Large‐eddy simulation of subtropical cloud‐topped boundary layers: 1. A forcing framework with closed surface energy balance

Abstract

Large‐eddy simulation (LES) of clouds has the potential to resolve a central question in climate dynamics, namely, how subtropical marine boundary layer (MBL) clouds respond to global warming. However, large‐scale processes need to be prescribed or represented parameterically in the limited‐area LES domains. It is important that the representation of large‐scale processes satisfies constraints such as a closed energy balance in a manner that is realizable under climate change. For example, LES with fixed sea surface temperatures usually do not close the surface energy balance, potentially leading to spurious surface fluxes and cloud responses to climate change. Here a framework of forcing LES of subtropical MBL clouds is presented that enforces a closed surface energy balance by coupling atmospheric LES to an ocean mixed layer with a sea surface temperature (SST) that depends on radiative fluxes and sensible and latent heat fluxes at the surface. A variety of subtropical MBL cloud regimes (stratocumulus, cumulus, and stratocumulus over cumulus) are simulated successfully within this framework. However, unlike in conventional frameworks with fixed SST, feedbacks between cloud cover and SST arise, which can lead to sudden transitions between cloud regimes (e.g., stratocumulus to cumulus) as forcing parameters are varied. The simulations validate this framework for studies of MBL clouds and establish its usefulness for studies of how the clouds respond to climate change.

Feedback mechanisms of shallow convective clouds in a warmer climate as demonstrated by changes in buoyancy

“…Even though the uniform warming and constant RH conditions are widely used in studies of warm convective cloud feedbacks (Rieck et al 2012, Vogel et al 2016, they do not capture the range of predicted environmental changes in large-scale circulation and thermodynamic conditions. Within these predicted changes (with their given uncertainty) in the thermodynamic conditions, which impact the cloud response (Tan et al 2016), the exploration of the single cloud scale enables a better understanding of the whole system, since the interaction between the cloud scale and the larger scale processes will determine eventually the overall cloud response to warming.…”

“…Here we explore the smaller scale of a warm convective cloud (∼1 km) response to a warmer atmosphere. Focusing on single cloud scale processes allows a separation from larger scales processes, like changes in the thermodynamic conditions of the boundary-layer (Rieck et al 2012, Tan et al 2016 and hence better understanding of low-cloud feedback mechanisms that are hard to detect in an evolving cloud field. Moreover, studying cloud feedbacks on the cloud field scale (using LES), has been shown to be sensitive to the assumptions regarding the changes in large scale forcing and surface fluxes (Tan et al 2016).…”

Large‐eddy simulation of subtropical cloud‐topped boundary layers: 2. Cloud response to climate change

“…As in Tan et al . [], the subtropical reference RH is assumed to be uniformly 30% below the tropopause. It is 1% just above the tropopause and decreases from there to 0% at the top of the atmosphere.…”

“…Over the 15 day analysis period, the SST drifts by less than in the prescribed‐OHU cases, and the ‐mean surface energy imbalance varies by less than in the prescribed‐SST cases. The inversion height, identifiable by the sharp jump of specific humidity q t across the top of MBL, is diagnosed as the lowest level where q t is below 1.2 times the free‐tropospheric reference q t [ Tan et al ., ]; it varies by less than (i.e., 1.5 vertical levels) in the statistically steady states of all cases. The steady state conditions are summarized in Table .…”

Influences of Subsidence and Free‐Tropospheric Conditions on the Nocturnal Growth of Nonclassical Marine Stratocumulus

“…The LES technique has proven to be a successful tool in representing observed Sc-topped boundary layers (STBLs; e.g., Stevens, 2005). Numerous studies have used LESs to explore the responses of Sc to perturbations in various parameters such as jumps in the temperature and humidity across the STBL top (e.g., Noda et al, 2013Noda et al, , 2014van der Dussen et al, 2015;Xiao et al, 2011;Xu & Xue, 2015), large-scale subsidence (e.g., Chung et al, 2012;Noda et al, 2014;van der Dussen et al, 2016), sea surface temperatures (e.g., Bellon & Geoffroy, 2016;Chung et al, 2012), surface heat fluxes (e.g., Tan et al, 2016), and wind shear (e.g., Kopec et al, 2016;Wang et al, 2008Wang et al, , 2012. Here we continue along the lines of former studies such as van der Dussen et al (2016) and Xu and Xue (2015) and employ the LES technique to study the responses of MSc to perturbations in subsidence and free-tropospheric thermodynamic conditions.…”