the sensitivity of a simulated supercell to emulated radiative cooling beneath the anvil

The Sensitivity of a Simulated Supercell to Emulated Radiative

Cooling beneath the Anvil

Paul Markowski and Jerry HarringtonPenn State University

Motivation

• Even though numerical simulations often have produced convective storms that closely resemble those that have been observed, a potentially important forcing frequently has been ignored—the influence of radiative effects on storm dynamics

• Justifications for excluding radiative effects have included (i) “radiative processes are unimportant on the time scales of the model integration” and (ii) “convective storms are largely dynamically, not radiatively driven”

Motivation

• Some 2D simulations of squall lines have included LW radiation– total precip amounts and formation of “transition

zones” has been found to have sensitivity (Chen and Cotton 1988; Dudhia 1989; Churchill and Houze 1991; Tao et al. 1991, 1993)

– Full range of possible dynamical effects owing to radiative transfer processes, especially those owing to SW radiative transfer processes, is uncertain due to model dimensionality

Motivation

• Radiative effects only have occasionally been included in 3D simulations – Parameterizations have been rather crude (e.g.,

often clouds seen only as areas of very high water vapor content, with the radiative characteristics of condensed liquid and ice species not taken into account)

– Of the 2D and 3D simulations that have included radiation, no sensitivity analyses done to quantify the effects of radiation on storm dynamics

22 May 1995 8 June 1995

Markowski, Paul M., Rasmussen, Erik N., Straka, Jerry M., Dowell, David C. 1998: Observations of Low-Level Baroclinity Generated by Anvil Shadows. Monthly Weather Review: Vol. 126, No. 11, pp. 2942–2958.

8 June 1995

Motivation

• Dynamical importance of low-level temperature gradients associated with precipitation regions (both internal and external to the storm) has been well appreciated (e.g., Maddox et al. 1980 Klemp and Rotunno 1983), but what about low-level temperature gradients arising from radiative effects?

• Magnitude of T associated with anvil shading is perhaps not as large as along precip gradients, but spatial scale of T is larger; back-of-envelope calculations by Markowski et al. (1998) suggested this effect could be important vorticity modifier

• Storm longevity and degree of anvil shading of the storm inflow probably are positively correlated (both tend to increase as vertical shear increases)

Demonstration Simulations

• ARPS 4.5.2• 5/20/77 sounding (sorry)• No ice; x=y = 1 km; z = 150-500 m• Two simulations

– (i) no surface physics (control) – (ii) radiative cooling due to anvil shading emulated by prescribing a

cooling rate to the skin temperature of 5 K/h at any grid point at which qc is present overhead; low-level T is coupled to the skin cooling by the inclusion of QH using simple bulk aerodynamic drag laws

• Though this emulation of radiative cooling is admittedly very crude, it should suffice to illustrate the potential effects that radiative cooling under the anvil may have on storm dynamics

No radiative effects

Isentropes at 75 m drawn at 1 K intervals

No radiative effects Emulated radiative cooling

t = 1 h



t = 2 h

T deficits of 1-2 K beneath anvil


t = 2 h


t = 3 h

Markowski et al. (1998)

Closing thoughts• It might seem that the simulations exaggerated the effect of

anvil shading due to the nearly stationary storm motion

• But the surface temperature deficits and associated baroclinity, even after 3 h, actually were not as large as have been observed (1-2 K vs 4-5 K)

• Also, the hodograph structure was not ideal—low-level inflow parcels failed to spend much time in the anvil-generated baroclinic zone (orientation of baroclinic zone also unfavorable perhaps?)

• Size of anvil was likely underdone due to Kessler microphysics

• It is possible that the demonstration simulations are a conservative indication of the importance of radiative effects

Closing thoughts• Lots of unknowns!

– Do the effects of baroclinic vorticity depend on whether e is nearly conserved?

– Competing effects of increased CIN in inflow versus possibly augmented vertical shear?

– Effect of low-level cloud cover in inflow? Nighttime?– Importance of hodograph?

• How important is the relationship between inflow trajectories and the orientation of the baroclinic zone?

• How does orientation of baroclinic vorticity due to shading compare to orientation of horizontal vorticity associated with mean shear?

• Even ground-relative wind profile may be important; i.e., storm motion may be important because the development of surface cooling due to anvil shading is not Galilean invariant

Next step

• Use “real” radiation schemes and more sophisticated microphysics (Harrington and Frame).

• Establish bounds on the magnitude of the radiative effects on long-lived convective storms, as well as the nature of these effects and how these effects depend on storm morphology and the ambient environment.