Commonly located in the Great Plains of the United States is a phenomenon known as the Nocturnal Low-Level Jet, or sometimes referred to as the nocturnal low-level wind maximum. It is known to be one of the primary causes for convective initiation during the warm season that occurs during calm, quiescent conditions.
The Nocturnal Low-Level Jet is defined by the American Meteorological Society as a jet stream located within the lower 2-3 kilometers of the troposphere that occurs during the night. This jet is a fast-moving stream of air that rapidly transports warm, moist air from the Gulf of Mexico northward at speeds ranging anywhere from 25 to over 70 knots! This low-level jet forms as a result of the diurnal temperature change and the resulting interaction between the surface layer and boundary layer of the atmosphere. During the day the forces resulting from a southerly geostrophic wind that include friction, Coriolis, and pressure gradient forces are in balance. However, the frictional component goes away during the nighttime as the daytime vertical mixing ceases to be distributed throughout the boundary layer. This imbalance of forces results in winds becoming supergeostrophic and in the overall net acceleration of the wind creating this nocturnal jet.
The warm, moist air that’s transported ahead of approaching synoptic and mesoscale disturbances as a result of this jet can generate large amounts of convective available potential energy (CAPE) and increase low-level vertical wind shear. These ingredients all help in the initiation of severe weather. Also, as this low-level jet often intersects with frontal or outflow boundaries, it can trigger longer-lived mesoscale convective complexes (MCC’s) that can produce heavy rain, winds, and sometimes more severe convective storms that can form tornadoes.
This nocturnal wind maximum is often enhanced by the natural, downward sloping of terrain that occurs from West to East by the diurnal oscillation of the thermal wind. This is shown by the figure below.
As shown in (a) during the day the surface can quickly heat up, but the sloping terrain causes the air towards the West to be warmer than the air to the East at the same height. The warmer air to the West results in a higher pressure than the relatively cooler air to the East causing surface pressure to rapidly decrease with height. The Coriolis force then kicks in and deflects the air to the right (southward) creating this Northerly daytime low level jet. (Remember that the thermal wind always blows with colder air to the left.)
As shown in (b) during the night the process is reversed. Since the ground towards the West cools off more quickly due to radiational cooling, it is relatively colder than the air to the East at the same height. This causes the thermal wind to reverse and a southerly low-level jet will develop and known as the nocturnal low-level jet.
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© 2019 Weather Forecaster Christine Gregory