Analogies between Mass-Flux and Reynolds-Averaged Equations

In many large-scale models mass-flux parameterizations are applied to prognose the effect of cumulus cloud convection on the large-scale environment. Key parameters in the mass-flux equations are the lateral entrainment and detrainment rates. The physical meaning of these parameters is that they quantify the mixing rate of mass across the thermal boundaries between the cloud and its environment. The prognostic equations for the updraft and downdraft value of a conserved variable are used to derive a prognostic variance equation in the mass-flux approach. The analogy between this equation and the Reynolds-averaged variance equation is discussed. It is demonstrated that the prognostic variance equation formulated in mass-flux variables contains a gradient-production, transport, and dissipative term. In the latter term, the sum of the lateral entrainment and detrainment rates represents an inverse timescale of the dissipation. Steady-state solutions of the variance equations are discussed. Expressions for the fractional entrainment and detrainment coefficients are derived. Also, solutions for the vertical flux of an arbitrary conserved variable are presented. For convection in which the updraft fraction equals the downdraft fraction, the vertical flux of the scalar flows down the local mean gradient. The turbulent mixing coefficient is given by the ratio of the vertical mass flux and the sum of the fractional entrainment and detrainment coefficients. For an arbitrary updraft fraction, however, flux correction terms are part of the solution. It is shown that for a convective boundary layer these correction terms can account for countergradient transport, which is illustrated from large eddy simulation results. In the cumulus convection limit the vertical flux flows down the cloud gradient. It is concluded that in the mass-flux approach the turbulent mixing coefficients, and the correction terms that arise from the transport term, are very similar to closures applied to the Reynolds-averaged equations

Observed Lagrangian Transition of Stratocumulus into Cumulus during ASTEX: Mean State and Turbulence Structure

Aircraft measurements made during the "First Lagrangian" of the Atlantic Stratocumulus Transition Experiment (ASTEX) between 12 and 14 June 1992 are presented. During this Lagrangian experiment an air mass was followed that was advected southward by the mean wind. Five aircraft flights were undertaken to observe the transition of a stratocumulus cloud deck to thin and broken stratocumulus clouds penetrated by cumulus from below. From the horizontal aircraft legs the boundary layer mean structure, microphysics, turbulence structure, and entrainment were analyzed. The vertical profiles of the vertical velocity skewness are shown to illustrate the transition of a cloudy boundary layer predominantly driven by longwave radiative cooling at the cloud top to one driven mainly by convection due to an unstable surface stratification and cumulus clouds. During the last flight before the stratocumulus deck was observed to be broken and replaced by cumuli, the total water flux, the virtual potential temperature flux, and the vertical velocity variance in the stratocumulus cloud layer were found significantly larger compared with the previous flights. To analyze the cloud-top stability the mean jumps of conserved variables across the inversion were determined from porpoising runs through the cloud top. These jumps were compared with cloud-top entrainment instability criteria discussed in the literature. It is suggested that enhanced entrainment of dry air is a key mechanism in the stratocumulus-cumulus transition

Surface Energy Balance and Turbulence Characteristics Observed at the SHEBA Ice Camp During FIRE III

The Institute for Marine and Atmospheric Research Utrecht (IMAU) participated in the FIRE III (First ISCCP Regional Experiment, ISCCP International Satellite Cloud Climatology Project) experiment in May 1998. In this paper we describe surface layer measurements performed on the sea ice at the SHEBA (Surface Heat and Energy Balance of the Arctic ocean) camp and compare these with measurements collected above a grasscovered surface in Cabauw, the Netherlands. The observations consist of both highfrequency turbulence measurements and mean-profile measurements of wind, temperature and humidity. In addition, we measured the upward and downward components of both the longwave and shortwave radiation, and the snow and ice temperatures in the upper 40 cm. The observations give a detailed picture of all components of the energy balance of the Arctic sea-ice surface. The turbulence measurements are used to study the surface layer scaling of the turbulence variables in the stable boundary layer. More specifically, we showed that the integral length scale of the vertical velocity fluctuations serves as the relevant turbulence length scale. The monthly-averaged energy balance of the Arctic sea-ice was dominated by radiative fluxes, whereas, the sensible and latent heat flux and the energy flux into the surface were rather small. A detailed inspection of the diurnal variations in the turbulent fluxes however indicates that although the monthly-averaged values are small, the hourlyaveraged values for these fluxes are significant in the surface energy balance

Massflux Budgets of Shallow Cumulus Clouds

The vertical transport by shallow nonprecipitating cumulus clouds of conserved variables, such as the total specific humidity or the liquid water potential temperature, can be well modeled by the massflux approach, in which the cloud field is represented by a top-hat distribution of clouds and its environment. The budget of the massflux is presented and is compared with the vertical velocity variance budget. The massflux budget is computed by conditionally sampling the prognostic vertical velocity equation by means of a Large-Eddy Simulation of shallow cumulus clouds. The model initialization is based on observations made during BOMEX. Several different sampling criteria are applied. The presence of liquid water is used to select clouds, whereas additional criteria are applied to sample cloud updraft, downdraft and core properties. The massflux and vertical velocity variance budgets appear to be qualitatively similar. The massflux is driven by buoyancy in the lower part of the cloud layer, whereas turbulent transport is important in generating massflux in the upper part of the cloud layer. Pressure and subgrid-scale effects typically act to dissipate massflux. The massflux approach is verified for non-conserved variables. The virtual potential temperature flux and the vertical velocity variance according to the the top-hat approximation do not correspond very well to the Reynolds-averaged turbulent flux. The top-hat structure for the virtual potential temperature is degraded by lateral mixing and the subsequent evaporative cooling of cloud droplets which support the development of negatively buoyant cloud downdrafts. Cloudy downdrafts occupy about 20% of the total cloud area in the upper part of the cumulus layer, and are the cause that the vertical velocity variance is not well represented by the massflux approach, either

An Isotropic Light Sensor for Measurements of Visible Actinic Flux in Clouds

A low-cost isotropic light sensor is described consisting of a spherical diffuser connected to a single photodiode by a light conductor. The directional response to light is isotropic to a high degree. The small, lightweight, and rugged construction makes this instrument suitable not only for application on aircraft or under balloons but also on the ground in microclimatological studies. A vertical profile of actinic flux in the visible range (400-750 nm) in Arctic stratus, obtained with this instrument under a tethered balloon during the FIRE experiment in 1998, is presented

Surface and Tethered-Balloon Observations of Actinic Flux: Effects of Arctic stratus, Surface Albedo and Solar Zenith Angle

As part of the FIRE III (First ISCCP Regional Experiment) Arctic Cloud Experiment actinic flux measurements were made above the Arctic Sea ice during May 1998. FIRE III was designed to address questions concerning clouds, radiation and chemistry in the Arctic sea ice region. The actinic flux, which is also referred to as the 4p-radiative flux, is the relevant radiative parameter needed to determine photodissociation rates. Moreover, it is discussed that the actinic flux may be used to determine vertical absorption profiles of the net irradiance, provided that the single scattering albedo is known. The diurnal cycle of UV-A (wavelength about 365 nm) and visible (wavelength about 550 nm) actinic fluxes during clear and cloudy conditions was measured by two 4p- radiometers installed just above the ice surface. In addition, vertical profiles of the visible actinic flux through low arctic stratus clouds were observed by means of a tethered balloon. The cloud thermodynamic and microphysical structure was assessed from observations made by the NCAR C-130 aircraft. The liquid water path was retrieved by a microwave radiometer. During clear skies the diurnal variation of the actinic flux was controlled mainly by Rayleigh scattering. Above the cloud layer the actinic flux was found to be almost the same as during clear sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. The observed vertical actinic flux profiles in arctic stratus are discussed and compared with similar measurements made in Atlantic stratocumulus. In the arctic stratus clouds the actinic flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the actinic flux significantly increased with height. The role of the solar zenith angle and ground albedo on in-cloud actinic flux profiles is discussed. It is concluded that the observed strong increase of the actinic flux in the upper part of the arctic stratus layer is a typical feature associated with large solar zenith angles

A comparison of the ECMWF forecast model with observations over the annual cycle at SHEBA

A central objective of the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment was to provide a comprehensive observational test for single-column models of the atmosphere-sea ice-ocean system over the Arctic Ocean. For single-column modeling, one must specify the time-varying tendencies due to horizontal and vertical advection of air through the column. Due to the difficulty of directly measuring these tendencies, it was decided for SHEBA to obtain them from short-range forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast model, into which SHEBA rawinsonde and surface synoptic observations were routinely assimilated. The quality of these forecasts directly affects the reliability of the derived advective tendencies. In addition, the ECMWF-forecast thermodynamic and cloud fields, and radiative and turbulent fluxes present an illuminating comparison of the SHEBA observations with a state-of-the-art global numerical model. The authors compare SHEBA soundings, cloud and boundary layer observations with the ECMWF model output throughout the SHEBA year. They find that above the boundary layer, the model was faithful to the SHEBA rawinsonde observations and maintained a proper long-term balance between advective and nonadvective tendencies of heat and moisture. This lends credence to use of the ECMWF-predicted advective tendencies for single-column modeling studies. The model-derived cloud properties and precipitation (which were not assimilated from observations) are compared with cloud radar, lidar, microwave radiometer, surface turbulent and radiative measurements, and basic surface meteorology. The model s slab sea-ice model led to large surface temperature errors and insufficient synoptic variability of temperature. The overall height distribution of cloud was fairly well simulated (though somewhat overestimated) in all seasons, as was precipitation. However, the model clouds typically had a much higher ratio of cloud ice to cloud water than suggested by lidar depolarization measurements, and a smaller optical depth, leading to monthly biases of up to 50 W m^(-2) in the monthly surface downwelling longwave and shortwave radiation. Further biases in net radiation were due to the inaccurate model assumption of constant surface albedo. Observed turbulent sensible and latent heat fluxes tended to be small throughout SHEBA. During high-wind periods during the winter, the ECMWF model predicted sustained downward heat fluxes of up to 60 W m^(-2), much higher than observed. A detailed comparison suggests that this error was due to both inadequate resolution of the 31-level model and a deficient parameterization of sea-ice thermodynamics

Properties of the conditionally filtered equations: Conservation, normal modes, and variational formulation

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.Conditionally filtered equations have recently been proposed as a basis for modelling the atmospheric boundary layer and convection. Conditional filtering decomposes the fluid into a number of categories or components, such as convective updrafts and the background environment, and derives governing equations for the dynamics of each component. Because of the novelty and unfamiliarity of these equations, it is important to establish some of their physical and mathematical properties, and to examine whether their solutions might behave in counter-intuitive or even unphysical ways. It is also important to understand the properties of the equations in order to develop suitable numerical solution methods. The conditionally filtered equations are shown to have conservation laws for mass, entropy, momentum or axial angular momentum, energy, and potential vorticity. The normal modes of the conditionally filtered equations include the usual acoustic, inertio-gravity, and Rossby modes of the standard compressible Euler equations. In addition, they posses modes with different perturbations in the different fluid components that resemble gravity modes and inertial modes but with zero pressure perturbation. These modes make no contribution to the total filter-scale fluid motion, and their amplitude diminishes as the filter scale diminishes. Finally, it is shown that the conditionally filtered equations have a natural variational formulation, which can be used as a basis for systematically deriving consistent approximations.We are grateful to two anonymous reviewers for their constructive comments on an earlier version of this paper. This work was funded by the Natural Environment Research Council under grant NE/N013123/1 as part of the ParaCon programme

Refinement and application of a regional atmospheric model for climate scenario calculations of Western Europe

Het KNMI regionaal klimaat model RACMO wordt in toenemende mate gebruikt bij de detaillering van Klimaatscenario’s. Voorbeelden zijn de frequentie en intensiteit van hittegolven en de veranderingen daarin. Of te verwachten wijzigingen in het optreden van lokale neerslagextremen. In dit project zijn een aantal componenten van RACMO verder ontwikkeld. De bodemhydrologie van het model is verder verfijnd door ruimtelijke heterogeniteit in te voeren voor een aantal bodemparameters, zoals bodemtype en worteldiepte. Deze aanpassing resulteert in meer uitgesproken ruimtelijke structuren op regionale schaal

The impact of climate change on the critical weather conditions at Schiphol airport (Impact)

Schiphol is van groot belang voor de economische positie van Nederland. De luchthaven is erg gevoelig voor kritieke weersomstandigheden zoals mist, intensieve neerslag en hevige wind. Als gevolg van klimaatverandering verwachten we dat ook de variabiliteit van het weer op de luchthaven en de frequentie en intensiteit van kritieke weersomstandigheden zullen veranderen, maar een precieze kwantificering daarvan ontbreekt. De belangrijkste doelstelling van dit project is daarom het verstrekken en demonstreren van het volgende generatie weer‐ en klimaatmodel HARMONIE. Dit is een nieuw model dat beter geschikt lijkt om het effect van klimaatverandering op lokale kritieke weersomstandigheden op de luchthaven te kwantificeren en te begrijpen. Bovendien zal kennis uit dit project worden gebruikt om de kwaliteit van onze huidige en toekomstige weersvoorspellingen te verbeteren. In dit project wordt het potentieel van het HARMONIE model, om meer gedetailleerdere en nauwkeurigere weersvoorspellingen voor luchthaven Schiphol te leveren dan ons huidige operationele weermodel HIRLAM, nagegaan in het huidige klimaat