Observer Comments

12:17 Mon Jan 04, 2021

Terms Used In Forecasting: Advection

Hi MWObsians!

It has been a while since I have written anything in this space so here is something I have been meaning to share. I plan to discuss several topics in our forecasts, specifically, the terms we use and why we use them. We generally try to use basic language in our forecast discussions, but I realize that is not always the case. Understanding these many terms and processes can be useful as they are used in forecasts by us here at the Observatory, the National Weather Service (NWS), and by other third party organizations such as your local news station, or a phone app.

These posts will provide a quick explanation of some of the terms used in forecasts so that you can better understand and prepare for whatever reason you require the information. Today’s topic will be ADVECTION. I am going to define it scientifically and then break it down in a much more conceptual and ‘real’ way. It should be noted that advection is one of many other processes that causes the temperature to change in a region.

Advection is defined by the American Meteorological Society as the “The process of transport of an atmospheric property solely by the mass motion (velocity field) of the atmosphere; also, the rate of change of the value of the advected property at a given point.” That was copied from their online glossary and there is a fancy calculation along with it, but let’s think about it conceptually so we can apply the concept. First off, an example of an atmospheric property that we will discuss and is arguably the most used in forecasting is temperature. Secondly, the “solely by the mass motion (velocity field)” means that the temperature is being moved by the flow of the mass of the substance (the air) and not by heating from the sun or convection. Advection differs from convection in the sense that advection is a movement of temperature from the speed and direction of the air, its velocity, whereas convection is the movement of air usually due to a density gradient or difference. Therefore, to simplify, for forecasting reasons, advection is the movement of temperature by the wind.

There are two types of advection, Cold and Warm Air Advection (CAA and WAA respectively). The way advection is measured is explained in the second part of the AMS definition; “also, the rate of change of the value of the advected property at a given point.” To apply this to temperature advection is to say the change in temperature at a certain point over a certain amount of time. To apply it even more directly to our forecasts, I would say something like this. “Cold air advection will bring falling temperatures into the evening as northwesterly flow sets up behind the cold front.” OR I would say, “Warm air advection behind the warm front will bring rising temperatures and an increase in moisture to the region.” What forecasters mean when they write or say the first statement is that the temperature will drop because colder air from the north is moving south from the north WITH the wind where it is colder. The temperature is not dropping because it is nighttime and there are no clouds around. This is more evident during the day when it is sunny but temperatures are dropping. The second statement means that warmer temperatures are moving north from the south WITH the wind so temperatures will rise because of the flow of the wind and not because of daytime heating or latent heat release.

Lastly, to pull this all together, look at the chart below. The shaded blue is CAA and the shaded red is WAA and is expressed as the change in temperature in Kelvin/hr. A higher number of WAA or CAA could mean a number of things. It could mean that there is a higher flow of air or that there is high pressure or temperature gradients as well. Forecasters use advection to infer and/or to lead to other factors that influence the state of the atmosphere. It is also an encompassing term to explain a process, which is why it used so often.


Temperature advection (K/hr, shaded) at 850hPa with contours and wind barbs from the NAM3K model. Note the darker red and blue shading. These areas are higher temperature changes from wind flow per hour. Chart from www.tropicaltidbits.com.
 
I hope this post was helpful.  Look for more of these in the future.


Jay Broccolo, Weather Observer and Meteorologist
  
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