Dynamic Warming? And Sensible heat of rainfall

[RU848789]

There's always lots of discussion of dynamic cooling in borderling situations, in which it's cold aloft and above 32F at the surface, but with lots of lift leading to heavy snowfall generation rates well aloft.  Those heavy snowfall rates essentially transport cold air with the falling precip down to the surface, cooling the column enough that the snow no longer changes to rain near the surface.  

 

However, I don't recall hearing many people talk about "dynamic warming."  I could be wrong, but I assume that's what's going on now.  At my house at 10 am, we had about 1/10" of solid ice on every surface, even including the moderately traveled 2-lane street in front of my house and it was raining lightly to moderately with the air temp at 31F.  

 

90 minutes later, as of 11:30 am, the temp had risen to 33F here and most of the ice is now melted, as the rain had increased to moderate to heavy rates.  I don't know the thermal profile of the air column over that time period, but I'd guess that temps are in the upper 30s a thousand feet up (hence the rain) and that the heavier rainfall rates are transporting that warmth to the surface slowly warming things up.  Dynamic warming?  

 

In addition, even if the rain isn't transporting warmth to the surface (maybe the raindrops are only 33F - I just don't know - I doubt they're below 32F though), if the raindrops are a few degrees above 32F, then the "sensible heat" contained in those raindrops might still be enough to slowly melt the ice on the surfaces (just like cold water will slowly melt an ice cube), especially since the rain is falling heavily meaning more and more warmer water is contacting the ice.  

 

Comments?  

[USCG RS]

Today is a combination of two things. First, the warm air pushing up from the South is warming the entire column.

However, what you are talking about is essentially latent heat release. In a typical ice storm situation, where the temps aloft are just warm enough to support liquid, while the ground and surface temperatures support freezing, as rain hits the ground and freezes, it releases heat. Remember, energy is neither created nor destroyed, so energy must therefore be transferred. Likewise, cold air is just a lack of heat. So when the surface temperatures are around 30 degrees and the rain is hitting the ground and freezing, in order for this to happen, the liquid rain drop must release heat energy in order to change to a solid state. This heat energy is released into the atmosphere and thus will eventually warm the surrounding air, bringing it above freezing. Of course, this all depends how shallow the cold air is and and just how cold it is.

One last thing. Moderate to heavy rain tends to be harder to freeze on contact for several reasons. First, the process described above helps to warm the surface temperatures more quickly. Second, the sheer speed and inertia of the rain drop can make it hard to freeze. Rain drops are bouncing when they hit the ground, therefore, unless the ground temperatures are exceedingly cold, it is not easy for them to freeze on contact when they are very large (heavy rain drops are much larger than drizzle). In addition, large rain drops likewise have more surface area which need to freeze. So not only are the drops falling quicker and bouncing, they have more area which needs to cool to below the freezing point in order for it to become ice. Because it needs to freeze on contact, it becomes a much harder process. Lastly, this process needs to happen for every single rain drop and therefore, when the drops are large and coming down quickly, it becomes exponentially harder to freeze on contact. (The only process where this is a bit different is wet ice, which is extremely rare for falling precipitation. I could explain if you'd like, but it's a whole different topic.)

United States Coast Guard Rescue Swimmer

[RU848789]

Today is a combination of two things. First, the warm air pushing up from the South is warming the entire column.

However, what you are talking about is essentially latent heat release. In a typical ice storm situation, where the temps aloft are just warm enough to support liquid, while the ground and surface temperatures support freezing, as rain hits the ground and freezes, it releases heat. Remember, energy is neither created nor destroyed, so energy must therefore be transferred. Likewise, cold air is just a lack of heat. So when the surface temperatures are around 30 degrees and the rain is hitting the ground and freezing, in order for this to happen, the liquid rain drop must release heat energy in order to change to a solid state. This heat energy is released into the atmosphere and thus will eventually warm the surrounding air, bringing it above freezing. Of course, this all depends how shallow the cold air is and and just how cold it is.

One last thing. Moderate to heavy rain tends to be harder to freeze on contact for several reasons. First, the process described above helps to warm the surface temperatures more quickly. Second, the sheer speed and inertia of the rain drop can make it hard to freeze. Rain drops are bouncing when they hit the ground, therefore, unless the ground temperatures are exceedingly cold, it is not easy for them to freeze on contact when they are very large (heavy rain drops are much larger than drizzle). In addition, large rain drops likewise have more surface area which need to freeze. So not only are the drops falling quicker and bouncing, they have more area which needs to cool to below the freezing point in order for it to become ice. Because it needs to freeze on contact, it becomes a much harder process. Lastly, this process needs to happen for every single rain drop and therefore, when the drops are large and coming down quickly, it becomes exponentially harder to freeze on contact. (The only process where this is a bit different is wet ice, which is extremely rare for falling precipitation. I could explain if you'd like, but it's a whole different topic.)

United States Coast Guard Rescue Swimmer

 

I wrote up a whole mini-treatise (see the link below) on the effects of latent heat release on freezing rain accreting as ice, so I'm quite familiar with the concept.  I should have qualified my post above by stating that I'm pretty certain the latent heat effect was a minor component of what I was analyzing.  This morning, we already had accumulated a decent layer of ice as the rain fell lightly onto the sub-32F surfaces.  Then, the intensity of the rain increased and the temperature at the surface increased and the ice that was already there melted.  For there to be a latent heat release at that point, the rain would have to continue freezing in order to release that latent heat to the atmosphere. 

 

Clearly, by maybe 10 am, there was no additional ice accretion going on (the ice was starting to melt), so something else had to be going on.  I'm pretty sure it's what I said, that the sensible heat of the "warm" (35F?) raindrops, combined with the kinetic rate element of there being a lot more warm raindrops, with heavier precip, is what was melting the ice.  The other effect of the increased rainfall rate of warmer than freezing raindrops should have been what I called dynamic warming, in which the heavy precip rates transported some of the warmer air aloft to the surface.  

 

In situations where the air aloft, at least for the last few hundred feet, is below 32F, but not deep enough to freeze the supercooled raindrops into sleet (supercooling can occur since the water needs a seed nucleus/particle to freeze onto), then the raindrops aren't well above 32F (which is what I'm theorizing they were today - not 100% sure of that) and then the latent heat effect really comes into play, when the raindrops freeze.  But that effect only leads to slow to very slow ice accretion, not net melting of what is already there, since some of the rain is converting to ice (accretion), but much of that ice is immediately melting, due to the latent heat release, but not all of it.  Again, this is a relative rate phenomenon - at best the melting rate can equal the freezing rate and no additional ice accretion occurs.  

 

One more point: larger raindrops have more surface area than smaller raindrops, but more important than that is what I've often called the most fundamental concept in chemical eng'g scaleup in lectures I've given on the topic - the surface area to volume RATIO is what's critical.  The SA/V ratio for large drops is much smaller than it is for small drops (if one assumes spheres, V=4/3*pi*r³ while SA=4*pi*r², so the volume increases far faster than the SA with inreasing radius or size).  Therefore, the large drops have a much larger internal volume than the small raindrops and with less surface area in contact with the cold surface per unit volume, the large raindrops are harder to freeze.  

 

http://www.americanwx.com/bb/index.php/topic/45342-112-possible-overrunning-storm/page-2#entry3244726

[USCG RS]

Thank you for the response. I apologize, I was under the impression you were asking and not instructing.

United States Coast Guard Rescue Swimmer

[RU848789]

Thank you for the response. I apologize, I was under the impression you were asking and not instructing.

United States Coast Guard Rescue Swimmer

 

Didn't mean to come off as too preachy, sorry - your points about latent heat were excellent, in general, although I think they just didn't apply much in this specific case.  However, I am looking for confirmation about the dynamic cooling part of the discussion, which I think is right, but would love to have a pro confirm, if possible (or point out what I got wrong).  Don't know your background, but when you go through 9 years of chemical eng'g, phase changes, system energy/entropy, mass/heat/momentum transport, and thermodynamics become fairly 2nd nature.  Where I wish I knew more is the application of these chem eng'g fundamentals to meteorology, which uses very similar underlying science, but in a different way: huge, unbounded, non-linear, chaotic systems, whereas in chem eng'g we work on small, bounded, but usually also non-linear and chaotic systems with chemical reactions going on to complicate things (no reactions of consequence in meteorology).  

[gravitylover]

When I got in the car this afternoon the driveway surface was still frozen solid and slick as snot with heavy rain falling.  The thermo on the dash showed 35 so out of curiosity I hopped into my car to check what it showed right next to the first one.  My car the thermo is ~18" lower and it said 33 and this is about 12" off the ground.  On both cars I checked without starting the engine so there was no added heat to affect the reading.