Effects of the land surface on pre-existing organized convective systems

Karsten Peters and Cathy Hohenegger

Max-Planck-Institut für Meteorologie and Hans-Ertel Zentrum (HErZ) für Wetterforschung, Hamburg, Germany

Squall line-land surface interaction: a bit of theory and the modeling approach

References

Dipankar et al (2015): Large eddy simulation using the general circulation model ICON, J. Adv. Model. Earth Syst., doi:10.1002/2015MS000431 Weisman and Rotunno (2004): ‘‘A Theory for Strong Long-Lived Squall Lines’’ Revisited, J. Atmos. Sci., 61, 361-382. Weisman and Klemp (1982): The Dependence of Numerically Simulated Convective Storms on Vertical Wind Shear and Buoyancy, Mon. Wea. Rev., 110, 504-520.

362 VOLUME 61J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S

FIG. 1. (left) Cold pool spreads away from a decaying convectivecell in an environment with no vertical wind shear. (right) Low-levelvertical wind shear balances cold-pool circulation on the downshearside, enhancing the ability to regenerate convective cells throughdeeper lifting.

FIG. 2. Three stages in the evolution of a convective system. (a)An initial updraft leans downshear in response to the ambient verticalwind shear, which is shown on the right. (b) The circulation generatedby the storm-induced cold pool balances the ambient shear, and thesystem becomes upright. (c) The cold-pool circulation overwhelmsthe ambient shear and the system tilts upshear, producing a rear-inflowjet. The updraft current is denoted by the thick, double-lined flowvector, and the rear-inflow current in (c) is denoted by the thick solidvector. The shading denotes the surface cold pool. The thin, circulararrows depict the most significant sources of horizontal vorticity,which are either associated with the ambient shear or which are gen-erated within the convective system, as described in the text. Regionsof lighter or heavier rainfall are indicated by the more sparsely ordensely packed vertical lines, respectively. The scalloped line denotesthe outline of the cloud. Here, C represents the strength of the coldpool while Du represents the strength of the ambient low-level verticalwind shear, as described in the text (adapted from Weisman 1992).

environmental air being forced up and over a deeperhead region, and then subsiding over the main body ofcold air. When ambient shear is present (Fig. 1b), thecirculation associated with the shear will, on the down-shear side, counteract some of the circulation associatedwith the cold pool, producing deeper lifting there. Thedeepest lifting and largest potential for cell retriggeringoccurs when the cold pool and shear circulations are inbalance. Naturally, the shear layer occupying the samevertical levels as the cold pool was proposed to be themost important for this effect. This basic impact of shearon a density current has been reproduced in many otherrecent studies (e.g., Xu et al. 1996, 1997; Xue 2000),although differing interpretations of the effect have beenoffered, as is discussed below.The RKW cold-pool–shear interaction theory ad-

dresses the threshold question of how the shear affectsthe transformation of ordinary finite-lifetime thunder-storms into a long-lasting system of cells continuallyregenerating along a line. Our view is that only afterthis threshold question has been answered, does it makesense to consider questions concerning the collectiveeffects of many generations of cells. For example, ma-ture squall lines often have a rear-inflow jet (e.g., Smulland Houze 1987) which, according to numerical sim-ulations, can originate through the collective heating/cooling pattern of the convective cells (Lafore andMon-crieff 1989; Weisman 1992). Consider the evolution ofa squall-line circulation as presented schematically inFig. 2, where C is a velocity representing the strengthof the cold pool and Du represents the magnitude of thelow-level ambient vertical wind shear. Before a signif-icant cold pool develops (C K Du), the convective cellswithin the squall line tilt predominantly in a downsheardirection in response to the ambient shear (Fig. 2a); aftera cold pool has developed, its circulation may counterthat associated with the low-level shear to produce deep-er lifting at low levels and an overall more upright con-vective structure (C ; Du, Fig. 2b); finally, if the coldpool evolves to a state where C . Du, then the circu-lation associated with the cold pool dominates that ofthe shear, thereby sweeping the convective cells andassociated zone of heating/cooling rearward (toward theleft in the figure), where it can induce the generation ofa rear-inflow jet (Fig. 2c). The RKW ‘‘optimal’’ state

is envisioned when C/Du is close to 1, whereby thesystem maintains an upright configuration and the deep-est lifting is produced at the leading edge of the coldpool. However, for all except the most strongly shearedenvironments, squall lines tend to evolve through allthree depicted phases during their lifetimes, as coldpools usually strengthen over time and eventually be-come strong enough to overwhelm the ambient shear.The induced rear-inflow circulation during the mature

phase of a squall line might fundamentally alter the

a) shear dominates; b) shear and cold pool circu-lations balance aka “the optimal state”; c) cold pool circulation dominates (Weisman and Rotunno (2004))

vorticity

cold pool

wind shear

Cold pool strength vs low-level shear

C2 = 2 (−B)dz0

H

∫Its fairly established that squall line characteristics are strongly, but not exclusively, determined by the relative strengths of the cold pool C versus the wind shear over the depth of the cold pool ΔU Previous studies mainly investigated squall line sensitivity to ΔU by varying the environmental wind profile. This study focuses on ways of varying C by exposing the squall line to changing surface conditions.

with

Approach: high resolution modeling ICON-HIRES (Dipankar et al 2015)

Model setup -  doubly periodic domain (500x450 or 700x500 km2) -  model top @20km, tropopause @12km, 100 vertical levels

(20m-560m thick, 15 levels from surface to 1000m) -  no radiation, 2-moment microphysics, 3D Smagorinski turbulence Experiment setup -  warm bubble testcase following Weisman and Klemp (1982)

-  elliptical +2K warm bubble, centred 1400m above surface, 10km horizontal and 1.4km vertical radius

-  0.014 kg/kg surface specific humidity -  10 m/s max wind speed with 7 m/s @3km

-  develops an initial storm, the expanding cold pool of which triggers a squall-like structure which maintains itself

-  squall line/cold pool evolution will be investigated given altered surface conditions (altered SST, fixed fluxes, roughness lengths) and resolutions (~500m – 2km)

Results OR „What does it take for the land surface to affect a squall line?“

cold

poo

l edg

e

30km 50km 70km 90km

cold pool front centred analysis, i.e. conditional sampling of near surface properties and precip -60 min and +90 min w.r.t. passage of dashed line

analysing cold pool properties

buoyancy profiles @70km, 1km resolution

1km resolution

surface heat fluxes @70km, 1km resolution

Preliminary conclusions

-  ICON-HIRES suitable for simulating idealized squall-lines consistent with previous literature

-  interactive surface fluxes have to be substantial and positive to have any thermodynamic effect on cold pool properties

-  mixing of near surface air up to higher levels appears critical

squall line accumulated rainfall (left), propagation speed (right)