10
Severe ATM and Unusual Weather
http://atm.ucdavis.edu/~grotjahn/course/atm10/index.html
Prof. Richard Grotjahn
Lecture topics:
What happens when air rises? Stability
What is it? Why is it important? How is it classified? How do you know what type is present? Use of a chart of temperature in vertical Etc.
Vertical Movement & Temperature
A rising air parcel encounters less pressure so it expands. Expansion uses energy to push out, adiabatically cooling the air. A sinking parcel encounters greater pressure and that higher pressure does work on the parcel thereby heating it up. Recall: P = R T
Figure 7.2
Stability & Clouds: Introduction
Stability is related to weather several ways:
Stability & Clouds: Introduction
Three examples of weather related to stability:
Clouds types linked to stability of the air. Stratus clouds found in stable conditions, cumulus are in unstable conditions. Thunderstorms in very unstable conditions. Lenticular wave clouds form in stable air Downslope winds, like the Santa Ana: stable air stratiform
cumuliform
lenticular
But what is stability?
What is Stability?
Stability indicates to what extent the temperature CHANGE IN A LAYER OF AIR resists vertical motion. Fig. 7.1 illustrates the concept w/ an analogy.
Stable returns to Original position.
Unstable keeps going In direction of push.
Figure 7.1
Why examine stability?
Different types of cloud form for different stability conditions Most clouds form as air rises -- leading to cooling -- leading to condensation. So, since stability affects the ability of air to rise, stability affects cloud growth.
How is T structure classified?
Stability related to temperature change in the vertical. Temperature change in the vertical is called the lapse rate. Lapse rate is: = - T / Z T is T at top of layer minus T at bottom of layer. Z is the thickness of layer. >0 usually
Fig: T= -15 C, Z=1000m, = 0.015 C/m
If I know , then I know the stability?
Almost Stability depends on: - value AND on - whether the air is saturated or not Why saturated? Because if air that is saturated is cooled further, then some vapor must condense. Recall that the adds condensation heat to the air.
How many types of stability are there?
Three basic types, (plus some special cases given specific names) 1. Absolutely stable 2. Conditionally unstable 3. Absolutely unstable Special case of #3 is inversion: T increases with elevation Neutral a special case related to saturation
How do you tell them apart?
Compare the actual change of temperature IN A LAYER with two calculated lapse rates called the dry adiabatic and moist adiabatic lapse rates: d & m. Example: in clear air, deduce stability from smoke plume
What are d and m ?
dry adiabatic lapse rate: d = 10 oC/1000m of elevation change moist adiabatic lapse rate: m < d.
m depends on amount of water vapor to dry air which in turn depends on T and P. T change for parcel that is saturated.
Am I now able to figure out the stability classes?
Yes! but it is easier to do this using a chart Compare the actual T change with height to the lapse rates d (red line) and m (blue line) Figures 7.3, 7.6, and 7.7 work out the three categories. Figure 7.8 is a summary:
Figure 7.8
Absolutely Stable
to the right of m Unsaturated (left), saturated (right) Recall: P = R T
Absolutely Unstable
to the left of d Unsaturated (left), saturated (right) Recall: P = R T
Conditionally Unstable
betwe...