Drylines By: Allie Vegh

Definition: A dryline is a zone of strong horizontal moisture gradient separating warm, moist air from hot, dry air in the boundary layer. The warm, moist air originates from the Gulf of Mexico. The hot, dry air originates in the elevated terrain of southwestern United States and northern Mexico.

The hot, dry air caps the warm, moist air to create an inversion referred to as the dry front. The intersection of the dry front with the topography is known as the dryline.

Drylines tend to form east of the Rocky Mountains in the western Great Plains. Drylines are present on approximately 40% of spring and early summer days. They are typically about 500 to 1000 km in length and 1 to 20 km wide. Drylines appear on surface charts as sharp gradients in the dewpoint temperatures up to ~ 18 K in a distance of 1 to 10 km. The depth of the low-level moisture out in front of the dryline will determine its speed. The less moisture in the lower atmosphere the faster the dryline will move.

Types of Dryline Environments The two types are: Synoptically Active Quiescent

Synoptically Active The dryline typically extends from a surface low pressure center. Its motion is generated by turbulent vertical mixing in the boundary layer due to diurnal heating and horizontal and vertical motions from a mid- or upper-level short-wave trough. This allows for drier air aloft to mix with the moist boundary layer air allowing the dryline to propagate eastward.

Quiescent These drylines form in an environment that does not contain synoptic-scale forcing. The dryline’s motion is mainly due to turbulent vertical mixing as a result of diurnal heating. During the late morning and early afternoon hours the dryline propagates in an eastward direction, and during the late evening and overnight hours the dryline retrogrades back to the west.

Most drylines tend to be quiescent in form with a diurnal pattern. Eastward movement during the day and westward retrogression during the night. The overall net motion of the dryline is typically to the east. The slope of the dry front strengthens during the day and weakens at night. Surface winds in the moist air however maximize around midnight and are weaker during the day. This is due to the nocturnal low level jet which can form as a result of the dryline earlier in the day.

Drylines tend to be areas of locally enhanced convergence at the surface. Due to the enhanced convergence drylines tend to be a favorable location for convective initiation and isolated severe storms. Approximately 70% of the time convective storms form within 200 nm of the dryline. These storms account for a large percentage of the precipitation the Great Plains receive during the spring and summer months.

Dryline bulges: Area of strong surface winds that accelerate a portion of the dryline farther east than the rest of the dryline. Bulges tend to be areas of increased convergence and have a greater potential for convective activity.

References Journal of the Atmospheric Sciences Monthly Weather Review Weather & Forecasting Weatherwise