Dry Line Initiation Video URL http www nssl

Dry Line Initiation Video URL: http: //www. nssl. noaa. gov/users/ziegler/public_html/initiation. html

Dry Lines or

Dry Lines • Associated with large horizontal gradients in moisture, but not necessarily temperature. • Results from the interaction of cyclones and fronts with large-scale terrain. • Found over the U. S. Midwest, northern India, China, central West Africa and other locations. • Acts as a focus for convection, and particularly severe convection. • Most prevalent during spring/early summer in U. S.

Dry Line • Surface boundary between warm, moist air and hot, dry air. Surface dry line Well-mixed warm air Inversion or cap

Typical Dryline Temperatures in degrees Celsius © 1993 Oxford University Press -- From: Bluestein, Synoptic-

Dry Line Southern Plains Dry Line © 1993 Oxford University Press -- From: Bluestein, Synoptic. Dynamic Meteorology in Temperatures in degrees Celsius

Trajectories • Fundamentally the dry line represents a trajectory discontinuity between moist southerly flow and flow descending from higher elevations. • Can only happen relatively close to the upstream barrier (no more than 1000 km) since otherwise air would swing southward behind the low system and thus would be cool and somewhat moist.

DRY LINE L Warm, Moist

L NO DRY LINE — Get Cold Front

Dry Line: Tends to Move Eastward During the Day and Westward At Night • After sunrise, the sun will warm the surface which will warm the air near the ground. • This air will mix with the air above the ground. • Since the air above the moist layer is dry, the mixed air will dry out. • The dry line boundary will progress toward the deeper moisture.

Dry Line Top of moist layer before mixing Boundary after mixing Hot, Dry Air—Usually Well Mixed Warm, Moist Air Initial Position of the Dry Line after mixing

Dry Line • After sunset, a nocturnal inversion forms and the winds in the moist air respond to surface pressure features. • The dry line may progress back toward the west.

West East Note weak inversion or “cap” over low-level moist layer east of the surf

NCAR Sounding West of the Dryline West Winds Very Dry Albuquerque, NM 12 Z -- 26 June 1998

NCAR Sounding East of the Dryline South Winds Moist Oklahoma City, OK 12 Z -- 26 June 1998

Aircraft Study of the Dry Line

Convection Tends to Focus On the Dryline

Storm Initiation Along a Dry Line

Simulation of a Thunderstorm Initiation Along Dryline in TX Panhandle Storm Note converging winds and rising motion