Dear Lakshmi,

You can use those terms as you see fit, as long as you explain what you mean by it. However, giving names of objects or processes is not, by itself, of particular relevance or interest. You may want to focus more on why you would want to distinguish these 'direct' versus 'indirect' effects on the amount of radiation reaching the surface: In other words, what is(are) the advantage(s) and implication(s) of making this distinction?

This being said, please note that aerosols are not limited to the boundary layer: volcanoes and large fires are two examples of processes that can send great quantities of aerosols at high altitude, including in the lower stratosphere in the case of large volcanic eruptions.

If there are multiple clouds layers in the atmosphere, there will be complex radiative interactions within the atmosphere, as well as between the latter and the surface. Again, I'm not sure whether or why the bottom cloud layer would be called the 'first' and the top the 'second', or what is the purpose of calling one 'direct' and the other 'indirect'. In any case, you will need to account for multiple (scattering) interactions between the surface and all atmospheric constituents, including gases, aerosols and clouds.

Cheers, Michel.

Dear Lakshmi,

You can use those terms as you see fit, as long as you explain what you mean by it. However, giving names of objects or processes is not, by itself, of particular relevance or interest. You may want to focus more on why you would want to distinguish these 'direct' versus 'indirect' effects on the amount of radiation reaching the surface: In other words, what is(are) the advantage(s) and implication(s) of making this distinction?

This being said, please note that aerosols are not limited to the boundary layer: volcanoes and large fires are two examples of processes that can send great quantities of aerosols at high altitude, including in the lower stratosphere in the case of large volcanic eruptions.

If there are multiple clouds layers in the atmosphere, there will be complex radiative interactions within the atmosphere, as well as between the latter and the surface. Again, I'm not sure whether or why the bottom cloud layer would be called the 'first' and the top the 'second', or what is the purpose of calling one 'direct' and the other 'indirect'. In any case, you will need to account for multiple (scattering) interactions between the surface and all atmospheric constituents, including gases, aerosols and clouds.

Cheers, Michel.

Hi Madhavan,

I agree to the previous comment, in general. It is contextual and needs to specified in a given context.

I also caution you on the second point. Significant amounts of aerosols could be found above the boundary layer. There are several observations showing substantial aerosol loading above the ABL.

These arise from multiple processes, over and above those given above by Michel.

• Lofting of near-surface aerosols by strong convective updrafts, and orographic winds.
• Direct injection of aerosols through contrails, and other aircraft emissions. 
• Lifting of aerosols by cumulus clouds, which could be at time strong enough even to lift them above the tropopause where and when these activities are intense. 

In the tropics, on several occasions, the AOD due to "above-the-boundary layer-aerosols" could be as high as 40% of the columnar AOD. Please examine yourself the CALIPSO data over tropics, especially during March to July in the northern hemisphere, and estimate the fraction of extinction in the altitude range 3 km (typically the ABL is below this altitude) to 10 km to the total columnar. 

I think the phrases you are looking for is "direct and diffuse" rather than "direct and indirect".  The first refers to light that is unscattered versus light that re-enters the beam by secondary scattering.  The second is often used to refer to the interaction of radiation with aerosols versus the effects aerosols have on clouds that in turn affect radiation.  You really don't want to get the two concepts confused.

I also am concerned with your focus on gaps between clouds.  If you are talking about 1 dimensional radiation theory in which clouds are in layers, then the gaps do no matter; all that matters is the relative order of vertical properties.  A layer of broken clouds can be treated correctly in this theory if new scattering functions are introduced to model 3D effects due to shadowing and illumination with sun-satellite angle.  This is rarely done in practice (if ever), but conceptually is helpful to realize that the exact vertical nature of layering does not matter to the surface shortwave radiation.