Steven K. Esbensen

Determination of Bulk Properties of Tropical Cloud Clusters from Large-Scale Heat and Moisture Budgets

Abstract The bulk properties of tropical cloud clusters, such as the vertical mass flux, the excess temperature, and moisture and the liquid water content of the clouds, are determined from a combination of 1) the observed large-scale heat and moisture budgets over an area covering the cloud cluster, and 2) a model of a cumulus ensemble which exchanges mass, heat, water vapor and liquid water with the environment through entrainment and detrainment. The method also provides an understanding of how the environmental air is heated and moistened by the cumulus convection. An estimate of the average cloud cluster properties and the heat and moisture balance of the environment, obtained from 1956 Marshall Islands data, is presented.

Scale Dependence of Air-Sea Fluxes over the Western Equatorial Pacific

Abstract The goal of this study is to examine the horizontal scale dependence of vertical eddy flux in the tropical marine surface boundary layer and how this scale dependence of flux relates to the bulk aerodynamic relationship and the parameterization of subgrid-scale flux. The fluxes of heat, moisture, and momentum are computed from data collected from 27 NCAR Electra flight legs in TOGA COARE (The Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment) with flight elevations lower than 40 m and flight runs longer than 60 km. The dependence of the fluxes on two length scales are studied: the cutoff length scale, defining the averaging length over which mean components are obtained in order to partition field variables into mean and perturbation components; and the flux averaging length scale, defining the length over which products of perturbations are averaged in order to estimate vertical fluxes. Based on the characteristics of the scale dependence of fluxes, the total flux of ...

Numerical simulation of boundary layer structure and cross-equatorial flow in the Eastern Pacific

Abstract Recent observations from spaceborne microwave sensors have revealed detailed structure of the surface flow over the equatorial eastern Pacific in the boreal fall season. A marked acceleration of surface wind across the northern sea surface temperature (SST) front of the cold tongue is a prominent feature of the regional climate. Previous studies have attributed the acceleration to the effect of enhanced momentum mixing over the warmer waters. A high-resolution numerical model is used to examine the cross-frontal flow adjustment. In a comprehensive comparison, the model agrees well with many observed features of cross-equatorial flow and boundary layer structure from satellite, Tropical Atmosphere Ocean (TAO) moorings, and the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. In particular, the model simulates the acceleration across the SST front, and the change from a stable to unstable boundary layer. Analysis of the model momentum budget indicates that the hydrostatic ...

Bulk Thermodynamic Effects and Properties of Small Tropical Cumuli

Abstract Separate diagnostic models of shallow and inversion-penetrating, trade wind cumulus clouds are combined with large-scale BOMEX heat and moisture budgets to obtain the thermodynamic effects and properties of the shallow clouds. The deeper trade wind cumulus effects are obtained using a modified version of Nittas (1975) spectral diagnostic method, while a bulk model based on the bubble theory of convection is used for the shallow clouds. Results show that the buoyancy of the small cumuli is an order of magnitude less than the buoyancy of the deeper clouds, and suggest that their effects on the temperature stratification are small compared with radiative cooling and subsidence warning due to deep clouds. Also, the results show that the primary role of the small cumuli is to support the growth of deeper clouds through the transport of water vapour from the subcloud layer into the lower cloud layer. The deduced shallow cloud properties are found to be consistent with observations and Sommerias (1976...

Estimation of Surface Heat Flux

Abstract The authors reconsider the problem of estimating the sensible heat transfer at the earths surface from direct measurements of turbulent fluxes in the atmospheric boundary layer. For simplicity, only horizontally homogeneous conditions are considered for a thin atmospheric layer containing no liquid water, adjacent to the earths ground surface. Applying the first law of thermodynamics to the thin interfacial layer, an expression is obtained for thermal conduction at the surface in terms of the traditionally defined sensible heat flux by turbulence and a set of correction terms including the so-called moisture correction term. A scale analysis is presented to suggest that the magnitudes of the miscellaneous correction terms are usually negligible. Previous literature on estimation of the sensible heat flux is critically reviewed in light of the new result.

Subgrid Surface Fluxes in Fair Weather Conditions during TOGA COARE: Observational Estimates and Parameterization

Abstract Bulk aerodynamic formulas are applied to meteorological data from low-altitude aircraft flights to obtain observational estimates of the subgrid enhancement of momentum, sensible heat, and latent heat exchange at the atmospheric–oceanic boundary in light wind, fair weather conditions during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment). Here, subgrid enhancement refers to the contributions of unresolved disturbances to the grid-box average fluxes at the lower boundary of an atmospheric general circulation model. The observed subgrid fluxes increase with grid-box area, reaching 11%, 9%, 24%, and 12% of the total sensible heat, latent heat, scalar wind stress, and vector wind stress magnitude, respectively, at a grid-box size of 2° × 2° longitude and latitude. Consistent with previous observational and modeling studies over the open ocean, most of the subgrid flux is explained by unresolved directional variability in the near-surface wind field. The auth...

A Composite Life Cycle of Nonsquall Mesoscale Convective Systems over the Tropical Ocean. Part I: Kinematic Fields

Abstract The wind fields associated with cloud clusters observed during the Global Atmospheric Research Programs Atlantic Tropical Experiment (GATE) are investigated. A compositing procedure is devised to isolate the cluster circulations. Satellite-observed cloud cover estimates by Cox and Griffith form the basis for the identification and classification of clusters and for the determination of their life cycles. The compositing criteria focus on the upper-tropospheric portions of anvil clouds that are a prominent feature of cloud clusters. The compositing procedure is applied to a set of objectively analyzed upper-air winds for Phase 3 of GATE prepared by K. V. Ooyama and J.-H. Chu. The results show that slow-moving cloud clusters tend to form in regions of relatively small vertical wind shear and that the shear at the cluster center decreases during the cluster life cycle. Squall clusters, on the other hand, have significantly larger lower-tropospheric shear. Changes in the total horizontal wind field ...

An Analysis of Subeloud-Layer Heat and Moisture Budgets in the Western Atlantic Trades

Abstract Using rawinsonde, radiation, and surface heat and moisture flux data from the Barbados Oceanographic and Meteorological Experiment (BOMEX), the budgets of heat and moisture in the subcloud layer are calculated. The results show that the vertical eddy flux of dry static energy at the top of the mixed layer is small compared with the sensible beat flux at the sea surface during a relatively undisturbed period. The eddy flux of moisture at the top of the mixed layer is large and nearly balances the sea-surface evaporation. The large eddy moisture flux dominates the eddy flux of dry static energy across the top of the layer and results in a positive eddy buoyancy flux during relatively undisturbed synoptic conditions. For weakly unstable boundary layers topped by cumulus convection this suggests that the commonly used representation of the eddy buoyancy flux at the mixed layer top as a small negative fraction of the sea surface flux may be valid only in the environment away from the convection. Large...

A Composite Life Cycle of Nonsquall Mesoscale Convective Systems over the Tropical Ocean. Part II: Heat and Moisture Budgets

Abstract The heat and moisture budgets associated with five large nonsquall cloud clusters observed during Phase 3 of the Global Atmospheric Research Programs Atlantic Tropical Experiment (GATE) are investigated. The input data for the budget computations are objectively analyzed fields of wind, temperature and relative humidity that were based on conventional upper-air soundings. Estimates of the radiative heating rate were obtained from Cox and Griffith. A compositing technique is used to summarize the budget results for the growing, mature and dissipating stages of the clusters. The budgets in the growing stage are characterized by a very large low-level, apparent moisture sink separated in height from the region where the apparent heating is realized. In the mature stage, the apparent heating maximum shifts upward, accompanied by the development of a corresponding secondary maximum of apparent drying. A composite of radiative heating estimates from Cox and Griffith shows that the horizontal radiative...

The Consistent Parameterization of the Effects of Cumulus Clouds on the Large-Scale Momentum and Vorticity Fields

Abstract A physical and mathematical framework for the mutually consistent parameterization of the effects of cumulus convection on the large-scale momentum and vorticity fields is proposed. The key to achieving consistency is the understanding that the vorticity dynamics of the clouds below the spatial resolution of a large-scale dynamical model may be neglected in the vorticity budget when the clouds are considered to be independent buoyant elements sharing a common large-scale environment This simplified approach is used to obtain a consistent pair of large-scale momentum and vorticity equations based on Ooyamas theory of cumulus parameterization. The results focus attention on the need to obtain a better understanding of the detrainment process and the pressure interactions between the clouds and their environment.