|VERTICAL VELOCITY ON THE 700 MB MODEL PROGS
METEOROLOGIST JEFF HABY
Wind flows in the horizontal at a much higher average wind speed than in the vertical. Vertical motion is roughly
two orders of magnitude smaller than horizontal motion. Wind speeds in the horizontal are commonly over 50 knots at
some point in the atmosphere above a location. The average vertical wind speed is only a few centimeters per second!
This seems unusual considering
thunderstorm updrafts can have vertical velocities over 100 miles per hour. The deal
is, thunderstorms only encompass a tiny surface area of the earth compared to regions not having thunderstorms. Well
less than 1% of the time are vertical velocities greater than 1 mile per hour above any point location. All the uplift
from low pressure and fronts only produce vertical upglide of a few to sometimes greater than 20 centimeters per
second on the synoptic scale. That's it. Why then does it rain? Well, an uplift of 6 centimeters per second leads
to a pretty significant distance given enough time. In fact, moving 6 centimeters per second in one hour produces
216 meters of vertical distance. Give it a few hours, and that parcel of air can rise in the vertical over a
kilometer. An upward vertical velocity of just 6 centimeters per second can produce a large volume of precipitation
moisture is present to be condensed. Let's apply upward vertical velocity to interpreting
the synoptic scale forecast models (NAM,GFS etc.). Upward vertical velocity is plotted on the 700-mb prog.
Go ahead and look at a 700 mb forecast panel on your computer. This prog is available on
UNISYS weather at:
You will see a panel full of colors, wind vectors, and height contours. The colors are the upward and downward
vertical velocities. Notice the color scale below the panel. The scale will range from the lowest to highest
forecasted synoptic scale vertical velocity on the panel. On the legend at the top you will notice a -ub/s.
This stands for negative microbars per second. The negative sign is used because pressure decreases with height
in the atmosphere (usually when graphing, up is positive, but in this case, up leads to LOWER pressure). ub/s is
made negative so upward vertical velocity can be given a positive sign. What is a ub/s anyway? A bar of pressure
is equal to 1000 millibars. A ub (called a microbar), is a millionth of a bar and a thousandth of a millibar.
You probably know that a thousandth of a bar is a millibar. This is a fairly small pressure change over time but
can lead to large changes in pressure given enough time. The model is using pressure as vertical distance instead
of height. Conveniently as the math works out, a ub/s is just about the same vertical velocity as a centimeter
per second. A vertical velocity of 6 -ub/s is significant while a vertical velocity of 10 or greater is very
significant (also need moisture!). As you look at the 700 mb forecast panel you will notice bullseyes of upward
vertical velocity (denoted UVV for short). These are regions where mechanisms such as
low level WAA,
low level convergence,
jet streak divergence,
orographic uplift, etc. are causing the air to rise in the vertical.
Sinking mechanisms such as CAA,
downsloping and NVA causes downward VV's. The vertical velocity value for any
one point is the compilation of ALL upward and downward vertical velocities added and subtracted at that point.
UVV will be maximized in regions lacking downward VV mechanisms while having uplift mechanisms in place. The
average of all upward and downward motions is zero averaged across the entire earth. If upward motion constantly
were larger than downward motion, then the atmosphere would lose its mass. There is a conservation of mass for
the atmosphere; What air goes up, eventually has to come back down. You will notice that by areal coverage, the
regions of near zero and negative vertical velocity (downward motion) encompass a larger area than the regions
experiencing UVV. Also, the UVV maximums tend to be higher in magnitude than the DVV maximums. This is partly
because high pressure encompasses a larger region than low-pressure regions. Having a larger area of downward
motion is offset by a smaller but more intense upward motion; In the end, the mass of the atmosphere is conserved.