Observations of Near-Surface Vertical Wind Profiles and Vertical Momentum Fluxes from VORTEX-SE 2017: Comparisons to Monin–Obukhov Similarity Theory

Abstract

Observations of near-surface vertical wind profiles and vertical momentum fluxes obtained from a Doppler lidar and instrumented towers deployed during VORTEX-SE in the spring of 2017 are analyzed. In particular, departures from the predictions of Monin–Obukhov similarity theory (MOST) are documented on thunderstorm days, both in the warm air masses ahead of storms and within the cool outflow of storms, where MOST assumptions (e.g., horizontal homogeneity and a steady state) are least credible. In these regions, it is found that the nondimensional vertical wind shear near the surface commonly exceeds predictions by MOST. The departures from MOST have implications for the specification of the lower boundary condition in numerical simulations of convective storms. Documenting departures from MOST is a necessary first-step toward improving the lower boundary condition and parameterization of near-surface turbulence (“wall models”) in storm simulations.

Evaluation of Monin–Obukhov and Bulk Richardson Parameterizations for Surface–Atmosphere Exchange

“…In Eq. (1) through (3), we used 6.5 m for z (i.e., the mean of 3 and 10 m AGL) and used the mean values from 3 and 10 m AGL for computing u * , w 0 u 0 y , and w 0 q 0 following, for example, Markowski et al (2019). We computed the gradients in Eqs.…”

“…We attribute this scatter to the uncertainties present in u * , which are exacerbated because z is a function of the cube of u * . Following Markowski et al (2019), we estimate the uncertainty in u * as du * 5 1 2 ju 0 w 0 j 21/2 du 0 w 0 .…”

“…The values for z 0 and d were again consistent with the expectation from, for example, Baldocchi (1997) that they are '10% and '60%, respectively, of the height of the vegetation surrounding each tower. Additional specific details about the sites and the datasets obtained appear in Lee et al (2019a,b) and Markowski et al (2019).…”

“…Another limitation of MOST is that MOST is normalized by a velocity scale u * , which appears both in the bulk-flux equations and also in z, which can result in spurious self-correlation (e.g., Andreas and Hicks 2002). Furthermore, the M-O length scale L is a function of the cube of u * , and thus errors in measured u * (e.g., Markowski et al 2019) can increase L errors.…”

The Effect of Surface Drag Strength on Mesocyclone Intensification and Tornadogenesis in Idealized Supercell Simulations

“…This is noteworthy because the literature on supercell dynamics has long emphasized the need for downdrafts to generate ''near ground'' cyclonic vorticity in a supercell. The basis for this argument is typically that tilting of horizontal vorticity in near-ground parcels ascending into an updraft cannot commence quickly enough to generate meaningful vertical vorticity until some appreciable height AGL (e.g., Rotunno and Klemp 1985;Davies-Jones and Brooks 1993;Markowski et al 2008; Davies-Jones 2015). However, Rotunno et al (2017, hereafter RMB17) have cast some doubt on this notion, using highly idealized numerical experiments (in which a pseudostorm updraft and downdraft were forced by persistent specified heating and cooling sources, respectively, and the lower boundary was free slip) to demonstrate how near-ground parcels with initially negligible vertical vorticity can in fact acquire cyclonic vorticity ''near the ground'' (e.g., 1 m AGL) immediately upon ascent.…”