Lightning NOx EmissionsGEOS5 v8-01-01

vs.

GEOS4 v7-04-13

Lee T. Murray, ltmurray@fas.harvard.edu

Difference in 2005 average flash density

The lightning distribution should be insensitive to globally-wide differences in CTH magnitude between meteorology because we (theoretically) scaled everything to an annual average flash rate of 46 fl/sec regardless.

However, redistributing locally in GEOS5 is losing lightning flashes (~11%). The redistribution scales down lightning in places to “move it” elsewhere by scaling those other boxes up. In regions with no parameterized lightning but observed lightning in the climatology, we can not scale up nothing, so this represents a loss from the total annual flashes calculated.

For GEOS4 this wasn’t a problem -- we had >99% yield in redistribution since the horizontal distribution each month matched the lightning climatology for that month. GEOS5 has a much smaller geographic extent in distribution (apparent in the following monthly breakdowns), so we are getting lower yields (~89% for 4ºx5º). My overall scaling to 46 fl/sec was based on the parameterized flash rate before redistribution.

GEOS5 2005 puts no lightning in E Europe / Central Asia, which is an important source region for the Middle East O3 hotspot -- and hence the N MidLat background? (Especially if benchmark ran over three 2005s)

GEOS4 puts lightning in Central Asia, but GEOS5 doesn’t.

As long as CTH is not zero (which it isn’t over most of the world), we should theoretically have some lightning everywhere. However, the flash rate parameterization is hardwired to zero if:(1) Surface is colder than the required negative charge layer of the cloud (-15ºC). This is to prevent polar and winter lightning.(2) The vertical extent of the cloud never reaches -40ºC, the range of the cold cloud depth assumed necessary for the establishment of the dipole [Williams, 1985]. This was to turn lightning off in persistent shallow clouds, particularly in the marine environment, and greatly helped GEO4 w/o having to redistribute. However, with our lower CTHs (or possibly also from changes in lapse rates over the subtropical subsidence zones?), this must be the culprit for our GEOS5 problem.

Grayed-out areas have no parameterized lightning

Throughout 2005, the GEOS5 geographic extent is much less than GEOS4.

Already, the lack of any lightning over Turkmenistan / Central Asia is appearing

Differences in Amazon and Congo between GEOS4/5 are probably due to comparing climatology to one year. (See slide note)

Australia and Argentina are underestimated during Austral Spring for the same reason as N MidLats -- GEOS5 puts no lightning over the interior continent.

Monthly Breakdown

Considering now NOx emissions versus flash densities, the majority of the global underestimate of lightning NOx emissions wrt GEOS4 occurs during Northern Summer and Fall, particularly July and August.

Regional Breakdown

-15%

-27%

-6%

-5%

-7%-24%

-26%

0%

The lack of lightning over E Europe / Central Asia is impacting the LNOx emissions.

This is particularly affecting the the N MidLats where we emit 500 mol N/flash (>23N in the Americas, >35N in Eurasia) versus 260 mol N/flash everywhere else.

The differential emissions rates follows the work of Rynda and Randall, and are at the upper-bound of the literature values so I’d be nervous to increase the NOx-per-flash value to match regional totals when it appears to be the geographic distribution that is the problem.

North America

North America is surprisingly well-matched. I haven’t seen the ozonesondes or MOZAIC profiles for the benchmark, but if we’re underestimating the US wrt GEOS4 and it is due to lightning, it must because of changes to the background, not local lightning emissions.

Latin America

The distribution is fairly well-matched and mostly within the range of GEOS4, and fluctuates as to which is greater -- again, I think this is from scaling a single year to the climatology.

Africa

I probably made this region too big. Shows signs of both the loss from redistribution (as observed in previous plots) and problems in matching temporal variability due to scaling a year to the climatology.

Europe

GEOS5 puts no lightning over Eastern Europe during July and August 2005, resulting in large underestimates wrt GEOS4 mean and 2005 value.

Central Asia

GEOS5 also puts no lightning over Turkmenistan and surroundings. Also, none over Northern Pakistan over July, which is one of the single largest lightning boxes in the monthly climatology.

East Asia

GEOS4 put a lot of lightning in Siberia this summer, so it makes sense that the GEOS5 follows the mean more closely.

Indonesia

Indonesia appears very well-matched between GEOS4 and GEOS5, though GEOS5 2005 follows the mean more closely than GEOS4 2005, as would be expected.

Australia

Australia has no lighting over its interior during its spring, so it is also losing lightning to the redistribution.

Recommendations

• Until development of new parameterization– Remove shallow-cloud inhibition for GEOS5 if local scaling

is turned on (the redistribution should take care of turning off lightning in shallow cloud areas as well), and if necessary, the surface temperature inhibition as well.