Hi,

Firstly these are great images :-)

I would agree that it is certainly plausible that dust aerosols could be having an impact on the evolution of these clouds. Cirrus clouds are subject to the same physical processes that occur in the more commonly observed stratocumulus at lower altitude. So, they are subject to long-wave radiatice cooling at cloud top and warming at cloud base which generates turbulent mixing within teh cloud layer. During the daytime, absorption of short-wave radiation offsets the long-wave cooling at cloud top and reduces the generation of turbulence.

Several years ago I was involved in a comparison study of 2-d and 3-d numerical simulations of cirrus. At that time, it was not uncommon for small pristine ice crystals to be considered as non-precipitating in some of the models - analogous to the situation of small cloud droplets in water cloud. These cirrus models produced cloud layers that had similar properties to stratocumulus. Because the cloud layer was strongly mixed, the ice water content increased linearly form cloud base to top - very similar to stratocumulus. Because the cloud ice was considered non-precipitating, the ice water path was much higher, leading to even higher levels of radiative heating/cooling and turbulence generation.

The possible link with dust aerosols here is that the high aerosol concentrations could be providing an unusually large number of ice-nucleating aerosol - IN. So, in these clouds the ice water is distributed amongst a larger number of smaller crystals than in a regular cirrus. Hence, these crystals precipitate more slowly, retain a higher ice water path within the cloud and so generate stronger radiatively-driven turbulence.

Just one point of caution. During the daytime with higher solar zenith angles, the conditions for viewing the cellular structure would be less-favourable. Your best examples are where the sun is low and casting stronger shadows in the cellular structure. So, this could be a cuase of the apparent reduction in cellulat structure during the daytime, as well as the reduced cloud-top radiative cooling.

Hi,

Firstly these are great images :-)

I would agree that it is certainly plausible that dust aerosols could be having an impact on the evolution of these clouds. Cirrus clouds are subject to the same physical processes that occur in the more commonly observed stratocumulus at lower altitude. So, they are subject to long-wave radiatice cooling at cloud top and warming at cloud base which generates turbulent mixing within teh cloud layer. During the daytime, absorption of short-wave radiation offsets the long-wave cooling at cloud top and reduces the generation of turbulence.

Several years ago I was involved in a comparison study of 2-d and 3-d numerical simulations of cirrus. At that time, it was not uncommon for small pristine ice crystals to be considered as non-precipitating in some of the models - analogous to the situation of small cloud droplets in water cloud. These cirrus models produced cloud layers that had similar properties to stratocumulus. Because the cloud layer was strongly mixed, the ice water content increased linearly form cloud base to top - very similar to stratocumulus. Because the cloud ice was considered non-precipitating, the ice water path was much higher, leading to even higher levels of radiative heating/cooling and turbulence generation.

The possible link with dust aerosols here is that the high aerosol concentrations could be providing an unusually large number of ice-nucleating aerosol - IN. So, in these clouds the ice water is distributed amongst a larger number of smaller crystals than in a regular cirrus. Hence, these crystals precipitate more slowly, retain a higher ice water path within the cloud and so generate stronger radiatively-driven turbulence.

Just one point of caution. During the daytime with higher solar zenith angles, the conditions for viewing the cellular structure would be less-favourable. Your best examples are where the sun is low and casting stronger shadows in the cellular structure. So, this could be a cuase of the apparent reduction in cellulat structure during the daytime, as well as the reduced cloud-top radiative cooling.

Many Thanks for your detailed answer!

If the larger number of smaller crystals is the dominant effect in the radiatively-driven coludtop cooling and turbulence, then, then this effect should work both daytime and nighttime, am I right?

You are right about the different visual appearance of the clouds because of the Sun angle. I checked it many cases, but it really seems that the structure of the cloud itself changes during daytime. It may means that the dust aerosols absorbs solar radiation at the cloud top during daytime and it is comparable with the indirect effect of dust aerosols (stronger radiative cooling). (Also in some cases we can clearly see diurnal variation of the temperature in the infrared images for these clouds.)

Is there any possibility to check these processes with a proper NWP model with adequate microphysics and radiation paramerization?

It is fresh and perfect case to study 28.02.2014.:

http://mrstratus.blogspot.hu/2014/02/20140228-closed-cell-convection-at-top.html

a fullHD video from MSG HRV images:

http://youtu.be/RrV_QaRbVLs

Hello Mr. Kornel.

I think that a lot of detailed information you find in these manuscripts;

1) I.S.A. Isaksen, C. Granier, G. Myhre, T.K. Berntsen, S.B. Dalsřren, M. Gauss et al. Atmospheric composition change: Climate–Chemistry interactions. Atmospheric Environment 43 (2009) 5138–5192,

2) P.C.S. Devara, M.G. Manoj. Aerosol–cloud–precipitation interactions: A challenging problem in regional environment and climate research. Particuology 11 (2013) 25– 33,

3) Catherine L. Muller, Chris Kidd, Ian J. Fairchild, Andy Baker. Investigation into clouds and precipitation over an urban area using micro rain radars, satellite remote sensing and fluorescence spectrophotometry. Atmospheric Research 96 (2010) 241–255,

4) Andrea I. Flossmann, Wolfram Wobrock. A review of our understanding of the aerosol–cloud interaction from the perspective of a bin resolved cloud scale modelling. Atmospheric Research 97 (2010) 478–497.

P.S. If you have troubles finding them, let me know, I'll send them as PDF files.

Regards.

I totally agree with Mr Brown. Dust are generally good IN and this should result in larger ice crystal concentration but smaller in size. Radiative cooling at the op pf the cloud should therefore be enhanced.

While i totally agree with Mr Brown, I think the concept of "convection" is equally plausible. Somewhat similar situation, but not with cloud, is occurring in the southeast Atlantic region. In this case, the biomass emission over the adjacent southern Africa is advected westward and offshore, with relatively high water vapor content within the same air-mass, to an otherwise dry layer (~550-750hPa). There is a strong radiative cooling at the top of this "offshore-biomass" layer, that give a strong temperature-moisture inversion capping, similar to that of the underlying stratocumulus cloud. Just below this inversion is a layer of constant specific humidity and potential temperature, which suggest vertical mixing.

I have been staring at my figures for a while now, trying to come up with explanation as part of my research. So I think "convection" would definitely play a role in the cooling as you have mention, just as it appears to be in the southeast Atlantic case.

This has been proposed as a potential intervention in climate - not advocating it as a choice, simply pointing you in that direction.

Mitchell, D.L. and W. Finnegan, 2009: Modification of cirrus clouds to reduce global warming. Environ. Res. Lett., 4, 045102 (8 pp), doi:10.1088/1748-9326/4/4/045102.

I wanted to add that the atmospheric dust appears to be the strongest impact on clouds and rainfall on the planet, if you overlay the dust over the monsoonal moisture between India and Arabia each year, for example. The Pakistan-Arabia Dust Cloud forms a wall that either stalls the monsoon in India or Pakistan and causes floods, or the lands on the other side of the dust wall experiences drought.

Overlaying the Pakistan-Arabia Dust Cloud with the moisture of the category-5 cyclone GONU, you can see the direct effect of the dust, ripping the cyclone in two on June 6, 2007 at 06:00Z and eating the cyclone like icecream. You can see this epic battle at http://www.ecoseeds.com/newGONU.html and there are other dust-rainfall battles at http://www.ecoseeds.com/contents.html.

The amazing action of the atmospheric dust, is that it can have an measurable effect on rainfall and droughts at very low levels, like only 40 micrograms per cubic meter. And it is also remarkable that any amount of dust has the power to extinguish a Category-5 cyclone, that is a real eye-opener.

We need to start incorporating the atmospheric dust into all of our computer climate models, and when we do so, we may get a really big surprise.

The interface between the Indian monsoon summer moisture and the Pakistan-Arabia Dust Cloud would be a good place to start, because the interaction is so easily and clearly available for modeling, and that interface interaction means drought or floods for over a billion people on the planet.

Conversely, if we could eliminate the Pakistan-Arabia Dust Cloud by ecological restoration with local native plants of the source countries producing that dust, that could modify weather in a very positive direction for the whole northern hemisphere, and can help contribute to subtract some of the global climate warming.

I just came upon this recent paper:

http://www.atmos-chem-phys.net/14/3027/2014/acp-14-3027-2014.html

It suggests that dust aerosols may actually decrease the ice concentration in cirrus rather than increase it. This is due to the competition between the processes of heterogenous ice nucleation on dust against homogenous freezing by soluble haze aerosols.

If this is true then you may expect the mean ice particle size to increase and so also the ice flux out of the cloud layer. You may then expect this to decrease the intensity of radiatively driven convection.

Going back to the original question, it would be interesting to know if there are examples of similar cloud systems that have been observed with and without the influence of dust aerosols. If so, there is then the possibility to compare their different behaviour.