Dryline Gust fronts Coastal fronts Topographically induced fronts

The Dryline The dryline can be defined as the near surface convergence zone between moist air flowing off the Gulf of Mexico and dry air flowing off of semi-arid high plateau regions of Mexico and the southwestern United States:

The Dryline The dryline is observed generally from April through June about 40% of the time It’s observed from the southern Great Plains up into the Dakotas – from the Rockies eastward to about 96 degrees W longitude. It is easiest to identify with a moisture variable such as mixing ratio: From Shaefer (86)

The Dryline From Shaefer (86) The 9 gm kg-1 isohume or 55 degree F isodrosotherm are often used to indicate dryline position Wind shift and moisture gradient are not always collocated Forms is synoptically quiescent regions, sometimes in a trough Td gradient is often 15 degrees C per 100 km, can be larger! From Shaefer (86)

Convective Initiation: Dryline Significance Convective Initiation:

Convective Initiation: Dryline Significance Convective Initiation:

Convective Initiation: (from Murphy et al. 2005, meso/radar conf) Dryline Significance Convective Initiation: (from Murphy et al. 2005, meso/radar conf)

Dryline Significance Agriculture: Hot/dry to the west – need to irrigate more Warm/humid to the east

Dryline Evolution and Movement Here is the situation in the morning hours before surface heating creates thermals in the boundary layer:

Dryline Evolution and Movement With time:

Dryline Evolution and Movement With time, the dryline moves east as thermal turbulence mixes out the western part of the shallow moist layer.

Dryline Evolution and Movement This idea of eastward dryline movement is supported by observations showing that it moves suddenly, very quickly eastward. This sudden movement can not be explained by advection alone. Mid to late afternoon, the dryline moves back to the west. Why? There are two theories:

Dryline Evolution and Movement Theory #1: It is advected back to the west by an enhanced ageostrophic flow produced by a deepening lee trough The lee trough is deepening in response to strong solar heating of the higher terrain

Dryline Evolution and Movement Theory #2: It’s moving back to the west like a density current:

Is the dryline dynamically similar to a density current? From Atkins et al. 97- MWR

Is the dryline dynamically similar to a density current? Over the scale near the dryline where the virtual potential temperature gradient was large, dryline speed match density current theoretical prediction. From Atkins et al. 97- MWR

From Miao and Geerts 07 – MWR Showed that propagation speed was consistent with density current dynamics

From Geerts 2008 - WAF Dqv = qve - qvw

Note good correlation between observations and theoretical estimates of dryline movement

Dryline Evolution: A Schematic Model From Parsons et al. (2000, MWR)

Dryline Structure What does the dryline look like, anyway? Is it a broad synoptic-scale (meso-a) confluence zone as suggested on synoptic charts? Or is it a smaller scale (meso-b or meso-g, sharper boundary such as a density current?? Let’s take a look:

Dryline Structure Aircraft observations:

Dryline Structure Lidar observations:

Dryline Structure Data from another case study by Atkins et al. (97)

Dryline Structure Notice: Dryline is associated with a sharp thin line Boundary layer convection (HCRs) on both sides of the dryline Begins moving westward at later times, notice the localized bulge Atkins et al. (97) From Atkins et al. (97 – MWR)

Dryline Structure Check out the incredible along-dryline variability in the vertical velocity field From Atkins et al. (97 – MWR)

Dryline Structure Notice: The large gradient in mixing ratio collocated with the thin line The qv gradient – implies a density gradient! Froude # calculation suggests that the dryline was moving like a density current! Well defined circulation in winds From Atkins et al. (97 – MWR)

Dryline Structure So, how is it that the dryline can evolve into a very sharp boundary when it is within a broad confluence zone between two synoptic-scale air masses? One idea is the “inland” sea breeze effect:

Dryline Bulges Somewhat common 80-100 km in scale Preferred locations for convective initiation Shaefer (86)

Multiple Dryline Structure MWR

Weiss et al. 2006 – MWR

Gust Fronts Wakimoto (82, MWR)

Gust Fronts From Droegemeier and Wilhemson (87, JAS)

Gust Fronts From Mahoney (88, MWR)

Gust Fronts From Mueller and Carbone (JAS 87)

Gust Fronts From Mueller and Carbone (JAS 87)

Gust Fronts From Droegemeier and Wilhemson (87, JAS)

Gust Fronts Sequence of events for gust front passage: Pressure rises and winds decrease Change in wind direction and speed Decrease of temperature Precipitation From Wakimoto (82)

Gust Fronts Notice the pressure rise both ahead and behind the leading edge of the gust front: Hydrostatic pressure rise: behind the gust front in cold air Non hydrostatic pressure rise ahead – a dynamic effect Pnh = 1/2rV2 is created by the collision of two fluids From Wakimoto (82)

Gust Fronts From Wakimoto (82, MWR)

Coastal Fronts First discussed by Bosart et al. (72, JAM) Refers to a boundary separating an easterly maritime air flow off Atlantic from cold, northerly outflow of an anticyclone Data on coastal fronts was collected during the New England Winter Storms Experiment (NEWSEX): From Nielsen (89, MWR)

Coastal Fronts – what do we know about them? Form near the coast in late fall, early winter From northern New England down to the Carolinas (See Appel et al. 2005 - WAF for climatology of Carolina coastal fronts) Boundary between rain and freezing rain/snow DT = 5-10 degrees Celsius over 5-10 km How do they form? Important factors include: Surface friction Orography Coastal configuration Land-sea thermal contrast

Coast Front Formation Three scenarios (a,b,c) – Type a is most common and shown below: Forms during cold air outbreak Takes place during transition from offshore to onshore winds caused by passage of a ridge of high pressure from west to east Inland winds fail to shift – generates a convergence zone From Nielsen (89, MWR)

From Nielsen (89, MWR)

Vertical Structure of Coast Fronts From Nielsen and Nielley (90, MWR)

Vertical Structure of Coast Fronts From Nielsen and Nielley (90, MWR)

Coastal fronts, a density current? From Nielsen and Nielley (90, MWR)

Topographically-induced boundaries – The Denver Convergence Zone Cheyenne Ridge Palmer Divide

The DCZ forms when the ambient flow is S, or SE From Wilson et al. (92, MWR)

Clouds and the DCZ From Wilson et al. (92, MWR)

Fine-scale structure along the DCZ- misocyclones From Wilson et al. (92, MWR)

DCZ Formation – modeling study by Crook et al. (90, JAS) From Crook et al. (90, JAS)

DCZ Formation – modeling study by Crook et al. (90, JAS) A cloud model was initialized with a sounding launched on an observed DCZ day along with the observed topography From Crook et al. (90, JAS)

DCZ Formation – modeling study by Crook et al. (90, JAS) Vortex formation only occurs when the froude number is small: F=U/Nh U = upstream velocity (upstream of the Palmer Divide) h = height of the obstacle (Palmer Divide in this situation) N = Brunt-Vaisala frequency that is given by: So, low froude number flows are those that have small upstream velocities and are stably stratified These types of flow will want to flow around an obstacle, or will be blocked by a long, linear one

END

From Schultz et al. 2007 MWR

From Schultz et al. 2007 MWR