Convergence is air streams flowing into one another or stronger wind moving into weaker wind. When air piles together in the low levels of the atmosphere it has no place to go but up (it obviously can not go through the earth's surface). Low level convergence can result from fronts, low pressure frictional convergence, WAA, moisture advection, orographic convergence, and mesoscale convergence boundaries (sea breeze, temperature discontinuities). An important ingredient that goes along with low level convergence is low level moisture and RH. Since convergence causes rising air, the RH will increase therefore causing the air to come closer to saturation as it rises. A dark hatching that partially hides the background colors on the model images shows convergence. Strong WAA over a frontal boundary will often produce high values of UVV. Another term for this is isentropic lifting. Low-level convergence produces lift (lift and moisture are the two critical ingredients for producing precipitation). Convergence is displayed on the 1000-mb and surface NAM model output. http://weather.unisys.com/nam/sfc.php |