Abstract

Mixing of entrained air in stratus clouds is an important but poorly understood process. It is a crucial ingredient of cloud-top entrainment instability (CEI). CEI has been proposed as a breakup mechanism for stratus clouds. A recently developed model called the linear eddy model was used to simulate mixing of air entrained into stratus clouds. The linear eddy approach involves stochastic simulation on a one-dimensional domain with sufficient resolution to include all physically relevant length scales. In each realization, molecular diffusion is implemented explicitly, while a sequence of statistically independent “rearrangement events” represents the effect of turbulent eddies. Inertial range scaling is incorporated.

The linear eddy model was used to simulate the mixing of one or more wisps of entrained air with a specified volume of cloud-topped boundary layer (CTBL) air. The volume was idealized to be a horizontal slab of fluid that travels from the top of the CTBL down to the surface in the descending branch of a large convective eddy. The probability density function of the mixing fraction of entrained air was determined from linear eddy model simulations as a function of time for a mean mixing fraction of 0.05 and three wisp sizes. The effect of the mixing on the mean buoyancy of the downdraft could then be calculated given a specification of the buoyancy as a function of mixing fraction.

In the simulations, the entrained air did not completely mix with cloudy air just below the CTBL top, nor was uniform saturation maintained. Furthermore, when buoyancy functions typical of observed CTBLs were used, the mean downdraft buoyancy due to entrainment and mixing integrated over the cloud layer remained positive. This suggests that CEI is unlikely in stratocumulus.

An additional conclusion is that using reduced spatial resolutions typical of published large-eddy simulations (LES) of CTBLs in mixing simulations significantly underestimates the buoyancy in the cloud layer near cloud top. This may explain why low-resolution LFS simulations have exhibited CEI under conditions for which CEI is not observed in the atmosphere.

Abstract

A two-dimensional numerical model suitable for simulating an ensemble of cumulus clouds has been developed. It differs from similar models in its greater emphasis on turbulent processes in the boundary layer and in clouds. In the model, cloud-scale dynamics are coupled with a third-moment turbulence closure. A turbulence-scale condensation scheme is used to parameterize the cloud water mixing ratio.

We studied the response of tropical cumulus clouds to imposed large-scale vertical advection by doing two simulations: one with upward large-scale vertical velocity (the “disturbed” case) and one without any (the “undisturbed” case) but otherwise identical. Deep cumulus clouds formed in the disturbed case, while only shallow clouds formed in the undisturbed case.

Time-averaged and horizontally averaged heat and moisture budgets from the simulations show that the subcloud layer (SCL) in the disturbed case was strongly affected by cumulus circulations and rain evaporation. Both cumulus updrafts and downdrafts were important contributors to the SCL sensible and latent heat fluxes. Seventy percent of the fluxes were due to the strongest drafts, which covered only 24% of the area at 500 m. The strongest downdrafts formed in rainshafts and carried air from the updrafts and from the environment at midlevels (1–4 km) into the SCL. These downdrafts were driven by rain water loading above the SCL; only within the SCL were they ever negatively buoyant. Such downdraft created gust fronts and cool outflow regions. In the cloud-free regions, the SCL warmed and dried due to compensating subsidence, which occurred despite the large-scale upward motion. Cumulus-scale circulations in the disturbed SCL determined where new clouds formed by creating convergence zones and moisture anomalies. These circulations also strongly affected the surface fluxes of sensible and latent heat.

Abstract

No abstract available.

Abstract

Diagnostic cloudiness parameterizations in large-scale models are evaluated by using a two-dimensional numerical cumulus ensemble model. The model covers a large horizontal domain (512 km) but resolves individual clouds. This study explores the dependence of diagnostic relations (between cloud amount and a large-scale variable) on cloud regime, horizontal averaging distance, and cloud type for tropical convective cloud regimes. Large-scale variables, including relative humidity, cumulus mass flux, large-scale vertical velocity and surface precipitation rate, are examined.

It is shown that the total cloud amount can be better estimated as the sum of separate estimates of stratiform and convective cloud amounts using different large-scale variables than by an estimate of the total cloud amount using any single large-scale variable. The stratiform cloud amount can best be estimated by using relative humidity. The convective cloud amount can be diagnosed by using cumulus mass flux. Neither set of diagnostic relations depends significantly on the simulated cloud regime or horizontal averaging distance, but other diagnostic relations do show some such dependence. These results are interpreted and their implications for cloudiness parameterization are discussed.

Abstract

This study describes and demonstrates a new method for identifying deficiencies in how cloud processes are represented in large-scale models. Kilometer-scale-resolving cloud radar observations and cloud-resolving model (CRM) simulations were used to evaluate the representation of cirrus clouds in the single-column model (SCM) version of the National Centers for Environmental Prediction Global Forecast System model for a 29-day period during June and July 1997 at the Atmospheric Radiation Measurement Program site in Oklahoma.

To produce kilometer-scale cirrus statistics from the SCM results, synthetic subgrid-scale (SGS) cloud fields were generated using the SCM’s cloud fraction and hydrometeor content profiles, and the SCM’s cloud overlap and horizontal inhomogeneity assumptions. Three sets of SCM synthetic SGS cloud fields were analyzed. Two NOSNOW sets were produced in which clouds did not include snow; one set used random overlap, the other, maximum/random. In the SNOW set, clouds included snow and random overlap was used. The three sets were sampled in the same way as the cloud-radar-detected cloud fields and the CRM-simulated cloud fields.

The mean cirrus cloud occurrence frequency for the SCM NOSNOW cloud fields agrees with the observed value as well as the CRM’s does, while that for SCM SNOW cloud fields is only half that observed. In most aspects, the SCM’s cirrus properties differ significantly from the cloud radar’s and the CRM’s, which generally agree.

In comparison, there are too many physically thin SCM NOSNOW cirrus layers (most occupy only a single model layer) and too many physically thick SCM SNOW cirrus layers (most are thicker than 4 km). For the optically thin subset of cirrus layers, 1) the mean, mode, and median ice water path, and layer-mean ice water content (IWC) values for the SCM are significantly larger than the observed and CRM values; 2) the SCM layer-mean IWCs decrease with cloud physical thickness, opposite to the observations and CRM results; and 3) the range of layer-mean effective radii in the SCM thin cirrus is too narrow.

Abstract

Cloud amount statistics from three different sources were processed and compared. Surface observations from a National Centers for Environmental Prediction dataset were used. The data (Edited Cloud Report; ECR) consist of synoptic weather reports that have been edited to facilitate cloud analysis. Two stations near the Southern Great Plains (SGP) Cloud and Radiation Test Bed (CART) in north-central Oklahoma (Oklahoma City, Oklahoma and Wichita, Kansas) were selected. The ECR data span a 10-yr period from December 1981 to November 1991. The International Satellite Cloud Climatology Project (ISCCP) provided cloud amounts over the SGP CART for an 8-yr period (1983–91). Cloud amounts were also obtained from Micro Pulse Lidar (MPL) and Belfort Ceilometer (BLC) cloud-base height measurements made at the SGP CART over a 1-yr period. The annual and diurnal cycles of cloud amount as a function of cloud height and type were analyzed. The three datasets closely agree for total cloud amount. Good agreement was found in the ECR and MPL–BLC monthly low cloud amounts. With the exception of summer and midday in other seasons, the ISCCP low cloud amount estimates are generally 5%–10% less than the others. The ECR high cloud amount estimates are typically 10%–15% greater than those obtained from either the ISCCP or MPL–BLC datasets. The observed diurnal variations of altocumulus support the authors’ model results of radiatively induced circulations.

Abstract

A two-dimensional cumulus ensemble model is used to study the interactions of radiation and convection in tropical squall cloud clusters. The model includes cloud-scale and mesoscale dynamics, an improved bulk ice microphysics parameterization, and an advanced interactive radiative transfer scheme. The life cycle of a tropical squall line is simulated over a 12-h period using thermodynamic and kinematic initial conditions as well as large-scale advective forcing typical of a GATE Phase III squall cluster environment. The focus is on the interaction and feedback between longwave (or IR) radiation and cloud processes.

It will be shown that clew-sky IR cooling enhances convection and, hence, surface precipitation. Simulation results reveal an increase of surface precipitation by ∼15% (∼1.7 mm) over a 12-b period due to this clear-sky cooling. With fully interactive IR radiative heating, direct destabilization of clouds via IR radiative top cooling and base warming generates more turbulence and contributes to the longevity and extent of the upper-tropospheric stratiform (anvil) clouds associated with deep convection. The greater extent of anvil clouds decreases the outgoing IR flux at the top of the atmosphere by as much as 20 W m⁻².

With fully interactive IR radiative heating, the anvil cirrus reduces the IR cooling of the troposphere with respect to the clear-sky values. This cloud IR radiative forcing has a negative feedback on tropical deep convection, which will be referred to as “anvil cloud IR radiative feedback.” This feedback decreases surface precipitation by ∼10% (∼1.3 mm). It will also be shown that IR radiative processes modify the hydrometer profiles by affecting convection. On changing the cloud particle size distributions prescribed in radiation calculations, it is further demonstrated that the size distributions significantly influence the convective activity through their effects on the cloud IR radiative forcing.

The impact of clear-air IR cooling and cloud radiative forcing on deep convection is further examined by using the cloud-work function, which is a generalized number of the moist convective instability in die large-scale environment. The clear-air IR cooling tends to increase the cloud-work function, but the cloud IR radiative forcing tends to reduce it, especially for the deposit clouds.

Abstract

A model used to study entrainment and mixing of thermodynamic properties in the stratus-topped boundary layer has been extended to represent these processes in cumulus clouds. The new model, called the “explicit mixing parcel model” (EMPM), depicts finescale internal structure of a rising thermal in a cumulus cloud using a 1D domain. The EMPM links the conventional parcel model, which has no internal structure, and multidimensional cloud models, which resolve cloud-scale structure produced by large eddies. In the EMPM, the internal structure evolves as a consequence of a sequence of discrete entrainment events and an explicit representation of turbulent mixing based on Kerstein’s linear eddy model. In this version of the EMPM, subgrid-scale (eddy) diffusion is found to be adequate for representing the effects of the smallest turbulent eddies. In addition, a simple parameterization is used to determine the local condensation or evaporation rates. If the grid size is reduced so that the Kolmogorov scale is resolved and a droplet growth model is incorporated, the EMPM can predict the local microphysical environments of individual cloud droplets.

To evaluate its entrainment parameterization, the EMPM was used to predict the bulk properties of Hawaiian cumulus cloud main turrets observed by aircraft. All of the quantities required by the EMPM except for the entrained blob size were obtained from the observations. Profiles of in-cloud means and variances of thermodynamic properties calculated by the EMPM for entrained blob sizes of 50 m, 100 m, and 200 m and by a parcel model with instantaneous mixing were compared to those observed. The observed mean conserved scalar profiles are reproduced by both mixing representations, but the observed mean liquid water mixing ratio and buoyancy profiles, all of the observed variance profiles, and the observed nonbuoyancy level are better reproduced by the EMPM. For entrained blob sizes of 100 m and 200 m, undiluted cloud base air reaches the inversion base in the EMPM, as was observed. These results indicate that the EMPM’s entrainment parameterization is adequate for these cloud turrets, and that the characteristic entrained blob size is about 100 m. The model results also demonstrate that the finescale structure represented by the EMPM’s 1D domain can be directly compared to high-frequency aircraft measurements.

Abstract

A stratus-to-cumulus transition (SCT) that resembles observations occurred in Lagrangian numerical simulations of the subtropical marine boundary layer over the northeastern Pacific Ocean southwest of California. The Lagrangian approach involves translating the domain along the climatological boundary-layer trajectory at a rate equal to the observed surface wind speed. The SST is increased at a corresponding rate. The simulations did not include drizzle, the diurnal cycle, divergence changes, or mesoscale circulations and thus demonstrate that these processes are not essential for an SCT.

A 2D numerical cloud model that can explicitly represent large convective eddies is used. Turbulence at scales smaller than the large eddies is parameterized using a third-moment turbulence closure. This type of model requires no cloud-regime-specific input and is computationally economical for multiday simulations.

The results suggest that there are four stages in the transition from the stratus-topped boundary layer (STBL) to the trade cumulus boundary layer (TCBL). The simulated transition involves two intermediate stages: the deep stratus-topped boundary layer (DSTBL) and the “cumulus-under-stratocumulus” boundary layer (CUSBL). The DSTBL, like the STBL, is well mixed. The CUSBL has a two-layer structure, like the TCBL, with a well-mixed subcloud layer and a stratified (partly mixed) cloud layer. The transition to a typical TCBL structure preceded the transition to a typical TCBL cloud fraction by about two days.

Sensitivity tests indicate that by using diurnally averaged solar radiation with the daytime-averaged solar zenith angle, the model is able to reproduce the diurnally averaged cloud-top height. Tests also suggest that the boundary-layer structure is sensitive to the above-inversion thermodynamic structure.

Abstract

The progression from the stratus-topped boundary layer (STBL) to the trade cumulus boundary layer (TCBL) during a simulated stratus-to-cumulus transition (SCT) involves two intermediate stages: the deep stratus-topped boundary layer (DSTBL) and the “cumulus-under-stratocumulus” boundary layer (CUSBL). The DSTBL, like the STBL, has an active circulation that extends from the surface to the cloud top. The CUSBL, like the TCBL, has an active subcloud-layer circulation that is linked to the cloud layer by narrow cumulus updrafts. It is called a “cumulus-coupled” boundary layer.

A generally applicable convective updraft/downdraft partitioning scheme based on trajectory analysis was developed and used to analyze the boundary-layer circulation changes during the simulated SCT. The circulation analysis revealed that as the SST increased and the boundary layer changed from an STBL to a TCBL the updraft fraction in the cloud layer decreased, the convective updrafts strengthened, and the convective downdrafts weakened. The convective mass flux in the cloud layer decreased significantly as SST increased, while in the subcloud layer it changed little. The differences between updraft and downdraft properties and cloud-base levels gradually increased as SST increased.

An analysis of the vertical acceleration components of the convective updrafts and downdrafts suggests that there are three steps in the transition from an STBL circulation to a TCBL circulation. First, the STBL deepens due to increased surface buoyancy fluxes as it moves over increasing SST but remains well mixed. Next, the DSTBL gradually changes into the two-layer CUSBL. During this step, negative buoyancy in downdrafts originating near cloud top becomes less important, while positive buoyancy in (cumulus) updrafts becomes more important. This indicates that cloud-top entrainment instability does not play a significant role in the SCT. Finally, the overlying stratocumulus deck gradually dissipates and only the underlying cumulus clouds of a typical TCBL remain. This general sequence of events is supported by recent observational evidence.