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Author Topic: Intensity of Insolation  (Read 1953 times)

Niall Dollard

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Intensity of Insolation
« on: July 24, 2019, 01:29:00 AM »
Earlier in the melting thread I aroused a certain amount of debate on the role of the sun and insolation at high lattitudes.

And following proper ASIF protocol, hopefully anyone who wishes to make any further comment will do so here and not derail the melting thread.

As the big dipole approaches I spent a lot more time than I normally would teasing through the many timelapse charts at the ESRL website.

What I was particularly interested in was watching the dynamic of the melting as the dipole unfolded. Top melt, bottom melt, precipitation, wind direction, melt ponding, albedo changes. These are all available at ESRL.

They are only forecasts and cannot at all be assumed to be highly accurate.

The ESRL charts showed the melting evolving over time, peaking about 5 days from now (on that forecast time period). As expected the weak edges of the ice took a battering where winds were onshore (to ice edge) but there was bands of higher melt rate up the middle of between the two dipole centres.

Underneath the HP centre, though it didnt show much melt. This got me thinking what will melt conditions be like under the anticyclonic conditions at 85N between the pole and CAA. The albedo charts indicate that the ice quality is a bit better there, once you move away from the edge.

In four to five days time the dipole will be well established. At the same time we will be 38 days past the solstice (sun angle equivalent to approximately May 16th). 

This page has a slider that shows intensity of direct radiation in kW/m² throughout the day. Day 1 on the left is the solstice.

It is the amount of power that would be received by a tracking concentrator in the absence of cloud. It does not take into account such things as atmospheric effects, cloud and albedo quality.

Is there a threshold intensity above which good quality ice snow will more readily melt.? If the albedo/ice quality is good enough, there will come a level which the clear sky solar intensity is not sufficient to melt.

Think of any practical experiences you may have observed. Given temperatures near zero, when sun angle is near 20 degrees, typically I dont see much melt on snow/ice on flat ground.  By the time you get to sun angle of 30 degrees, it's radically changed.

Binntho showed the graph of daily insolation at different lattltudes but that's the level that's spread out over a full day.The heat will not build up if it is radiated back. Intensity is different and there will always be a point where the albedo of the ice surface is not sufficient to prevent solar melting from occuring.

We will be shortly approaching the time when intensity of insolation will not be enough to melt good quality ice above 85 N. That's the way the planet moves - but of course this year even above 85N there is a dearth of quality ice. And any holes/melt ponds will lower the albedo and allow melting to continue.

I had nt intended either to address the other atmospheric issues, clouds, smoke, aerosols, black soot which will also encourage melting but the purpose of this post was to address the intensity of insolation and its effect on good quality snow/ice.

This is not meant to be a comment on the current state of the ice (hence moving it away from the melt season thread) but more a theoretical look at interplay between albedo and intensity of insolation.


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Re: Intensity of Insolation
« Reply #1 on: April 29, 2020, 03:33:44 PM »

The incoming solar radiation spectrum displayed ranges from ultraviolet to the near infrared.  Several absorption bands are shown in the near infrared, which start at 700 nm, corresponding to carbon dioxide and water.  Both of these gases have been increasing since the start of the industrial age, and increased concentrations will increase absorption of infrared radiation at the corresponding wavelengths.  This decrease in solar insolation is evident in temperature data during the summer, as detailed in the paper for United States temperature changes over the past century and graphs of DMI Greenland temperatures. 
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