Deep Convective Adjustment of Temperature and Moisture

Abstract

Simple process models and complex climate models are remarkably sensitive to the time scale of convective adjustment τ, but this parameter remains poorly constrained and understood. This study uses the linear-range slope of a semiempirical relationship between precipitation and a lower-free-tropospheric buoyancy measure BL. The BL measure is a function of layer-averaged moist enthalpy in the boundary layer (150-hPa-thick layer above surface), and temperature and moisture in the lower free troposphere (boundary layer top to 500 hPa). Sensitivity parameters with units of time quantify the BL response to its component perturbations. In moist enthalpy units, BL is more sensitive to temperature than equivalent moisture perturbations. However, column-integrated moist static energy conservation ensures that temperature and moisture are equally altered during the adjustment process. Multiple adjusted states with different temperature–moisture combinations exist; the BL sensitivity parameters govern the relationship between adjusted states, and also combine to yield a time scale of convective adjustment ~2 h. This value is comparable to τ values used in cumulus parameterization closures. Disparities in previously reported values of τ are attributed to the neglect of the temperature contribution to precipitation, and to averaging operations that include data from both precipitating and nonprecipitating regimes. A stochastic model of tropical convection demonstrates how either averaging operations or neglected environmental influences on precipitation can yield τ estimates longer than the true τ value built into the model. The analysis here culminates in construction of a precipitation closure with both moisture and temperature adjustment (q–T closure), suitable for use in both linearized and nonlinear, intermediate-complexity models.

Convection–Kelvin Wave Coupling in a Global Convection-Permitting Model

“…The CCKW-associated precipitation in this DYNAMO case features a nearly monotonic relationship with saturation fraction, while a hysteresis-like behavior occurs with DCIN and surface fluxes ( Figure 11); the implication is that saturation fraction is the primary driver of precipitation intensity. This conclusion is also supported within the framework of Ahmed et al (2020), in which the lower tropospheric plume buoyancy ( ) is represented as (their Eq. 7):…”

“…For and Π, the subscripts and indicate averages over the boundary layer (1000-850-hPa) and the lower-free-tropospheric layers (850-500-hPa), respectively. and are determined following Ahmed et al (2020). Eq.…”

“…While these (and other) recent studies have emphasized the roles of CIN and column moisture on CCKW coupling, their relative importance is still largely unexplored. Ahmed and Neelin (2018) and Ahmed et al (2020) recently proposed a buoyancy-based model in which these two parameters affect precipitation via lower tropospheric plume buoyancy. Their model enables examining the relative roles of lower tropospheric stability and moisture on precipitation within a single framework.…”

Interactions between Water Vapor, Potential Vorticity, and Vertical Wind Shear in Quasi-Geostrophic Motions: Implications for Rotational Tropical Motion Systems

“…12 in Adames and Ming 2018b). In the tropics, the main source of diabatic heating is deep cumulus convection, whose buoyancy is sensitive to the thermodynamic environment rather than the large-scale ascent (Brown and Zhang 1997;Mapes 2000;Ahmed and Neelin 2018). Adames and Ming (2018b) showed that anomalous precipitation in simulated MLPSs is more closely in phase with anomalous water vapor, rather than large-scale lifting (their Fig.…”

“…The role water vapor plays in modulating the occurrence and organization of tropical deep convection is well documented. For example, a humid free troposphere reduces the dilution that updrafts experience as they ascend (Kuo et al 2017;Ahmed and Neelin 2018). The central role moisture has in tropical convection has led to the hypothesis that waves may exist where water vapor plays a central role in their dynamics (Sobel et al 2001).…”