Hello

1. It would be very useful to calculate and disseminate the ERF for each country of the northern hemisphere : there are still obvious differences (Spain / Norway)
2. Moreover, a study carried out on the Tibetan plateau, where warming accelerated during the winter months at the end of the 20th century, shows that increased surface humidity leads to an increase in long-wave radiation (= heat) and can locally and temporarily raise temperatures at altitude. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2009GL037245

Is it possible to have induced ERF at an altitude between 1500 2500m ?

Thanks a lot

All the best

Louis 

Hi

the monthly distribution of the contrail coverage is slightly different from the one reported for year 2002 in the publication below, from Stuber & Forster : 

https://acp.copernicus.org/articles/7/3153/2007/

page 4

but 2002 was a special year, just after september 11

Louis

About the monthly distribution, please find another source : 

https://commons.wikimedia.org/wiki/File:Contrails_1994_1995_Nasa_US_Air_Force.jpg?uselang=fr

Both statistics show a rebound in october and a high level, rather in February-April. This is important because one can link it with the first snows in october and the melting in march-april.

Louis

This is a much needed bottom up flight by flight assessment of the global annual radiative impact from aircraft contrails. The work that has gone into the analysis is impressive. Congratulations ot the authors.

One aspect that does require a little more explanation is the use of a global climate model water vapour dataset, adjusted through a statistical fit against IAGOS in situ data, to drive highly specific trajectory based analyses of radiative forcing from COCIP. 

The data presented in supplementary tables shows "ratio compares the false positive and false negative rate and is computed by (NIAGOS/YHRES (%) − 1). A positive value indicates YIAGOS/NHRES (%) that the ERA5 HRES underpredicts contrails, a value of zero indicates a symmetrical false positive and false negative rate, while a negative value indicates that the ERA5 HRES overpredicts contrails."

On semantics, this metric predicts ISSR occurrence, not contrails. The FP and FN rates are hidden in the ratio. The ETS values appear quite low.

Contrail formation and persistence will be highly sensitive to the vertical distribution of water vapour in the atmosphere. Given the authors have access to all the IAGOS data, it would be helpful and instructive to present more data on TP/FP/TN/FN rates (e.g., in the vertical and also by region) and to relate this to the challenges of doing a trajectory based COCIP analysis with all the associated specifics of aircraft type, engine emissions, etc. How does water vapour uncertainty propagate into a calculation of radiative impact at the global scale? There are no errors presented on annual net RF values. What are these errors and how much is attributed to water vapour uncertainty?