METEOROLOGIST JEFF HABY
You may run across this term in a
NWS forecast discussion. Omega forcing deals with
synoptic scale rising or sinking of air. The two terms in the omega equation that deal with
vertical motion are
thermal advection and
positive differential vorticity advection. Thermal advection can be either cold air advection
or warm air advection.
Cold air advection in the low levels of the atmosphere causes sinking while warm air advection in the low levels of
the atmosphere causes rising air. PDVA causes rising air while NDVA causes sinking air. PDVA means higher values of
positive vorticity are being advected and that vorticity is increasing with height. A
will usually have vorticity increasing with height since winds in the upper levels are generally stronger than
low-level winds. The omega equation takes into account the magnitude of sinking or rising due to thermal advection
and couples that with the magnitude of sinking or rising due to PDVA or NDVA and determines if the resultant
vertical motion is upwards or downwards.
Upward motion implies a greater chance for: increasing
RH through a large depth of the atmosphere, clouds, and precipitation. The closeness of the air to saturation and the amount
of uplift can be used to forecast
forecast precipitation, and forecast precipitation amounts.