I discuss the implementation of an H2O transport scheme in the Global Modeling Initiative (GMI) model

Considine, David B.

NASA Technical Reports Server

Transporting Stratospheric Water Vapor in 
the GMI Combo Model
David B. Considine
NASA Langley Research Center
GMI Science Team Meeting
March 17, 2007
Motivation:
Stratospheric water vapor in the current Combo model is climatological,
and accounting for PSC processes as well as changes in CH4 emissions
is clumsy. Can we improve this by transporting water in the stratosphere?
https://ntrs.nasa.gov/search.jsp?R=20080014822 2019-08-30T04:14:24+00:00Z
Methodology
1. Advection
2. Convection
3. Deposition
4. Add Trop Water
5. Chemistry
H2O(t+dt) = H2O(t) + A x ( MetH2O(t) - H2O(t) )
Currently A = 1, meaning Met H2O
replaces transported water below 
tropopause at every time step
Independently Transported 
Species:
H2O
H2OCOND
HNO3
HNO3COND
Need to transport separately to allow
for separate nucleation and evaporation
temperatures
Trop to Strat:
No discontinuity
Strat to Trop:
Troposphere sees
strat met H2O field,
not combo model H2O Might be a
problem,
depending
on what strat 
metH2O is
Run Description
• 5 year run.
• GEOS4 AGCM with 1994 - 1998 SSTs data set.
• 2 x 2.5 resolution.
• Combo model is modified version of 10/31/07 download.
• 1st 4 years run with bad “ilat” specification, resulting in
PSC scheme only being called at pressures lower than 120 hPa,
with max pressure specification at high SH lats being sometimes
as low as 36 hPa.
• Reran last year several times (~4 days/year @ 64 nodes); best
results with latitudinally-invariant pmax of 300 hPa. (Operationally
probably best to set lower bound as tropopause.)
• Also ran 1 year of GEOS 4 DAS winds (year 2004), initialized from
December 1998 of AGCM run.
MetH2O stratosphere is HALOE/MLS climatology. Transported H2O in stratosphere
agrees pretty well overall. Transported H2O tends to be higher just above tropopause 
at mid-to-high lats. Diffs could also be due to interannual variability.
Transported
Met H2O
HALOE v19 H2O data, 1994 - 1998, 4o
latitude bins, at equator, regressed to 
annual and semiannual harmonics only, 
as in Randel et al. [2001].
Model exhibits lower stratospheric ascent
rate of ~ 0.3 mm/s, consistent with HALOE
obs, with ascent rates increasing above
20 - 25 km as observed. Tape recorder signal 
remains coherent to higher altitudes (> 30 km) 
than observed, due to lack of QBO and 
perhaps a weak SAO. US values are slightly 
smaller than HALOE observation, and there may
be a small decreasing trend. Lower 
stratospheric minima and maxima agree well 
with obs.
At base of overworld, GMI transported H2O
shares similarities with MLS H2O.
Evidence of poleward transport extending
from tropical minimum in February is weak.
NH H2O Max (associated with NH summer
monsoon in atmosphere) is right magnitude,
but occurs 2 months too early and ~20o
poleward of observed position. SH H2O max
in model occurs too late. Model exhibits high
H2O along stormtracks in SH, which is not
observed. SH polar minima are higher than
observed in MLS, perhaps due to interannual
variability, but timing is correct.   
(2007)
In July, MLS H2O maxima occur over 
Central America, western Pacific and 
Tibetan plateau. Model has larger 
maxima over CONUS and western 
Pacific as far north as 70o. The MLS 
Central American maximum is stronger 
in September and is apparently 
responsible for annual cycle peak.
GEOS 4 Heating rates at 390K are 
positive between ~ (30S - 40N), and 
negative poleward. Morphology of high 
H2O values does not reflect heating rates. 
Vertical pressure velocities are generally 
negative (i.e. upwelling) in NH where 390K 
H2O mixing ratios are large. 
The 350 K surface is approximately coincident
with the 215 hPa surface, and crosses from
the tropical UT into the lowermost stratosphere
poleward of the tropopause. Model H2O is higher
than observed in tropics, where water vapor is 
met water field, and shows substantially higher 
values than observed by MLS in the LMS.
MLS values are extremely low, lower
even than at the tropical cold point from which
they are supposed to have been transported.
In NH LMS, MLS mixing ratios are < 15 ppmv. Combo model H2O
mixing ratios range from 25 - 100 ppmv in LMS. Model H2O in LMS
appears to have been transported from maximum over northern India
and Tibetan plateau, with secondary contribution from west coast
of northern and central America. Model shows ridge of high H2O
running along west coast of South America that is not observed. 
Plotting fraction of H2O maximum on 350 K surface shows that Combo model 
distribution is more sharply peaked than observed by MLS. In NH LMS, model 
H2O fraction is smaller than seen in MLS. This indicates that model tropopause 
transport barrier may be fine, and that excessive H2O in the LMS is due to 
excessive vertical transport of H2O in tropics over Tibetan plateau.
Convective heating rates, cloud mass 
fluxes and negative pressure velocities 
maximize over Tibetan plateau & southeast 
Asia, indicating both large-scale and 
convective contributions to vertical 
transport to 350 K isentrope. In LMS, 
negative heating rates and zero convective 
mass fluxes indicate any transport from 
below which might contribute to high water 
vapor mixing ratios is anomalous. 
• At 350 K, Combo Model and MLS O3 are in good agreement at high lats, as shown
for v 1.5 MLS data in Strahan et al. [2007]. High lat fall O3 minimum not as low in 
Combo model as is observed (Consistent with Strahan et al. [2007].) 
• High bias in Combo H2O at high lats would suggest concurrent low bias in O3 if purely 
excessive isentropic transport caused H2O high biases.
• Summer/fall O3 high-bias in model may be due to insufficient isentropic transport
from lower latitudes. 
• Note good agreement in tropics; Strahan et al. [2007] found v 1.5 MLS high-biased
with respect to Combo model.
• At 350 K, Combo Model and MLS O3 are in good agreement at high lats, as shown
for v 1.5 MLS data in Strahan et al. [2007]. High lat fall O3 minimum not as low in 
Combo model as is observed (Consistent with Strahan et al. [2007].) 
• High bias in Combo H2O at high lats would suggest concurrent low bias in O3 if purely 
excessive isentropic transport caused H2O high biases.
• Summer/fall O3 high-bias in model may be due to insufficient isentropic transport
from lower latitudes. 
• Note good agreement in tropics; Strahan et al. [2007] found v 1.5 MLS high-biased
with respect to Combo model.
Effects of GEOS 4 DAS met fields. GEOS 4 DAS uses TOVS moisture 
retrievals above 200 hPa, where it is very dry and there is no information.
GEOS 4 DAS cold point temps are lower than GEOS4 AGCM, leading to
low water vapor values in tropical lower stratosphere.
EFFECTS OF GEOS 4 DAS MET DATA
When driven by the GEOS 4 DAS met data,
annual cycle of H2O at base of overworld shows
much lower than observed mixing ratios in the
tropics due to the ~5 degree colder 
temperatures than seen in GEOS 4 AGCM.
This degrades agreement with observations
shown above.
(2007)
GEOS 4 DAS (2004)
GEOS 4 DAS H2O on the 350 K surface agrees 
better in the tropics with MLS obs than GEOS 4
AGCM. However, There is more H2O in summer
extratropics than in GEOS 4 AGCM or seen in
MLS observations. Could be excessive isentropic
transport or anomalous vertical transport.
GEOS 4 DAS

Conclusions
1. Using G4AGCM met data, good simulation of H2O in stratospheric overworld.
• Model generates reasonable polar dehydration.
• Tape recorder signal is good:
• Overly coherent signal above 30 km understandable 
given lack of QBO in met data (what about SAO?)
• Stratospheric H2O mixing ratios not bad.
• Annual cycle of H2O at base of overworld (390 K) resembles
patterns seen in MLS and HALOE obs, but suggestion of weaker 
isentropic poleward transport than seen in observations.
2. Simulation of H2O in LMS shows deficiencies.
• Mixing ratios appear to be too high (but MLS is dry-biased 
relative to AIRS by 50% at low H2O, so might not be so bad).
• High H2O appears to be due to excessive UT H2O
in met H2O field, rather than excessive isentropic cross-
tropopause transport - O3 is not correspondingly low.
3. HNO3 annual cycle in lower stratosphere compares well to MLS:
• Polar denitrification in Combo model is reasonable. 
• Summer high lat NH mixing ratios 2x high - why?
4. GEOS 4 DAS LMS more high-biased relative to obs, very low tropical
lower stratosphere mixing ratios. Not as good a simulation as GEOS 4 AGCM


Transporting Stratospheric Water Vapor in the GMI Combo Model

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