AOS 100: Weather and Climate

AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell

Miscellaneous Exam Breakdown: • Average: 85.73% • Median: 87.5%

Review of September 8th: Fronts and Forces • Besides surface observations, it is also possible to detect cyclones and fronts in satellite or radar images • In order for cyclones to intensify, you must decrease the surface pressure • The only way to accomplish this is to remove mass from the column of air above the cyclone • This means you need a net divergence of air in the column to intensify cyclones, and a net convergence to intensify anticyclones

Review Continued • First, we need to understand some fundamental atmospheric forces: the Pressure Gradient Force (PGF), the Coriolis Force, and Friction • The PGF will direct air from high pressure towards low pressure • A stronger PGF will cause a stronger wind • However, the Coriolis Force will act to turn that air to the right in the northern hemisphere, until it becomes parallel to the isobars

Review Continued When the PGF and Coriolis force are balanced, the atmosphere is said to be in “geostrophic balance”- The resultant wind is called the “geostrophic wind”

What direction would you expect the geostrophic wind to blow in? www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtmlwww.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

Almost … but why the difference? www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

The Friction Force • Close to the surface, geostrophic balance is not a very good approximation • This is because friction is quite strong near the Earth’s surface • The Earth’s surface is very rough (buildings, trees, mountains, etc.), which induces friction • Therefore, as you get farther away from the Earth’s surface, the friction force decreases • This means that geostrophic balance becomes more realistic as you move away from the Earth’s surface • Friction always acts to oppose the wind (with a strength proportional to the strength of the wind)

Friction Continued • However, if friction acts to slow the wind, then the Coriolis Force will weaken • At the same time, the PGF remains the same strength • Therefore, geostrophic balance is no longer in effect • This causes the wind to blow slightly across isobars, towards low pressure

The New Force Balance From: www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Spiral_Winds/Spiral_Winds.html

Constant Pressure vs. Constant Height Maps • So far we’ve looked at Sea Level Pressure maps (so pressure varies while the height is constant everywhere - 0 meters) • However, meteorologists often look at constant pressure maps (so the height changes, rather than the pressure) • As we’ll learn more about later, you can think of “high” heights as being analogous to high pressures, and “low” heights as being analogous to low pressures • Similarly, the geostrophic wind will blow parallel to lines of constant height, with lower heights to the left of the direction of the wind

Heights and winds at 200 mb Notice how much closer the winds are to geostrophic balance at this level, compared with the surface www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml

A Summary

Now Let’s Revisit Divergence …

Upper Levels Similarly to lower levels, at upper levels of the atmosphere, there is often a series of high pressures (high heights) and low pressures (low heights)

Upper Levels Ridge Trough Ridge Convergence Divergence

Why Do These Patterns Occur? • These patterns of convergence and divergence have to do with vorticity advection • If there is positive vorticity advection, divergence occurs • If there is negative vorticity advection, convergence occurs • Let’s explain vorticity …

Vorticity Vorticity is simply a measure of how much the air rotates on a horizontal surface Positive vorticity is a counterclockwise (i.e. cyclonic) rotation Negative vorticity is a clockwise (i.e. anticyclonic) rotation Therefore, troughs contain positive vorticity, and ridges contain negative vorticity

Vorticity Vorticity is simply a measure of how much the air rotates on a horizontal surface Positive vorticity is a counterclockwise (i.e. cyclonic) rotation Negative vorticity is a clockwise (i.e. anticyclonic) rotation Therefore, troughs contain positive vorticity, and ridges contain negative vorticity Trough Ridge

Let’s Revisit … Vorticity < 0 Vorticity < 0 Vorticity > 0 Convergence Divergence

Let’s Revisit … Vorticity < 0 Vorticity < 0 Vorticity > 0 Negative Vorticity Advection Positive Vorticity Advection

Diagnosing Vorticity Advection • To determine vorticity advection, first find the locations of maximum (positive) vorticity and minimum (negative) vorticity • Then, determine what direction the wind flow is • Areas of negative vorticity advection (NVA) will be just downstream of vorticity minima, and areas of positive vorticity advection (PVA) will be just downstream of vorticity maxima

The Connection Based on what we learned earlier, areas of convergence at upper levels favor surface anticyclones, and areas of divergence at upper levels favor surface cyclones

The Connection Given what we learned earlier, areas of convergence at upper levels favor surface anticyclones, and areas of divergence at upper levels favor surface cyclones

A 3-D Look

The Connection As the locations of upper level troughs and ridges change, we might expect the positions of surface cyclones and anticyclones to change

The Big Picture

Cyclone Growth And Decay • Based on what we’ve learned, the position of the surface cyclone in relation to the upper level structure is key to development • A cyclone will grow if it is below an area of PVA, and weaken if below an area of NVA or neutral vorticity advection • Commonly, a cyclone will intensify until it becomes situated in an unfavorable location in relation to the upper levels

An Example:Time 1 Above: Upper Level Height and Wind Speed Right: Surface Pressure

Time 2 Above: Upper Level Height and Wind Speed Right: Surface Pressure

Time 3 Above: Upper Level Height and Wind Speed Right: Surface Pressure

Summary of Event • At time 1, the upper levels and lower levels are perfectly set up for the surface cyclone to intensify • At time 2, the upper trough is almost above the surface cyclone, so the intensification slows • By time 3, the upper trough is exactly over the surface cyclone, so the intensification has halted

AOS 100: Weather and Climate