WEATHER and CLIMATE 1A
I'll try to answer a few of the questions from the last blog. Gravity waves --they are called gravity waves because it is the force of gravity that causes the wave-like motion. If you took an introductory course in meteorology then you should have seen buoyancy waves, which are a type of gravity wave. They occur when air which is a little cooler and heavier than the surrounding air is pulled down. The air overshoots a little, it's then warmer and lighter than the surrounding air, and it is lifted back up by the buoyancy force. I've adapted a picture from one of my lectures to try to make the idea more clear. Remember, as air rises it cools, as it sinks it warms, so it's the difference between the parcel's temperature and the background temperature that's important. Figure 1:
Schematic to demonstrate the role of gravity as a restoring force to a parcel of air. A basic concept in what makes a gravity wave.
I want to go back to the figure I showed last time. For you who asked for the data and related figures--well, that figure was made a long time ago and I had to scan it. In the future I will see if I can put some digital information with the figures so you can play with the numbers. Back to the figure--I reproduce it below with some annotation that I hope makes is clearer. (Really, though I want to show Jeff Masters that I figured out how to do that.) Figure 2:
Wavelet analysis of north-south moisture flux at San Antonio, Texas.
Since a couple of people asked me to clarify the picture a little bit. The wavelet analysis tells us what period oscillations are evident in the observations. So following the time axis it tells us that on, say, June 1 that the 4-8 day period wave was important. On, for example, August 1 the 1 day period wave was important. The one day period is labeled as the "diurnal time scale." The 4-8 day period is labeled as "baroclinic time scales." Baroclinic and synoptic are terms to describe the waves that are responsible for the ordinary highs and lows we associate with weather over the United States.
I find it especially interesting to think about the diurnal period moisture flux and climate. The low level diurnal jet stream is confined in the lower couple of kilometers of the atmosphere and is responsible for a night time river of moist air into the Great Plains. It is bound to the west by the increasing altitude of the plains and, ultimately, the Rockies. It's tremendously important for summertime moisture, and those thunderstorms that excite WU devotees. Because it is to some extent defined and anchored by the topography, it requires pretty high resolution models to represent it. When I start to think about climate predictions that are good enough for water resource managers to use, I want to know how this low level jet, this climate mechanism, will change. I want to know if it will start earlier in the year and last later. Will it penetrate further into the continent? Will it transport more water because the Gulf of Mexico is warmer? More questions? Seriously, if you were in the position to have to use a climate forecast in your job or life to make a decision--what would you want to know? What would give you confidence in the information in the forecast?
1993? Do any of you remember what was special about the summer of 1993?