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We use 3-D cloud-resolving model (CRM) simulations of two mesoscale convective systems at mid-latitudes and a simple statistical ensemble method to diagnose the scale dependency of convective momentum transport (CMT) and CMT-related properties, and evaluate a parameterization scheme for convection-induced pressure gradient (CIPG) developed by Gregory et al. (GKI97). GKI97 relates CIPG to a constant coefficient multiplied by mass flux and vertical mean wind shear. CRM results show that mass fluxes and CMT exhibit strong scale dependency in temporal evolution and vertical structure. The prevalent understandings of CMT characteristics in terms of upgradient/downgradient transport are applicable to updrafts but not downdrafts across a wide range of grid spacings (4-512 km). For the small-to-median grid spacings (4~64 km), GKI97 reproduces some aspects of CIPG scale dependency except for underestimating the variations of CIPG as grid spacing decreases. However, for large grid spacings (128~512 km), GKI97 might even less adequately parameterize CIPG because it omits the contribution from either the nonlinear shear or buoyancy forcings. Further diagnosis of CRM results suggests that inclusion of nonlinear shear forcing in GKI97 is needed for the large grid spacings, and use of the three-updraft and one downdraft approach proposed in an earlier study may help a modified GKI97 capture more variations of CIPG as grid spacing decreases for the small-to-median grid spacings. Further, the optimal coefficients used in GKI97 seems insensitive to grid spacings, but they might be different for updrafts and downdrafts, for different MCS types, and for zonal and meridional components.

This paper examines the "too bright" issue pertaining to non‐planetary boundary layer (PBL) clouds over the South Pacific trade‐wind region and its potential link to the falling ice radiative effects (FIREs). We run sensitivity experiments with CESM2‐CAM6 (CESM2) global climate model with FIREs on (SON) and off (NOS). The model exhibits more in‐cloud liquid water content (CLWC) and droplet above the PBL in NOS, leading to larger shortwave (SW) reflectivity at the top of the atmosphere than in SON over the trade wind regions. CMIP6 models are divided into three subsets: separately calculates the radiative effects of cloud ice and falling ice (SON2), combined (SON1) and without falling ice (NOS). SON2 models exhibit improved CLWC and SW reflectivity similar to CESM2‐SON, while NOS and SON1 models are akin to CESM2‐NOS owing to weaker surface wind stress and warmer ocean surface, caused by the lack of FIREs over the convective zones. Plain Language Summary: This paper examines why some clouds simulated by global climate models (GCMs) tend to reflect too much sunlight back into space (i.e., appear too bright) in the South Pacific trade‐wind region. It explores whether this is linked to the effects of falling ice (FIREs) over the convective zones of the Pacific Intertropical Convergence Zone (ITCZ). Using the CESM2‐CAM6 GCM, we conducted experiments with FIREs turned on (CESM2‐SON) and off (CESM2‐NOS), primarily occurring in the ITCZ. The results showed that when FIREs are turned off, the model predicts more liquid water and cloud droplets above the planetary boundary layer, causing these clouds to reflect more sunlight over the trade‐wind regions. Additionally, we analyzed different climate models from the Coupled Model Intercomparison Project (CMIP6), categorizing them into three groups: those that separately account for the radiative effects of cloud ice and falling ice (SON2), as in CESM2‐SON; those that combine these effects (SON1); and those without falling ice (NOS). The SON2 models showed a better vertical spatial distribution of liquid water content and sunlight reflectivity over the trade‐wind region, while the NOS and SON1 models performed similarly to CESM2‐NOS. Key Points: Cloud properties in CESM2 with falling ice radiative effects (FIREs) (SON) are compared to without FIREs (NOS) over the trade‐wind regionCESM2 NOS exhibits more liquid water content above the planetary boundary layer (PBL) with more SW reflectivity at top of the atmosphere (TOA) over the trade‐wind region compared to CESM2 SONSimilar improvements are found in CESM2‐SON‐type subset of CMIP6 models, linked to stronger surface wind stress and cooler ocean surface [ABSTRACT FROM AUTHOR]

Cloud radiative effects (CREs) and cloud‐type mean CREs depend upon how clear‐sky fluxes are computed over a large area: those of the immediate environment of clouds or the regional mean clear‐sky fluxes. Five convectively active regions in the Tropics, two over land (Africa and Amazon) and three over ocean (eastern and western Pacific and Atlantic), are selected to understand the influence of immediate environment of clouds on CREs. Fluxes derived from 19 years of high‐resolution CERES satellite data, categorized by cloud type, are utilized. The cloud types are classified based on the joint cloud top pressure and cloud optical depth distribution. For the entire tropical region, differences in cloud‐type mean CRE with regional mean and immediate environment clear skies range from −7.8 to 10.7 Wm−2 for shortwave (SW), 2.9 to 15.8 Wm−2 for longwave (LW), and 6.1 to 17.9 Wm−2 for net, respectively. The oceanic and Amazonia regions have negative (positive) SW (LW) CRE differences, typically 2–6 Wm−2 in SW but 7–10 Wm−2 in LW, whereas Africa has positive SW and LW CRE differences (typically 20–30 Wm−2, up to 40–50 Wm−2). The influence of immediate environment reduces the regionally averaged, that is, cloud‐type mean CREs weighted by cloud fractions, SW cloud cooling, and LW cloud warming in four of the five regions except for Africa. For Africa, it increases the SW cloud cooling and greatly reduces the LW cloud warming, resulting in net cloud cooling as in other regions instead of warming. The implications of these findings for observational and modeling studies are discussed. Plain Language Summary: Clouds reflect solar radiation back to space and trap infrared emission, that is, cooling effect from solar radiation and warming effect from infrared radiation. The amount of net cloud radiative effect depends upon what kinds of clouds are present in the atmosphere. A new satellite data product provides in‐cloud radiative property information by cloud types that are classified by cloud optical thickness and cloud top height in addition to clear skies counterpart. This study found that regionally averaged CREs depend upon how the fluxes of clear skies are computed either averaged clear skies over the entire region or clear skies in the vicinity of a specific cloud type. Among the five tropical convectively active regions, Africa is the only region that exhibits net cloud warming using the traditional expression of clear skies. Using our new expression that uses clear skies in the vicinity of a cloud type, we find reduced infrared warming and increased solar cooling over Africa, leading to net cloud cooling effects in the region. Thus, all five convectively active regions have net cloud cooling effects. Key Points: Examine the influence of immediate environment on the cloud‐type mean cloud radiative effects (CREs) in the Tropics with a CERES data setUse of regional mean clear‐sky flux causes too much longwave warming and insufficient shortwave cooling and net cloud warming over AfricaCREs resemble each other more among five convectively active regions when considering the influence of immediate environment of clouds [ABSTRACT FROM AUTHOR]

This study investigates the physical mechanisms of the low cloud feedback through cloud-resolving simulations of cloud-radiative equilibrium response to an increase in sea surface temperature (SST). Six pairs of perturbed and control simulations are performed to represent different regimes of low clouds in the subtropical region by specifying SST differences ([DELTA]SST) in the range of 4 and 14 K between the warm tropical and cool subtropical regions. The SST is uniformly increased by 2 K in the perturbed set of simulations. Equilibrium states are characterized by cumulus and stratocumulus cloud regimes with variable thicknesses and vertical extents for the range of specified [DELTA]SSTs, with the perturbed set of simulations having higher cloud bases and tops and larger geometric thicknesses. The cloud feedback effect is negative for this [DELTA]SST range (-0.68 to -5.22 W [m.sup.-2] [K.sup.-1]) while the clear-sky feedback effect is mostly negative (-1.45 to 0.35 W [m.sup.-2] [K.sup.-1]). The clear-sky feedback effect contributes greatly to the climate sensitivity parameter for the cumulus cloud regime whereas the cloud feedback effect dominates for the stratocumulus regime. The increase of liquid water path (LWP) and cloud optical depth is related to the increase of cloud thickness and liquid water content with SST. The rates of change in surface latent heat flux are much higher than those of saturation water vapor pressure in the cumulus simulations. The increase in surface latent heat flux is the primary mechanism for the large change of cloud physical properties with +2 K SST, which leads to the negative cloud feedback effects. The changes in cloud fraction also contribute to the negative cloud feedback effects in the cumulus regime. Comparison of these results with prior modeling studies is also discussed. DOI: 10.1175/2009JAS3239.1

Formulating the contribution of subgrid-scale (SGS) variability to microphysical processes in boundary layer and deep convective cloud parameterizations is a challenging task because of the complexity of microphysical processes and the lack of subgrid-scale information. In this study, a warm-rain microphysics parameterization that is based on a joint double-Gaussian distribution of vertical velocity, liquid water potential temperature, total water mixing ratio, and perturbation of rainwater mixing ratio is developed to simulate drizzling boundary layer clouds with a single column model (SCM). The probability distribution function (PDF) is assumed, but its parameters evolve according to equations that invoke higher-order turbulence closure. These parameters are determined from the first-, second-, and third-order moments and are then used to derive analytical expressions for autoconversion, collection, and evaporation rates. The analytical expressions show that correlation between rainwater and liquid water mixing ratios of the Gaussians enhances the collection rate whereas that between saturation deficit and rainwater mixing ratios of the Gaussians enhances the evaporation rate. Cases of drizzling shallow cumulus and stratocumulus are simulated with large-eddy simulation (LES) and SCM runs (SCM-CNTL and SCM-M): LES explicitly resolves SGS variability, SCM-CNTL parameterizes SGS variability with the PDF-based scheme, but SCM-M uses the grid-mean profiles to calculate the conversion rates of microphysical processes. SCM-CNTL can well reproduce the autoconversion, collection, and evaporation rates from LES. Comparisons between the two SCM experiments showed improvements in mean profiles of potential temperature, total water mixing ratio, liquid water, and cloud amount in the simulations considering SGS variability. A 3-week integration using the PDF-based microphysics scheme indicates that the scheme is stable for long-term simulations.