Sunday, February 19, 2017

Summer greater cloudiness thermal flux mystery resolved

~Has huge implications with respect to sea ice melt season.
~ Partial cloud cover reduces sea ice albedo,  this was captured by horizon refraction method.

"Spectral albedo and transmittance of thin young Arctic sea ice "


Torbjørn Taskjelle1, Stephen R. Hudson2, Mats A. Granskog2, Marcel Nicolaus3, Ruibo Lei4, Sebastian Gerland2, Jakob J. Stamnes1, and Børge Hamre1 1


 A most interesting paper,  well apt study with respect to greater current thinner sea ice conditions, has made at least 2 major discoveries:  

15 cm sea ice has :

"Integrated albedo and transmittance for photosynthetically active radiation (400–900 nm) were in the range 0.17–0.21 and 0.77– 0.86, respectively. The average albedo and transmittance of the total solar radiation energy were 0.16 and 0.51, respectively. V"

     That is a rather big discovery-observation,  it means that thin sea ice even with a bit of snow has 4 to 4.5 times less albedo than thicker ice,  likely in excess of 1 meters thickness.  

   The rather much larger discovery:

"Under a cloudy sky we found molecular oxygen absorption bands in the atmosphere to favor light traveling less obliquely and thus slightly increase the fraction of light penetrating the ice within these bands compared to the penetration of light at wavelengths outside the bands. For large solar zenith angles, a cloud layer was found to increase the ice transmittance at all wavelengths, because it shifts the average direction of the light toward the vertical."

This very important observation/conclusion solves a once puzzling Horizon Refraction mystery
found in the High Arctic Nunavut Canada:

All these spring April to May 2011 horizon sequences,  one day per row, from shortly after Local Apparent Noon towards end of Arctic day till sunset,   have one thing in common,  clear skies and rising horizon from afternoon to evening.  This is explained by sea ice approximately 1.5 meters thick mostly covered with snow 10-20 cm,  stretching all the way well beyond the horizon line, and the lowering sun as evening progressed.  Transmittance of solar energy,  said to be much reduced by albedo as much as 90%,  with only 10% transmittance of short wave sunlight.  

       However weakened by the very sun ray reflectiveness of sea ice and snow cover,  the surface of sea ice temperature became equal to surface air after Local Apparent Noon,  with Astronomical Horizon held as long as the sun further warmed the ice.  Then after,  as the sun elevation lowered,  the surface of sea ice and snow cooled along with air interface by conduction,   while the air immediately above (a few meters above the surface to air interface) remained warmer, this caused surface inversions becoming steeper as the sun descended further.  We can judge the thickness of sea ice by the horizon rebound height.  From April (top sequence) to May (bottom) the ice thickened,  hence by conduction,  the mid or coldest layer of sea ice  embedded within its column cooled the top of sea ice quicker than the rate of cooling of air having a much lower heat capacity.

These observations are a constant feature unless clouds (sometimes winds) play havoc with the thermal fluxes: 

May 17 2011.  a fine sunny noon Astronomical Horizon (extreme left) ,  yet the horizon line largely stayed steady afterwards,  because low clouds covered sky thereafter.  One reason for the steady horizon, Taskjelle, T., S. R. Hudson et all suggest that the albedo of sea ice itself may be weakened by sun rays hitting the ice more perpendicularly,  the other explanation is the well known  lessening of energy escaping to space,  long wave radiation bouncing back and forth from cloud bottom to top of sea ice.   The former implies a neat possibility of extra but slight transmittance gain of energy to sea ice by albedo change,  combined with clouds causing a lack of surface cooling,  confirmed by the multitude of persistent near Astronomical  Horizon height horizons observed,  which of course means a significant drop or cancellation of sea ice accretion.  

And so on May 2 , 2011 the horizon height never changed at all,  this despite the clouded sun 30.8 degrees elevation (left) giving the same energy or horizon thermal flux result as when the still covered sun was 10.81 degrees elevation later in the evening.  This seem to contradict basic thermal evaluations until we consider that the light getting through the later evening cloud horizon gap  (right) has had an impact,  reducing the cooling if not slightly increasing the net warming,  because the sea ice albedo changed from more vertically directed photons hitting the icy/snowy surface.  This kind of late evening horizon observation was not at all rare.  

 Conclusion and also in appreciation:

     A strange phenomena was perceived throughout many evening horizon refraction observations,  in Southwest Cornwallis Island,  Nunavut Canada,  there was a lesser rise of horizon,  no change in height or even a drop in horizon heights,  unexplained by any other mean but an assumption that the distant horizon had different weather.  

         But a paper written by Taskjelle, T., S. R. Hudson et all  revealed a likely change in albedo of sea ice increasing transmittance of thermal rays reflected by clouds redirecting the rays at a lesser zenith angle.  This explains a couple of very interesting horizon refraction observations,  namely when cloudy there is definitely a greater thermal transfer to sea ice because of cloud cover,  but also in addition by the reflection of rays at a lower zenith angle (higher horizon elevation) .  The two,  cloud cover and more direct ray bombardment of sea ice explains the lower sea ice horizon when cloudy,  and especially when partially cloudy.  Horizon refraction observations shortly after local apparent noon during fully sunny skies have similar horizon heights than when cloudy (usually lowest during clear air).  With overcast conditions towards sunset time, the lack of horizon rise as always observed with clear air,  suggests more energy towards sea ice despite a lower sun elevation.  However, there is more energy transferred to sea ice, lesser escaping it,  when cloudy,  this has a huge over all accretion/melting  impact needed to be considered.  

          The over all melting of 2016 Arctic sea ice,  2nd place in extent,  surprised many considering the greater cloudiness by persistent  Cyclones during much of June and July,   sometimes not so opaque,  which I called "see through".   More vertical redirection of sun rays usually low most times during the Arctic horizon summer may indeed give more melting than calculated.   

      Finally the effect of lowering horizon with partially cloudy  skies when nearing sunset time should have a Norwegian name given the discovery of reduction albedo as discovered by Taskjelle, T., S. R. Hudson et all...   

      WDFebruary 19, 2017


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