Steering and the Met Math That Can Explain It (Kinda)
Jul 8, 2021 15:12:17 GMT -6
HarahanTim - Now in Covington!, grisairgasm, and 1 more like this
Post by thermalwind - Touro on Jul 8, 2021 15:12:17 GMT -6
I posted in my climate change thread using the QG height eqn to explain parts of why that ridge built in the NW a couple of weeks ago, but there are some useful applications for what it means for the subtropical ridge that so often is the main player in steering tropical systems that threaten the northern gulf coast.
This is intended for the average joe/jane interested in weather, so it is going to be oversimplified and I'll ignore some details that aren't useful to explain things. The goal here is to explain in English what the QG thermo equation tells us about what causes changes in ridge strength and positioning.
Using Elsa as an example, here's a look at how focusing on the evolution of the ridge gave a pretty good idea where Elsa was most likely to go. Do note, it wasn't this simple at the time with some significant questions about if it could escape farther west.
Some iteration of this happens with most storms, and you'll read in discussions that some trough is going to cause the ridge to erode and let the north...or something like that.
So we bring in the QG height eqn. It is done assuming a set pressure level, and the heights (solid lines) are essentially how high off the surface that pressure level is. Warmer the airmass, the higher the heights, stronger the ridge. The equation is trying to explain how the heights at a certain pressure level change with time at a location. It assumes nearly geostrophic flow, which just means it's not going to work with a hurricane or something resulting from smaller scale influences.
So there are a number of "terms" that influence the evolution of heights at a given location.
1) Vorticity Advection Term
The first is advection of cyclonic/anticyclonic vorticity (so much for plain english) at that location and specific pressure level of concern. Ok, so what you need to know is a trough advects in cyclonic vort on it's right side (for the northern hem at least) and a ridge advects in anticyclonic vort on it's right side. Advection of cyclonic vorticity (a trough moving in) will basically move the ridge out of the way, causing a drop in heights at the location. This hasn't changed anything about the strength of the ridge, just where it is.
So you get a shortwave diving out of the NW, the cyclonic vort advection pushes the ridge back east a little (or at least pushes back the edge) and the storm turns north around the edge. The details of this, of course, are never super cut and dry and they absolutely matter.
2)Differential Thermal Advection Term
Did that first one seem complicated? Well, bad news...
What this is saying is the difference in thermal advection between two pressure levels influences the strength of the ridge (how high the heights are). If warm air advection decreases with height (warm air is being blown into the bottom more than the top) or cold air advection increases with height (cold air is coming in more higher up than the surface), this leads to heights rising.
You don't look at this quite so directly as the edge getting pushed back, but it can be part of the explanation for why a ridge might be strengthening and expanding.
3)Differential Diabatic Heating
Basically energy gain/lost from solar radiation, radiational cooling at night, latent heat exchanges (evaporation/condensation). Add heat, ridge gets stronger. Remove it, ridge gets weaker. Or add heat, heights rise and remove heat, heights fall.
The point of this is to give you a little insight as to why the subtropical ridge will vary in strength and position, and that those evolutions often determine where a hurricane will end up.
This is intended for the average joe/jane interested in weather, so it is going to be oversimplified and I'll ignore some details that aren't useful to explain things. The goal here is to explain in English what the QG thermo equation tells us about what causes changes in ridge strength and positioning.
Using Elsa as an example, here's a look at how focusing on the evolution of the ridge gave a pretty good idea where Elsa was most likely to go. Do note, it wasn't this simple at the time with some significant questions about if it could escape farther west.
Some iteration of this happens with most storms, and you'll read in discussions that some trough is going to cause the ridge to erode and let the north...or something like that.
So we bring in the QG height eqn. It is done assuming a set pressure level, and the heights (solid lines) are essentially how high off the surface that pressure level is. Warmer the airmass, the higher the heights, stronger the ridge. The equation is trying to explain how the heights at a certain pressure level change with time at a location. It assumes nearly geostrophic flow, which just means it's not going to work with a hurricane or something resulting from smaller scale influences.
So there are a number of "terms" that influence the evolution of heights at a given location.
1) Vorticity Advection Term
The first is advection of cyclonic/anticyclonic vorticity (so much for plain english) at that location and specific pressure level of concern. Ok, so what you need to know is a trough advects in cyclonic vort on it's right side (for the northern hem at least) and a ridge advects in anticyclonic vort on it's right side. Advection of cyclonic vorticity (a trough moving in) will basically move the ridge out of the way, causing a drop in heights at the location. This hasn't changed anything about the strength of the ridge, just where it is.
So you get a shortwave diving out of the NW, the cyclonic vort advection pushes the ridge back east a little (or at least pushes back the edge) and the storm turns north around the edge. The details of this, of course, are never super cut and dry and they absolutely matter.
2)Differential Thermal Advection Term
Did that first one seem complicated? Well, bad news...
What this is saying is the difference in thermal advection between two pressure levels influences the strength of the ridge (how high the heights are). If warm air advection decreases with height (warm air is being blown into the bottom more than the top) or cold air advection increases with height (cold air is coming in more higher up than the surface), this leads to heights rising.
You don't look at this quite so directly as the edge getting pushed back, but it can be part of the explanation for why a ridge might be strengthening and expanding.
3)Differential Diabatic Heating
Basically energy gain/lost from solar radiation, radiational cooling at night, latent heat exchanges (evaporation/condensation). Add heat, ridge gets stronger. Remove it, ridge gets weaker. Or add heat, heights rise and remove heat, heights fall.
The point of this is to give you a little insight as to why the subtropical ridge will vary in strength and position, and that those evolutions often determine where a hurricane will end up.