)
qn+1
Operator splitting: Godunov splitting
q
q
n ,*
A
n1
AB
n
n
=q  t Aq 
n ,*
=q A  t Bq
n ,*

t A(qn)
qn
t B(qn ,* )
qn ,*
qn+1
Operator splitting: Strang splitting
q
n ,*
A/2
t
n
=q 
Aq  ,
2
n
q
n1
AB
=q
q
n , **
A/2B
n ,**
A/ 2B
n ,*
A
n ,*
=q  t Bq 
t
n , **

Aq A/2B 
2
t
A(qn)
2
qn
t B(qn ,* )
qn ,*
t
n , **
A(q
2
q
n ,*
*
)
qn+1
Processes and time steps in GEOS­Chem
Note: All examples and values are for a full chemistry run in resolution 4x5
Advection
Convection
PBL mixing
Wet deposition
“Transport”
30 min t.s.
Emission
Dry deposition
Chemistry
“Chemistry”
60 min t.s.
Implementation in GEOS­Chem
Base time loop over the transport time step:
Transport forced to be the smallest time step
Chemistry time step = 2x “base” time step
t=0
30
60
time step 1
time step 2
time step 3
Transport
Chemistry
Transport
Transport
Chemistry
Calculation of diagnostics
t=0
30
60
time step 1
time step 2
time step 3
Transport
Chemistry
Accum. Diag
Transport
Transport
Chemistry
Accum. Diag
Accum. Diag
Calculation of diagnostics
t=0
30
60
time step 1
time step 2
time step 3
Transport
Chemistry
Accum. Diag
Transport
Transport
Chemistry
Accum. Diag
Accum. Diag
Non­physical states !!!!
Change to accumulate only at the end of longest time step.
Categories of diagnostics affected
Concentrations of transported tracers and family members. ND33, ND35, ND45, ND47
Time series. ND48, ND49, ND50, ND51
Pressure ND31
Land water indexes ND30
Grid box height, air mass, water vapor ND68
Examples for daily output (July 1st)
Three different types of species in GEOS­
Chem
Ox
O3
OH
Transported tracer
Member of Ox family
Pure chemistry species
Conc. given in Concentration chemistry only explicitly but transport calculated by all important (long­
processes
lived) Conc. given in chemistry only and transport not important
Treatment of each type of species in diagnostics
Should we use the same treatment for Ox, O3 and OH ?
Ox
O3
OH
Conc. at the time of the diagnostics
Ox conc. times the O3 fraction from chemistry
OH conc. from the chemistry
STT(:,:,:,IDTOX)
STT(:,:,:,IDTOX) * FRACO3(:,:,:)
SAVEOH(:,:,:)
Local time selection
We want to output concentrations only bw 11 am and 4 pm local time.
Local time in GEOS­Chem is determined at the beginning of each transport time step.
The Earth is still during each transport time step and “shifts” from one time step to the next.
Transported tracers should use local time at the beginning of both transport t. s.
Daylight is calculated in the same way for chemistry: still daylight for each chemistry time step
Pure chemistry species and family members should be selected considering the time used for chemistry, i.e. local time at the beginning of the chemistry t. s.
Summary of local time selection
Ox
Transported tracer
O3
OH
Family member
Pure chemistry specie
Local time selection at Combine local time the chemistry time step
selection at 2 (coherence with transport time steps
FRACO3)
Local time selection at the chemistry time step
Consequences depend on the length of the averaging period and output frequency
Output
freq.
Avg.period
Monthly
Daily
24 hours
No change
Some change
“afternoon”
No change
Change
1 hour
Some change
Change
Hourly
Change
Examples
DAILY OUTPUT:
“afternoon” selection
DAILY OUTPUT:
hour selection