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

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

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

Turbulence characteristics of the stable boundary layer over a mid-latitude glacier. Part II: pure katabatic forcing conditions.

Observations obtained over a glacier surface in a predominantly katabatic flow and with a distinct wind maximum below 13-m height are presented. The data were collected using a 13-m high profile mast and two sonic anemometers (at about 2.5-m and 10-m heights). The spectra at frequencies below that of the turbulence range appear to deviate considerably from the curves obtained by Kaimal and co-workers during the 1968 Kansas experiment. The characteristics of these deviations are compared to the observations of others in surface-layers disturbed by any kind of large-scale outer-layer (or inactive) turbulence. In our case the disturbances are likely to be induced by the high mountain ridges that surround the glacier. Moreover, the deviations observed in the cospectra seem to result from an, as yet, unspecified interaction between the inactive outer-layer turbulence and the local surface-layer turbulence. Near the distinct wind maximum turbulence production ceased while turbulence itself did not, probably the result of turbulence transport from other levels. Consequently, we studied the local similarity relations using σ(w) instead of u(*) as an alternative velocity scale. Well below the wind maximum, and for relatively low stability (0 0.2), and near or above the wind maximum, the boundary-layer structure conforms to that of z-less stratification suggesting that the eddy size is restricted by the local stability of the flow. In line with this we observed that the sensible heat fluxes relate remarkably well to the local flow parameters

The observed katabatic flow at the edge of the Greenland ice sheet during GIMEX-91

Observations performed in the melting zone of the Greenland ice sheet and over the adjacent tundra in the summer of 1991 are described. The experimental area is the region near St ndre Stromfjord (67°N, 54°W), which is relatively dry and sunny, resulting in the highest mean temperature in Greenland in July. The katabatic wind is dominantly present over the ice; it influences the energy balance near the surface through the sensible and latent heat flux. With the aid of a tethered balloon it was observed that the thickness of the katabatic layer is typically 100 to 200 m. An interesting aspect of the katabatic wind appears to be the acceleration of the flow in the late afternoon due to the large temperature gradient at the border between tundra and ice. Further on the ice, this effect is no longer important for the dynamics of the katabatic flow. The net radiation is the main driving force there. An attempt is made to estimate the importance of these thermal wind effects compared to the buoyancy forcing. It is concluded that near the edge of the ice surface winds are driven by the horizontal pressure gradient, imposed by the thermal contrast between tundra and ice. A comparison is made between the observed katabatic wind and those in the Antarctic