|DYNAMIC PRECIP vs. CONVECTIVE PRECIP
METEOROLOGIST JEFF HABY
There are two basic precipitation generation types which are dynamic precipitation and convective
precipitation. Dynamic precipitation is also known as stratiform precipitation. Dynamic precipitation
results from a
forced lifting of air. These forcing mechanisms include processes that cause
low level convergence and
upper level divergence. As unsaturated air rises the
relative humidity of the air will increase.
Once the air saturates, continued lifting will produce
clouds and eventually precipitation. Dynamic precipitation
tends to have a less intense rain rate than convective precipitation and also tends to last longer.
Convective precipitation is also known as thermodynamic precipitation. While dynamic precipitation only needs
saturated air and lift, convective precipitation requires an additional component called
due to instability release occurs when the air rises on its own after being lifting to a certain point in the
troposphere. Instability is commonly assessed by examining the
Lifted Index (LI) and
CAPE (Convective Available Potential Energy). Both these indices can be used to assess the acceleration rate of air once air from the lower
troposphere is brought to a level in the troposphere where it will rise on its own due to positive buoyancy. Instability
causes the air to rise much faster than it would by forced lifting alone. Think of convective precipitation as
falling from thunderstorms with
strong updrafts while dynamic precipitation falls from a deck of stratus clouds.
Convective precipitation tends to have
heavy rain while dynamic precipitation is more of
a gentle long lasting rain with no lightning and thunder.
Below this short essay are 6 forecast model images from 0Z. These images will be referenced throughout the
rest of this essay.
Let us look at two regions of precipitation and determine which region will have dynamic precipitation and
which region will have convective precipitation. On the image labeled "precipitation" notice the area of
precipitation in Washington State and Oregon and also notice the region of precipitation over the plains states.
Both regions of precipitation have forced lifting which is causing the air to rise. See the image labeled
"forced lifting". The lifting in the plains is caused by the low pressure region,
warm air advection,
convergence along a dryline and front, and
positive vorticity advection. Look for these lifting mechanisms on the images
labeled "low level moisture and convergence" and "vorticity". The lifting in Washington and Oregon is caused
by topographic uplift, a frontal boundary coming off the ocean and positive vorticity advection. Look at the images
below to see these lifting processes.
The last two images assess instability. The image labeled "lifted index" shows the regions of instability in
regions with the dark hatching. This region of instability extends from the Gulf of Mexico and into the plains
states. The image labeled "CAPE" shows instability in this same region. Notice that over Washington and Oregon
that there is no instability.
Notice the low level dewpoints are much higher over the plains states than they are over the Pacific northwest.
High dewpoints are one factor that increases instability.
From this information we can determine that the precipitation over the plains states will have convection embedded
within it while the precipitation in Washington and Oregon will be stratiform in nature. There is saturated air,
lifting and instability over the plains states while there is just saturated air and lifting in Washington and
Examples are from UNISYS weather. For current forecast model products visit:
LOW LEVEL MOISTURE AND CONVERGENCE