Gravity Wave Interactions in the Stratocumulus-Topped Boundary Layer

TL;DR

This paper addresses how gravity waves can destabilize and breakup stratocumulus-topped boundary layers using high-resolution LES with an RCE baseline to isolate forcing effects. It introduces a nondimensional framework with parameters such as $Ja_m$, $Ro$, and $Fr$, and explores a localized gravity-wave forcing with amplitude, location, and spectrum. The key finding is a critical forcing amplitude around $\mathcal{A}\ge 2.5$ beyond which the STBL breaks up into patchy clouds, with bichromatic forcing markedly increasing clearing and longer-duration forcing promoting persistent breakdown; the study links these outcomes to TKE budgets and flow anisotropy via the Lumley triangle. The work implies that gravity-wave–cloud interactions could significantly affect cloud radiative forcing and should be accounted for in climate-model representations of STBL dynamics and cloud breakup processes.

Abstract

This work studies the breakup propensity of the stratocumulus-topped boundary layer (STBL) interacting with gravity waves using large-eddy simulation with a uniform vertical grid of $5$ m and horizontal spacing of $30$ m. A radiative-convective equilibrium (RCE) state is constructed to enforce stationarity in the STBL, and the gravity waves are introduced via a vertical momentum forcing mimicking a packet of plane waves. A nondimensionalization involving the inversion height and mean horizontal base wind as length and velocity scales is proposed to provide a framework to analyze the forcing parameter space. The magnitude of the scaled forcing amplitude ($\mathcal{A}$) is critical in understanding various STBL breakup conditions. Classification of breakup was based on the reduction of the liquid water path for each forced STBL case. We found that breakup did not occur for $\mathcal{A}<1$ and observed modest reductions in cloud for $1<\mathcal{A}<2$, but the deck recovered to the stationary state slowly after the single-period forcing ceased. Fixing $\mathcal{A}\sim 2$ showed that forcings with longer duration and wider locality promote breakup. However, when the forcing is a linear combination of waves of two different periods, the percentage of cleared cloud dramatically increases, though recovery of RCE is still observed in some cases. $\mathcal{A}\geq2.5$ marks a critical threshold by which the STBL breaks up entirely and remains patchy. We further explore the connection between these bulk breakup results and the turbulent state by examining energy budgets and the anisotropy induced by the forcing.

Figures (25)

• Figure 1: Initial profiles of potential temperature (black) and water vapor specific humidity (blue), which is the same as S05.
• Figure 2: Spatiotemporal contour of (a) equivalent potential temperature and (b) liquid water specific humidity for the Stevens2005LES case.
• Figure 3: Spatiotemporal contour of (a) equivalent potential temperature and (b) cloud condensate specific humidity for the current setup. The onset of radiative-convective equilibrium is denoted by the black dashed line.
• Figure 4: Vertical profiles of a) total water specific humidity and b) vertical velocity flux averaged over the same time window (from 3-4 hr) as S05 which is compared against the profiles averaged during the RCE period and the field data from DYCOMS-II first research flight (digitized from S05). The red "errorbar" in b) shows the widest spread in the LES ensemble range from S05 at $578$ m.
• Figure 5: Spatiotemporal contour of the nondimensional forcing for the amplitude simulations: Base, A1, A2, and A3. Note how the A3 forcing is depressed near $\Delta t = 15$ as a result of a lower $z_i$.