When my daughter and grandson visit, we always have a pancake breakfast. I am the Executive Chef; I wear a funny hat and talk with a French accent.
My grandson, now ten, is my sous chef. He wears a different funny hat and breaks the eggs (226 in a row without a speck of shell).
Our pancakes come in three sizes:
- The smallest size is the self-explanatory ‘silver dollar.’
- The largest size is Koplovian, named after a friend (Koplow) who was excused from the armed services for exceeding the maximum weight.
- The middle size is called mesoscale, which means just what it sounds like — middle size.
In the atmosphere, the names are analogous:
- Storm scale systems are small-scale storms such as thunderstorms with a size less than a few miles. The smallest storm scale phenomena are also called microscale.
- Synoptic scale systems are the customary high and low pressure systems that move generally from west to east in the mid-latitudes. They are larger than several hundred miles wide.
- Between storm scale and synoptic scale — and it’s a big between, encompassing systems from a few miles to a few hundred miles across — we have mesoscale systems.
The Atmosphere Moderates The Earth’s Temperature
The atmosphere is a complicated collection of gas molecules of primarily nitrogen, critically oxygen, significantly water vapor, increasingly importantly several greenhouse gases (CO2 and methane are the most important), and some other relatively insignificant stuff.
The earth and its atmosphere are in equilibrium with the solar energy received from the sun. Electromagnetic radiation comes in, mainly in the visible wavelengths, and terrestrial radiation goes out, mainly in the microwave wavelengths.
Since more solar radiation comes in at the equator than at the poles but roughly the same amount of terrestrial radiation goes out everywhere, there is an imbalance that must be corrected to achieve a temperature steady state. On an airless planet, the correction mechanism is conduction, a tedious process, which leaves the poles much colder than the equator because it acts so slowly.
On Mars, the summer equator is around 70 degrees Fahrenheit; winter at the pole is around minus 200 degrees.
On the earth, the temperatures are 100 at the equator and minus thirty at the north pole — about half the range of Mars. The moderation of earth’s temperature variation from pole to equator is accomplished by heat transport in the atmosphere.
The Process Of Atmospheric Heat Transport Depends On Rotation
If an intelligent person were to be shown an Earth-sized planet with an Earth-like atmosphere, not rotating but revolving around a sun-like star, he would see a circulation that would be easy to understand. Air would rise at the equator, move north aloft, sink at the poles, and return equatorward.
But if we asked our educated person to guess what would happen if we gave the planet a spin rate of 365 rotations for every revolution, it’s not likely he would predict a jet stream or equatorial easterlies.
Mid-latitude synoptic scale systems, driven by the jet stream, provide the bulk of the energy transport from equator to pole. Tropical systems add most of the rest.
There is something about the synoptic scale of a thousand miles or so that makes it most effective for heat transport, but it is not possible to say, looking at the equations that govern the atmosphere on a rotating planet, exactly why that is so.
Hurricanes Are Mesoscale Phenomena By NOAA’s Definition
Tropical cyclones, with a typical scale of a few hundred miles, are considered to be mesoscale phenomena. Decoded Science feels this is a misinterpretation of the spirit of the term.
The equatorial easterlies have a characteristic wavelength of a few hundred miles. It is no simpler to say why this should be so than it is to say why mid-latitude synoptic scale is a thousand miles.
Tropical waves and storms should be thought of as synoptic scale equatorial size — a few hundred miles across.
Mesoscale Convective Systems
The primary difference between synoptic scale and mesoscale features is that the former are baroclinic, while the latter are convective.
OK, there’s a big word here: baroclinic. It simply means derived from the potential energy of cold and warm air masses side-by side. When the cold air sinks and the warm air rises, a lower potential energy state is reached. This occurs in extratropical cyclones.
Convective overturning in the atmosphere is much more violent than the more gently rising and falling air associated with baroclinic systems. Convective activity occurs when the lapse rate (the decline of temperature with height) becomes too great and any displacement of an air parcel results in its continued upward or downward motion.
The most interesting mesoscale systems in the mid-latitudes are mesoscale convective complexes, derechos, squall lines, and supercells.
- A mesoscale convective complex (MCC) is a cluster of thunderstorms associated with a larger area of instability than just a single thunderstorm.
- A squall line is an elongated MCC. Squall lines often occur in association with cold fronts, but they can form along any boundary that separates two different types of air such as a sea breeze front.
- A derecho is really just a very large and powerful squall line. Derechos are characterized by damaging winds and long life.
- A supercell may form when a mesoscale convective area has vertical wind shear. The rotating structure can produce hail and tornadoes as it contracts.
The Role Of Mesoscale Systems In The Big Picture
Mesoscale systems play a subsidiary role in atmospheric energy transport, fairly insignificant in the total energy balance, but necessary for the smooth functioning of the system.
Consider the expenses of owning a car. You buy the car — the primary expense — but in order for the car to perform properly you have to purchase gas, change the oil, and occasionally have it washed. These are relatively minor expenses, but you couldn’t get around without them.
Analogously, the atmosphere probably cannot get along without mesoscale systems.