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What is a dryline MET 171B: Drylines z Mike Voss z An airstream boundary separating moist air originating over the Gulf of Mexico from dry air originating over the plateaus of southwestern U.S. and Mexico. The dryline typically forms along the high plains in the spring months and is a frequent site of convection initiation. Dry Lines Climatology of Dry Lines Defined: Narrrow zone of strong moisture gradient (dew points) at the surface …. Really a dry line is a dew point front… z Where??? z z z Why?? z This typically represents a boundary between air masses (moist maritime vs. dry continental) Typically located over the Plains from Texas northward to the Dakotas in spring and summer Rarely seen east of the Mississippi z Moist air masses that originate over the Gulf mingle with air masses that were modified and or develop over the interior West Topography Matters Typical Dryline Dryline Symbology 1. Descending air in lee of Rockies is warm and dry 2. Descending air also leads to lee side trough What process would initiate southerly flow from Gulf and westerly flow from SW over a place like KS/OK?? z Our friend lee… and a surface high over the SE US Temperatures in degrees Celsius ©1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic Meteorology in Midlatitudes, Volume II 1 z z z z Synoptic conditions associated with strong and weak periods in dryline are outlined in the handout journal paper Strong defined by: high Delta Td, high Delta U Weak defined by: opposite See paper for details z Air on upwind (upslope) side of dry line is represented by a deep mixed layer z z z z Air west of the dryline has broken the inversion and so upper level dry and warm air from the EML has mixed down The inversion breaks on its western extremity first because the inversion is closet to the surface there As the daytime surface mixed layer deepens to the inversion throughout the day, the inversion break (dryline position) works its way eastward West of the dryline, mixing of the deep EML with the shallow moist layer produces : z Gusty westerly surface winds z Very low humidities z Warm temperatures z Air east (downslope) of the dryline is relatively moist and cool, although there tends to be only a small virtual temperature contrast across the dryline z z z z Daily Evolution: z z z Typical Cross Section View of Dryline (see images from lecture!) Shallow moist boundary layer Source of moisture includes z Evapotranspiration z Advection from south east (Gulf) Moist boundary layer reaches depth of the inversion at the point of the dryline Late morning and afternoon: z Boundary layer grows below nocturnal inversion z dryline eats its way eastward Evening: z Nocturnal inversion forms z Moisture trapped under inversion Early morning z Plants become active and transpire raising moisture under inversion z Slope flow commences enhancing inversion 1. Elevated Mixed Layer advected east over moist layer 4. Heating of the day allows inversion to mix out from west to east. Dry line moves. Topography West Side East Side Mixed Layer, breaks through shallow inversion early in day Moist (cool) air in boundary layer, grows during nocturnal inversion, mixes with heating of day Interior West Plateau 3. The Intersection with the Topography to the West is the Dry Line 2. Warm Mixed Layer Overrides the Moist Air to create an Inversion Heating, no inversion 2 Quiescent (non-synoptic forced) Types of Dryline Environments z The two types are: z z Quiescent (absence of synoptic forcing) Synoptically Active z z z Synoptically Active The dryline typically extends from a surface low pressure center. Often positioned ahead of the cold front Vertical motions from a mid- or upper-level short-wave trough exacerbate quiescent diurnal pattern This allows for drier air aloft to mix with the moist boundary layer air allowing the dryline to propagate eastward. z z z z The dryline’s motion is mainly due to vertical mixing as a result of diurnal heating along topography Moves eastward during the late morning and early afternoon hours (advances mix the cool moist air with the well mixed component as heating progresses) + advection Moves westward during the late evening and overnight hours the dryline retrogrades Dryline Bulge z z Non uniform tropospheric flow can advance portions of the dryline. The combination of low-level convergence + shear can locally enhance convective uplift. Features are typically pronounced with synoptic forced dryline Drylines = Convergence z Drylines tend to be areas of locally enhanced convergence at the surface. z Approximately 70% of the time convective storms form within 200 km of the dryline. z These storms account for a large percentage of the precipitation the Great Plains receive during the spring and summer months. 3