AOS 100: Weather and Climate

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

Miscellaneous • New Homework

Review of September 8th: Forces and Vorticity • Geostrophic balance is a result of a balance between the PGF and the Coriolis Force, and the resultant wind is called the geostrophic wind • This is not a good approximation at the surface, because of friction • Friction always acts opposite the wind, and causes the wind to slow • Since the Coriolis Force is proportional to the strength of the wind, the Coriolis Force also slows

Review Continued • However, the PGF remains the same strength • This results in a new force balance, which causes the wind to blow slightly across isobars toward lower pressure • Upper levels are pretty close to geostrophic balance, due to the reduced friction • When looking at a map of constant pressure, we can use heights the same we use pressure when looking at a surface map • Vorticity advection determines where convergence or divergence will occur at upper levels

Review Continued

Review Continued 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

From: http://www.noaanews.noaa.gov/stories2009/20091007_septusstats.html

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 (read cyclone) 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

Cyclone Decay • Recall that due to friction, air blows across isobars near the surface • This means that the air is always converging at the center of low pressure areas • Therefore, unless there is at least enough divergence at upper levels to counteract the convergence at low levels, the surface cyclone will weaken because more mass will be added to the air column • This will force the surface pressure to rise

Revisited

About Contour Analysis …

An Example From Tuesday Morning: http://www.ral.ucar.edu/weather/surface/

Observations about Observations • Conventionally, only temperature, dewpoint, wind speed and direction, cloud cover, pressure, current weather, and visibility (if less than 10 miles) are shown • However, much of the planet goes unobserved • Large differences in temperature, dewpoint, etc. can exist from location to location • This is why we must do contour analysis in order to “fill in” the missing data

How Do We Do Contour Analysis? • You can think of it like a glorified version of “connect the dots” • However, for contour analysis, we have to “fill in” some of the missing data • The goal of contour analysis is to allow for easier interpretation of the current weather • Some things to remember: - Lines never cross - Always use a pencil so you can erase lines • Now for an example …

AOS 100: Weather and Climate