Transcript Further thoughts about dryline formation

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
other studies have documented a density
difference
Drylines in West Texas / Oklahoma
reference
approx. time
(UTC)
scale
Dqv (K) (estimated)
zi,d (km)
dryline motion (ms-1)
10 km
25 km
1.1
1.1
<0
Fig. 6 in NSSP Staff (1963)
2110
0.9
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 2330
0.5 early
1.5 late
1.0 early
3.0 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
The density difference across well-defined drylines is
small compared to strong density currents, but
comparable to sea breezes
a. Non-dryline boundaries interpreted as atmospheric density currents
reference
phenomenon
Dqv (K)
over ~10 km
Ddc
(k
m)
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
 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 finescale convergence?
(dry side)
6 May 1995
(VORTEX)
Atkins et al 1998, Mon. Wea. Rev.
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
IHOP (3 km averages)
VORTEX-II (10 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.
•
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