Using the Weak‐Temperature Gradient Approximation to Evaluate Parameterizations: An Example of the Transition From Suppressed to Active Convection

Abstract

Two single‐column models are fully coupled via the weak‐temperature gradient approach. The coupled‐SCM is used to simulate the transition from suppressed to active convection under the influence of an interactive large‐scale circulation. The sensitivity of this transition to the value of mixing entrainment within the convective parameterization is explored. The results from these simulations are compared with those from equivalent simulations using coupled cloud‐resolving models. Coupled‐column simulations over nonuniform surface forcing are used to initialize the simulations of the transition, in which the column with suppressed convection is forced to undergo a transition to active convection by changing the local and/or remote surface forcings. The direct contributions from the changes in surface forcing are to induce a weakening of the large‐scale circulation which systematically modulates the transition. In the SCM, the contributions from the large‐scale circulation are dominated by the heating effects, while in the CRM the heating and moistening effects are about equally divided. A transition time is defined as the time when the rain rate in the dry column is halfway to the value at equilibrium after the transition. For the control value of entrainment, the order of the transition times is identical to that obtained in the CRM, but the transition times are markedly faster. The locally forced transition is strongly delayed by a higher entrainment. A consequence is that for a 50% higher entrainment the transition times are reordered. The remotely forced transition remains fast while the locally forced transition becomes slow, compared to the CRM.

How Do Ocean Warm Anomalies Favor the Aggregation of Deep Convective Clouds?

“…Using a single column model (SCM) and CRM, Ramsay and Sobel (2011) and Wang and Sobel (2011) showed that precipitation rate increases over local warm SSTs and is determined by the temperature anomaly rather than by the mean SST. Daleu et al (2017) confirmed this result using two adjacent SCMs with different SSTs. The SST difference, if large enough, can suppress convection in the cold column and strengthen it in the warm column.…”

Estimating Convection’s Moisture Sensitivity: An Observation–Model Synthesis Using AMIE-DYNAMO Field Data

“…For the dynamicist, the precision and controllability of CCPMs remains extremely valuable for inferring coupling mechanisms, despite some systematic shortcomings of the models (e.g., Varble et al 2014). Dynamically ''open'' protocols allow CCPM-simulated convection to be interrogated cleanly for responses to certain forcings while coupled to parameterized or simplified large-scale dynamics (e.g., Sobel and Bretherton 2000;Raymond and Zeng 2005;Herman and Raymond 2014;Raymond and Flores 2016;Daleu et al 2016) that are less chaotic than our tangent linear GCM's dynamics. Direct if rather method-dependent contacts could then be made with observations, at least ''to some degree'' (Wang et al 2013(Wang et al , 2016 or statistically (Sentić et al 2015), using idealized probing signals for sensitivity characterizations (Sessions et al 2015).…”

“…We speculate that one major confounder (an important but unmeasured variable) shaping filter-scale Q variability in nature is filter-scale vertical velocity [w], which CCPMs (and therefore their causal response matrices M and G) lack for structural reasons. Advection of basic-state vertical gradients by a Qproportional [w] under the weak temperature gradient approximation could be incorporated as a linear effect in a modified matrix (Kuang 2012), or captured in other parameterized large-scale dynamics approaches (e.g., Daleu et al 2016Daleu et al , 2017, but here we shall instead utilize a more explicit mechanism for coupling CCPM sensitivities to larger-scale dynamics, in order to bring those sensitivities into an observation-comparable domain.…”

Probing the Response of Tropical Deep Convection to Aerosol Perturbations Using Idealized Cloud-Resolving Simulations with Parameterized Large-Scale Dynamics

“…where u and q are potential temperature and specific humidity of water vapor, respectively, and the subscript ''LS'' denotes large scale. Note that, unlike the conventional WTG (e.g., Daleu et al 2017) this method does not require a special treatment in the well-mixed boundary layer (e.g., Sobel and Bretherton 2000), as vertical gradients vanish, because the shallow modes with small phase speed are weakly relaxed toward the target profile.…”

“…The resulting large-scale vertical motion W then acts in a way to restore tropospheric temperature and maintain its horizontal homogeneity over large variations in surface forcings (e.g., Bretherton and Smolarkiewicz 1989). This mechanism of a two-way interaction between convection and large-scale motion has long been implemented in single-column models (e.g., Sobel and Bretherton 2000;Ramsay and Sobel 2011) as well as cloud-resolving models (e.g., Raymond and Zeng 2005;Wang and Sobel 2011;Romps 2012;Daleu et al;Anber et al 2014Anber et al , 2015a. Varying SST while strongly relaxing freetropospheric temperature toward a fixed profile can be thought of as varying the relative SST, or the difference between local and tropical mean conditions.…”