Further thoughts about dryline formation

Bart Geerts, University of Wyoming

Miao and Geerts (2006) provide rather strong evidence that the fine-scale convergence is driven by a solenoidal circulation and thus a density difference in the convective BL (CBL).

However, there may be other mechanisms:- topography (ridgeline)- contrast in CBL depth, and thus differential vertical momentum transfer during the deepening stage of the CBL- land use contrast ??

For all these mechanisms, scale matters: a better understanding of the relevant scales is important.

Miao, Q., and B. Geerts, 2006: Fine-scale vertical structure and dynamics of some dryline boundaries observed in IHOP. Mon Wea. Rev., accepted

Drylines in West Texas / Oklahoma

reference approx. time (UTC)

v (K) (estimated) zi,d (km) dryline motion (ms-1)

scale 10 km 25 km

Fig. 6 in NSSP Staff (1963) 2110 0.9 1.1 1.1 <0

Fig. 8 in Bluestein et al. (1990) 2230 1.5

Fig. 8 in Parsons et al. (1991) 0100 - 1.9 1.2 -7.5

Fig. 8 in Ziegler and Hane (1993) 2110 1.8 2.1 1.3 <0

Fig. 5 in Hane et al. (1997) 2220 1.0 1.2 1.0 ~0

Figs. 14 and 16 in Atkins et al. (1998); Fig. 12 in Ziegler and Rasmussen (1998); Fig. 1a (6 May 1995)

see Fig. 15 1.8 2-3 K 0.9 ~0 to -1

Fig. 7 in Ziegler and Rasmussen (1998); Fig. 1b (7 June 1994)

see Fig. 15 1.9 2.0 K 1.5 ~0

Fig. 2 in Ziegler and Rasmussen (1998) (15 May 1991)

1540 to 23300.5 early1.5 late

1.0 early3.0 late

0.6-1.2 +3 to 0

Miao and Geerts (2006), May 22 22-00 1.3 - 1.6 -3

Miao and Geerts (2006), 3 other cases 21-00 0.2-0.9 - 1.0-1.6 variable

other studies have documented a density difference

a. Non-dryline boundaries interpreted as atmospheric density currents

reference phenomenon v (K)

over ~10 km

Ddc (km)

Geerts et al. (2006) cold front 4.1 0.8

Charba (1974) Oklahoma gust front 6.1 1.3

Mueller and Carbone (1987) Colorado gust front 4.3 1.2

Atkins and Wakimoto (1997) 6 Florida gust fronts 2.5 1.1

Kingsmill and Crook (2003) 10 Florida gust fronts 4.5 1.1

Atkins and Wakimoto (1997) 18 Florida sea breeze fronts 1.1 0.5

Kingsmill and Crook (2003) 10 Florida sea breeze fronts 2.0 0.7

The density difference across well-defined drylines is small compared to strong density currents, but

comparable to sea breezes

at small scales (~3-30 km?) a dryline appears no different from other radar “fine-lines”.

Differential vertical momentum transfer due to different CBL depths: is this a cause of

fine-scale convergence?

Atkins et al 1998, Mon. Wea. Rev.

(dry side)

6 May 1995(VORTEX)

change in momentum towards the dryline, on the dry side of the developing dryline

Vertical momentum transfer?

For this process to be effective, we want westerly (“towards the

dryline”) momentum to exist over the depth over which the

dryline is deepening.

Ths process seems to be of little relevance in the VORTEX and IHOP cases examined, but

the possibility remains.

Even if the process is important, the question

remains what causes the CBL depth discontinuity in the first

place.

Temporal changes of differences across several drylines

VORTEX-II (10 km averages)IHOP (3 km averages)

So what drives the density difference?

• Even in a boundary layer with vigorous convective motions, relatively small horizontal θv differences drive a solenoidal circulation in which the less-dense air (usually the dry air) rises over the denser air.

• Fine-scale convergence lines (drylines or other radar fine-lines) are expected to form whenever the mesoscale θv gradient exceeds some threshold ( 1.0K/ 25km? Less?).

• The strong diurnal signal observed in all cases suggests that the driving force is the east-west gradient in daytime surface buoyancy flux over the southern/central Great Plains.

• On days with a large east-west gradient in daytime surface buoyancy flux, the threshold may be exceeded in several locations, and initially multiple fine-lines may form.

sonde 08

• combine fine-scale measurements with larger scale efforts (hi-res modelling and mesoscale transect of surface fluxes)

• mean dryline longitude: ~100.6W (101.0W in June) (Hoch and Markowski 2005, J Climate)

• strong assets present:– AMA and LBB radars close enough for clear-air obs– dense West Texas mesonet, denser towards Lubbock (LBB) where the

dryline tends to be best defined.

• see map next page