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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 if the moisture is present to be condensed. Let's apply upward vertical velocity to interpreting the synoptic scale forecast models (ETA, NGM, AVN, 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:

http://weather.unisys.com/nam/700.php

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, PVA, 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.