Atlantic Stratocumulus Transition Research Articles

A 19-Month Record of Marine Aerosol–Cloud–Radiation Properties Derived from DOE ARM Mobile Facility Deployment at the Azores. Part I: Cloud Fraction and Single-Layered MBL Cloud Properties

AbstractA 19-month record of total and single-layered low (<3 km), middle (3–6 km), and high (>6 km) cloud fractions (CFs) and the single-layered marine boundary layer (MBL) cloud macrophysical and microphysical properties was generated from ground-based measurements at the Atmospheric Radiation Measurement Program (ARM) Azores site between June 2009 and December 2010. This is the most comprehensive dataset of marine cloud fraction and MBL cloud properties. The annual means of total CF and single-layered low, middle, and high CFs derived from ARM radar and lidar observations are 0.702, 0.271, 0.01, and 0.106, respectively. Greater total and single-layered high (>6 km) CFs occurred during the winter, whereas single-layered low (<3 km) CFs were more prominent during summer. Diurnal cycles for both total and low CFs were stronger during summer than during winter. The CFs are bimodally distributed in the vertical with a lower peak at ~1 km and a higher peak between 8 and 11 km during all seasons, except summer when only the low peak occurs. Persistent high pressure and dry conditions produce more single-layered MBL clouds and fewer total clouds during summer, whereas the low pressure and moist air masses during winter generate more total and multilayered clouds, and deep frontal clouds associated with midlatitude cyclones.The seasonal variations of cloud heights and thickness are also associated with the seasonal synoptic patterns. The MBL cloud layer is low, warm, and thin with large liquid water path (LWP) and liquid water content (LWC) during summer, whereas during winter it is higher, colder, and thicker with reduced LWP and LWC. The cloud LWP and LWC values are greater at night than during daytime. The monthly mean daytime cloud droplet effective radius re values are nearly constant, while the daytime droplet number concentration Nd basically follows the LWC variation. There is a strong correlation between cloud condensation nuclei (CCN) concentration NCCN and Nd during January–May, probably due to the frequent low pressure systems because upward motion brings more surface CCN to cloud base (well-mixed boundary layer). During summer and autumn, the correlation between Nd and NCCN is not as strong as that during January–May because downward motion from high pressure systems is predominant. Compared to the compiled aircraft in situ measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX), the cloud microphysical retrievals in this study agree well with historical aircraft data. Different air mass sources over the ARM Azores site have significant impacts on the cloud microphysical properties and surface CCN as demonstrated by great variability in NCCN and cloud microphysical properties during some months.

The GASS/EUCLIPSE model intercomparison of the stratocumulus transition as observed during ASTEX: LES results

Large‐eddy simulations of a Lagrangian transition from a vertically well‐mixed stratocumulus‐topped boundary layer to a situation in which shallow cumuli penetrate an overlying layer of thin and broken stratocumulus are compared with aircraft observations collected during the Atlantic Stratocumulus Transition Experiment. Despite the complexity of the case and the long simulation period of 40 h, the six participating state‐of‐the‐art models skillfully and consistently represent the observed gradual deepening of the boundary layer, a negative buoyancy flux at the top of the subcloud layer and the development of a double‐peaked vertical velocity variance profile. The moisture flux from the subcloud to the stratocumulus cloud layer by cumulus convection exhibits a distinct diurnal cycle. During the night the moisture flux at the stratocumulus cloud base exceeds the surface evaporation flux, causing a net drying of the subcloud layer, and vice versa during daytime. The spread in the liquid water path (LWP) among the models is rather large during the first 12 h. From additional sensitivity experiments it is demonstrated that this spread is mainly attributable to differences in the parameterized precipitation rate. The LWP differences are limited through a feedback mechanism in which enhanced drizzle fluxes result in lower entrainment rates and subsequently a reduced drying at cloud top. The spread is furthermore reduced during the day as cloud layers with a greater LWP absorb more solar radiation and hence evaporate more.

A Cloud-Resolving Model with an Adaptive Vertical Grid for Boundary Layer Clouds

Abstract Accurate cloud-resolving model simulations of cloud cover and cloud water content for boundary layer clouds are difficult to achieve without vertical grid spacing well below 100 m, especially for inversion-topped stratocumulus. The need for fine vertical grid spacing presents a significant impediment to global or large regional simulations using cloud-resolving models, including the Multiscale Modeling Framework (MMF), in which a two-dimensional or small three-dimensional cloud-resolving model is embedded into each grid cell of a global climate model in place of more traditional cloud parameterizations. One potential solution to this problem is to use a model with an adaptive vertical grid (i.e., a model that is able to add vertical layers where and when needed) rather than trying to use a fixed grid with fine vertical spacing throughout the boundary layer. This article examines simulations with an adaptive vertical grid for three well-studied stratocumulus cases based on observations from the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) experiment, the Atlantic Stratocumulus Transition Experiment (ASTEX), and the Atlantic Trade Cumulus Experiment (ATEX). For each case, three criteria are examined for determining where to add or remove vertical layers. One criterion is based on the domain-averaged potential temperature profile; the other two are based on the ratio of the estimated subgrid-scale to total water flux and turbulent kinetic energy. The results of the adaptive vertical grid simulations are encouraging in that these simulations are able to produce results similar to simulations using fine vertical grid spacing throughout the boundary layer, while using many fewer vertical layers.

Fidelity of Analytic Drop Size Distributions in Drizzling Stratiform Clouds Based on Large-Eddy Simulations

Abstract Cloud microphysical parameterizations and retrievals rely heavily on knowledge of the shape of drop size distributions (DSDs). Many investigations assume that DSDs in the entire or partial drop size range may be approximated by known analytical functions. The most frequently employed approximations of function are of the type of gamma, lognormal, Khrgian–Mazin, and Marshall–Palmer. At present, little is known about the accuracy of these approximations. The authors employ a DSD dataset generated by the Cooperative Institute for Mesoscale Meteorological Studies Large-Eddy Simulation (CIMMS LES) explicit microphysics model for stratocumulus cases observed during the Atlantic Stratocumulus Transition Experiment (ASTEX) field project. The fidelity of analytic lognormal- and gamma-type DSD functions is evaluated according to how well they represent the higher-order moments of the drop spectra, such as precipitation flux and radar reflectivity. It is concluded that for boundary layer marine drizzling stratocumuli, a DSD based on the two-mode gamma distribution provides a more accurate estimate of precipitation flux and radar reflectivity than the DSD based on the lognormal distribution. The gamma distribution also provides a more accurate radar reflectivity field in two- and three-moment bulk microphysical models compared to the conventional Z–R relationship.

Study of the Aerosol Indirect Effect by Large-Eddy Simulation of Marine Stratocumulus

Abstract A total of 98 three-dimensional large-eddy simulations (LESs) of marine stratocumulus clouds covering both nighttime and daytime conditions were performed to explore the response of cloud optical depth (τ) to various aerosol number concentrations (Na = 50–2500 cm−3) and the covarying meteorological conditions (large-scale divergence rate and SST). The idealized First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) and the Atlantic Stratocumulus Transition Experiment (ASTEX) Lagrangian 1 sounding profiles were used to represent the lightly and heavily drizzling cases, respectively. The first and second aerosol indirect effects are identified. Through statistical analysis, τ is found be to both positively correlated with Na and cloud liquid water path (LWP) with a higher correlation associated with LWP, which is predominantly regulated by large-scale subsidence and SST. Clouds with high LWP occur under low SST or weak large-scale subsidence. Introduction of a small amount of giant sea salt aerosol into the simulation lowers the number of cloud droplets activated, results in larger cloud droplets, and initiates precipitation for nondrizzling polluted clouds or precedes the precipitation process for drizzling clouds. However, giant sea salt aerosol is found to have a negligible effect on τ for lightly precipitating cases, while resulting in a relative reduction of τ of 3%–77% (increasing with Na, for Na = 1000–2500 cm−3) for heavily precipitating cases, suggesting that the impact of giant sea salt is only important for moist and potentially convective clouds. Finally, a regression analysis of the simulations shows that the second indirect effect is more evident in clear than polluted cases. The second indirect effect is found to enhance (reduce) the overall aerosol indirect effect for heavily (lightly) drizzling clouds; that is, τ is larger (smaller) for the same relative change in Na than considering the Twomey (first indirect) effect alone. The aerosol indirect effect (on τ) is lessened in the daytime afternoon conditions and is dominated by the Twomey effect; however, the effect in the early morning is close but slightly smaller than that in the nocturnal run. Diurnal variations of the aerosol indirect effect should be considered to accurately assess its magnitude.

Retrieving Stratocumulus Drizzle Parameters Using Doppler Radar and Lidar

Abstract Stratocumulus is one of the most common cloud types globally, with a profound effect on the earth’s radiation budget, and the drizzle process is fundamental in understanding the evolution of these boundary layer clouds. In this paper a combination of 94-GHz Doppler radar and backscatter lidar is used to investigate the microphysical properties of drizzle falling below the base of stratocumulus clouds. The ratio of the radar to lidar backscatter power is proportional to the fourth power of mean size, and so potentially it can provide an accurate size estimate. Information about the shape of the drop size distribution is then inferred from the Doppler spectral width. The algorithm estimates vertical profiles of drizzle parameters such as liquid water content, liquid water flux, and vertical air velocity, assuming that the drizzle size spectrum may be represented by a gamma distribution. The depletion time scale of cloud liquid water through the drizzle process can be estimated when the liquid water path of the cloud is available from microwave radiometers, and observations suggest that this time scale varies from a few days in light drizzle to a few hours in strong drizzle events. Radar and lidar observations from Chilbolton (in southern England) and aircraft size spectra taken during the Atlantic Stratocumulus Transition Experiment have both been used to derive the following power-law relationship between liquid water flux (LWF) (g m−2 s−1) and radar reflectivity (Z) (mm6 m−3): LWF = 0.0093Z0.69. This relation is valid for frequencies up to 94 GHz and therefore would allow a forthcoming spaceborne radar to measure liquid water flux around the globe to within a factor of 2 for values of Z above −20 dBZ.

Observations and numerical simulations of the diurnal cycle of the EUROCS stratocumulus case

AbstractAs part of the European Project on Cloud Systems in Climate Models, the diurnal cycle of stratocumulus has been simulated with Large‐Eddy Simulation (LES) models and Single Column Models (SCMs). The models were initialized and compared with observations collected in marine stratocumulus in July 1987 during the First International Satellite Cloud Climatology Project Regional Experiment. The results of the six LES models are found to be in a fair agreement with the observations. They all capture the distinct diurnal variation in the cloud liquid‐water path, the turbulence profiles and clearly show a decoupled boundary layer during daytime and a vertically well‐mixed boundary layer during the night. Entrainment of relatively dry and warm air from just above the inversion into the boundary layer is the major process modifying the thermodynamic structure of the boundary layer during the night. The differences that arise in the liquid‐water path evolution can therefore be attributed mainly to differences in the entrainment rate. The mean entrainment rates computed from the LES model results are 0.58±0.08 cm s−1 and 0.36±0.03 cm s−1 for the night‐time and daytime periods, respectively. If the horizontal domain size in a LES model is enlarged, mesoscale fluctuations develop. This leads to a broader liquid‐water path distribution and a reduction of the cloud albedo.To assess the quality of the representation of stratocumulus in general‐circulation models, results from ten SCMs are compared with observations and LES results. The SCM latent and sensible heat fluxes at the surface agree fairly well with the LES results. Many of the SCMs predict a liquid‐water path which is much too low, a cloud cover smaller than unity, and cloud tops that are lower than the observations and the LES results. This results in a much larger amount of downwelling short‐wave radiation absorbed at the sea surface. Improvement of entrainment parametrizations is needed for a better representation of stratocumulus in SCMs.Observations and LES results of entrainment rates for different stratocumulus cases are compared. The observed entrainment rates in Atlantic stratocumulus clouds during the Atlantic Stratocumulus Transition Experiment (ASTEX) are larger than for the ones over the Pacific Ocean off the coast of California. Results from LES models corroborate these findings. The differences in the entrainment rate can likely be attributed to the smaller inversion jumps of the liquid‐water potential temperature for the ASTEX stratocumulus cases. © Royal Meteorological Society, 2004. A. P. Lock's contribution is Crown copyright

Three‐dimensional simulation of the ASTEX Lagrangian 1 field experiment with a regional numerical weather prediction model

AbstractThe Atlantic Stratocumulus Transition Experiment (ASTEX) first Lagrangian experiment (Lagrangian 1) is here simulated with a modified version of the regional forecast model HIRLAM (High Resolution Limited Area Model). The main modification is that moist turbulent fluxes are accounted for in the model. Trajectory calculations show good agreement with earlier estimations. The initially rather shallow stratocumulus topped marine boundary layer is deepening along the trajectory, and in the end cumulus clouds are formed that penetrate the boundary‐layer top. The model predicts this change in cloudiness, but the boundary layer is too shallow in the model. A simulation with modified initial conditions shows improved results, but is still too slow in increasing the boundary‐layer depth. Additional factors that influence the boundary‐layer growth are: the increase in sea surface temperatures, lower modelled wind speeds, low entrainment rates due to coarse vertical resolution, and synoptic‐scale subsidence.An anticyclone at the surface moved slightly northward during the simulation. The anticyclone was accompanied at 500 hPa by a deepening cyclone and, therefore, one would expect synoptic subsidence in the area of the Lagrangian 1. The modelled negative vertical wind component at the boundary‐layer top oscillates, and this is examined using spectral analysis. The results show that the vertical velocity is influenced by cumulus clouds on time‐scales up to 15 h with a peak at 9 h. The horizontal and vertical wavelengths of the vertical velocity disturbances are estimated from model output to be 400–500 km and 6–10 km, respectively. Using the estimated vertical wavelength and linear theory for hydrostatic inertia–gravity waves, a horizontal wavelength of 350–550 km was calculated for a frequency of 9 h. The model results thus indicate that these types of waves are responsible for the undulating vertical velocity.Finally, an estimation of the synoptic‐scale vertical velocity is calculated by filtering out all scales smaller than 15 h from the vertical velocity signal. This results in subsidence both at the beginning and the end of the Lagrangian with vertical velocities between −0.1 and −0.4 cm s−1. Copyright © 2004 Royal Meteorological Society

The structure of gradually transforming marine stratocumulus during the ASTEX first Lagrangian experiment

AbstractAn analysis of the mean and turbulent structure of the planetary boundary layer is presented, using aircraft data obtained during the Atlantic Stratocumulus Transition Experiment first Lagrangian experiment. The analysis of the mean vertical structure relies mostly on slant profile data, however, part of the analysis also uses so‐called ‘porpoise’ runs in the vicinity of the cloud top. The analysis of the turbulence structure utilizes all of the data, horizontal flight legs as well as slant profiles, and presents both scaled turbulence statistics and a spectral analysis. The purpose is to investigate details in the temporal development of the first Lagrangian experiment, and to study the turbulent structure in order to find scale relations that can be utilized in improved modelling of the stratocumulus‐capped boundary layer. The hypothesis is that so‐called ‘cloud decoupling’ has a significant impact on the turbulence scaling.The measured cloud water is corrected to become consistent between the two participating aircraft. The corrected maximum cloud water is more constant in time than in previous studies of the first Lagrangian; it is suggested that previously analysed temporal variability may be an artefact of using two different instruments to measure cloud water. The cloud‐top jump in equivalent potential temperature is positive in the first three flights, sufficiently large for a stable cloud top, but decreases and becomes negative for the last two flights in line with the breaking up of the cloud field. The upper part of the boundary layer warms and dries more rapidly than the lower, suggesting entrainment to be a dominant factor in the development of the cloud. The analysis of the porpoise flight legs illustrates how sharp features at the cloud‐top inversion are smoothed out while analysing several profiles, flight legs or even entire flights, rather than normalizing each profile with the height to the local cloud top.Scaling of the turbulent fluxes and the velocity variances shows that the cloud layer is decoupled throughout almost the entire first Lagrangian. It is also shown that while surface‐layer scaling is appropriate for the marine atmospheric boundary layer (MABL) the cloud‐layer turbulence scales with convective scaling. The low‐frequency spectrum has no spectral gap, with the exception of the vertical velocity, and continues to increase with decreasing frequency with only a change in slope; this indicates the presence of mesoscale motions. The cospectra of the turbulent fluxes are very noisy throughout. When averaging several spectra into height intervals, the scales of the turbulence as indicated by the peak in the averaged spectra appear to vary with height above the surface in the MABL but to be height‐independent in the cloud layer. An attempt to calculate a dissipation length‐scale shows scatter, but is consistent with a dissipation length‐scale increasing linearly with height in the MABL, but being roughly constant or possibly parabolic with height in the cloud layer. Copyright © 2003 Royal Meteorological Society

Simulations of aerosol‐cloud‐dynamical feedbacks resulting from entrainment of aerosol into the marine boundary layer during the Atlantic Stratocumulus Transition Experiment

Numerical simulations of a cloudy marine boundary layer (MBL) observed during the Atlantic Stratocumulus Transition Experiment were performed to study the influence of entrainment of free tropospheric cloud condensation nuclei (CCN) on cloud microphysics, dynamics, and radiative properties. The initial CCN concentration is 100 cm−3 in one simulation, while in the second simulation it varies from 100 cm−3 below the cloud top to a peak of 1200 cm−3 at the inversion. In the clean case, cooling from evaporating drizzle destabilizes the layer just below cloud base (not the entire subcloud layer) with respect to the surface, and promotes stronger penetrating cumulus. In the case with the elevated pollution layer, reduced drizzle at the cloud base results in weaker penetrating cumulus and a less effective supply of surface moisture to the cloud. This results in a much lower liquid water path (LWP) relative to the clean case that offsets the cloud albedo enhancement due to higher drop concentrations. Thus, although entrained CCN enhance the droplet concentration, the net effect on the cloud albedo is small. Additional simulations were performed to study the sensitivity of the MBL to varying levels of large‐scale subsidence. The change in large‐scale subsidence has a large effect on boundary layer dynamics, cloud microphysics, and the radiative budget. The simulations are used to separate the effects of enhanced albedo due to enhanced drop concentrations at constant LWP and those where LWP is modified due to dynamical feedbacks. For this case study, weaker subsidence results in a cloud with higher LWP and a cloud albedo that is enhanced over and above that due to enhanced droplet concentration. The simulations point to the complex dynamical‐microphysical‐radiative feedbacks in the MBL and how elevated polluted layers can change cloud radiative forcing in ways that would not be easily predicted by large‐scale models.

Diurnal Variability of the Marine Boundary Layer during ASTEX

The diurnal cycle of the marine boundary layer over the Atlantic Stratocumulus Transition Experiment (ASTEX) domain is examined during a two-week period of June 1992 in several fields, including: fractional low cloudiness, various parameters that enter into the heat and moisture budgets (e.g., temperature, water vapor mixing ratio, vertical motion), the budgets themselves, radiative heating, and the vertical eddy flux of moist static energy. Results show fractional low cloudiness varies over this region from a maximum of 54% in the predawn hours to a minimum of 39% in the midafternoon. These changes in low cloudiness are accompanied by an opposite trend in the boundary layer moisture, which shows a predawn drying and an afternoon moistening. Large-scale vertical motion also varies diurnally with a peak amplitude of 0.12 cm s 21 at 1.8 km, which is a ;20% variation from the mean subsidence rate. Cloud-radiative processes play an important role in determining the diurnal characteristics of the vertical eddy flux of moist static energy, with a peak amplitude in the cloud layer at 1.2 km and a variation at this level from 24 W m 22 at 1300 LST to 100 W m22 at 0400 LST. These diurnal changes suggest that the early morning peak in eddy flux results in a drying of the subcloud layer as moisture transport into the cloud layer is enhanced, leading to the observed low-cloud maximum. In contrast, during the daylight hours, solar heating within the cloud leads to a well-mixed cloud layer, a stabilizing of the subcloud layer with respect to the cloud layer, a decoupling of the cloud and subcloud layers, and ultimately, a thinning or reduction of the low clouds. Comparison of the diurnal cycle of summertime low-level divergence and vertical motion from this study with earlier published results from other regions in the North Atlantic shows a complex diurnal pattern with alternating phase in adjacent latitude belts across this region. The sense of these phase changes suggests that the northern branch of the Hadley circulation from roughly 58 Nt o 258N over the eastern Atlantic is pulsing diurnally with enhanced mass flow during the early morning hours.

Lidar and satellite retrieval of dust aerosols over the Azores during SOFIA/ASTEX

The synergy between active (airborne lidar) and passive (Meteosat) sensors is achieved with the help of a numerical transport model (TM2z) to derive optical properties of Saharan dust during a long range transport over the Azores. Measurements were taken in June 1992 during the surface of the ocean, fluxes and interaction with the atmosphere campaign, which took place during the Atlantic stratocumulus transition experiment. The dust source is identified to be in north Morocco from a TM2z back-trajectory analysis. Lidar observations over the Azores show that the dust is maintained in multiple thin layers (few hundred meters) up to 5 km altitude after a 4-day transport. Horizontal gradients are less marked, with a typical scale of variation of about 5 km. Lidar inversions yield dust optical thicknesses from 0.1 to 0.16 mainly due to two layers centered at 1.3 and 3.7 km. Since the weather was extremely cloudy over this region, the dust plume was not observable on the coincident Meteosat image. We thus processed the image taken two days earlier that clearly shows a dust plume between Azores and Spain. The Meteosat inversion was constrained by using the airborne lidar measurements in the marine boundary layer. The retrieved dust optical thicknesses are in good agreement with that retrieved from the lidar. Coherence of both lidar and radiometry measurements suggests that such a combined analysis is promising for retrieving the optical thickness of elevated dust layers as well as their spatial extent outside the source region even under cloudy conditions.

Upward Momentum Transfer in the Marine Boundary Layer

Abstract This paper focuses on the study of momentum flux between ocean and atmosphere in light winds and is based on the data collected during several field campaigns, the Atlantic Stratocumulus Transition Experiment, the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, and the San Clemente Ocean Probing Experiment. Weak wind at sea is frequently accompanied by the presence of fast-traveling ocean swell, which dramatically affects momentum transfer. It is found that the mean momentum flux (uw covariance) decreases monotonically with decreasing wind speed, and reaches zero around a wind speed U ≈ 1.5–2 m s−1, which corresponds to wave age cp/U ≈ 10 for wave/swell conditions of the experiments in this study. Further decrease of the wind speed (i.e., increase of the wave age) leads to a sign reversal of the momentum flux, implying negative drag coefficient. Upward momentum transfer is associated with fast-traveling swell running in the same direction as the wind, and this regim...

Microphysical and radiative properties of inhomogeneous stratocumulus: Observations and model simulations

Detailed microphysical observations made from aircraft during the Atlantic Stratocumulus Transition Experiment are used to examine the optical properties of marine stratocumulus. Special attention is given to the relationship between the droplet size distribution (droplet-number concentration, effective radius, and liquid-water content) and the optical extinction parameter. The extinction parameter calculated with measurements from the forward-scattering spectrometer probe (FSSP) differs only slightly from the extinction parameter obtained from the total droplet spectrum (measured with two optical probes, FSSP and 260X). In contrast, the results for the effective radius and the liquid-water content show differences up to 40% between the two spectra. A Monte Carlo radiative-transfer model is initialized with optical properties calculated from the observed microphysics. Monte Carlo simulations of radiative fluxes are in good agreement with observations for well-mixed boundary-layer cloud fields.

Microphysical and radiative properties of inhomogeneous stratocumulus: Observations and model simulations

AbstractDetailed microphysical observations made from aircraft during the Atlantic Stratocumulus Transition Experiment are used to examine the optical properties of marine stratocumulus. Special attention is given to the relationship between the droplet size distribution (droplet‐number concentration, effective radius, and liquid‐water content) and the optical extinction parameter. The extinction parameter calculated with measurements from the forward‐scattering spectrometer probe (FSSP) differs only slightly from the extinction parameter obtained from the total droplet spectrum (measured with two optical probes, FSSP and 260X). In contrast, the results for the effective radius and the liquid‐water content show differences up to 40% between the two spectra. A Monte Carlo radiative‐transfer model is initialized with optical properties calculated from the observed microphysics. Monte Carlo simulations of radiative fluxes are in good agreement with observations for well‐mixed boundary‐layer cloud fields.

Evaluation of radar retrieval algorithms in stratiform clouds using large‐eddy simulations

The performance of algorithms retrieving cloud liquid water content W from radar reflectivity factor Z is evaluated using data from a three‐dimensional large‐eddy simulation model with detailed size‐resolving microphysics. On the basis of case studies of a marine stratocumulus cloud observed during the Atlantic Stratocumulus Transition Experiment in June of 1992 and a continental stratus cloud observed over north central Oklahoma on April 30, 1994, it is shown that retrieval algorithms based only on Z are very sensitive to cloud microphysics and do not reproduce adequately the horizontally averaged vertical profile of W. Additional independent measurements are needed to constrain the retrieved W profile. Surprisingly, algorithms show little improvement when the exact concentrations of cloud droplets obtained from the model are included in the calculations. Vertically integrated liquid water path P is found to be a robust constraint that ensures a more accurate retrieval. In the two studied cases an algorithm based on the Z‐W relation of the form Z = aWb gives an optimal performance when a is inversely proportional to P and b = 1.32. The horizontal averaging of P over a computational domain of several kilometers does not affect the accuracy of the retrieval of the horizontally averaged profile of cloud liquid water content, which adds to the versatility of the algorithm.

Relationships between Total Water, Condensed Water, and Cloud Fraction in Stratiform Clouds Examined Using Aircraft Data

Abstract Relationships among total water, condensed water, and cloud fraction in boundary layer and cold tropospheric stratiform clouds are investigated using a large observational dataset collected by the U.K. Met. Office C-130 aircraft. Values of the above parameters are estimated using horizontal aircraft runs ranging from 40 to 80 km in length. Boundary layer (warm cloud) data were taken from the Atlantic Stratocumulus Transition Experiment (ASTEX) and First International Satellite Cloud Climatology Project (ISCCP) Research Experiment (FIRE). Free tropospheric (cold cloud) data were taken from the European Cloud and Radiation Experiment (EUCREX). In both warm and cold cloud a single reasonably well-defined relationship exists between the cloud fraction and the total water content (vapor + condensate) when normalized with the saturation specific humidity. A relationship exists between the condensed water content and the cloud fraction when appropriately scaled with the saturation specific humidity. Fun...

Modeling the chemistry of the marine boundary layer: Sulphate formation and the role of sea‐salt aerosol particles

A one‐dimensional model is presented that interactively simulates the dynamics and the gas‐aqueous phase chemistry of the cloud‐topped marine boundary layer. The model is described and tested using observations from the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) measurement campaign. The comparison generally indicates satisfactory agreement for dynamical properties and chemical species, except for SO2. We present several explanations for this discrepancy. However, a conclusive account is dependent on quantitative information about free tropospheric SO2 and H2O2 that is not available. Furthermore, a series of sensitivity runs is presented to explain the large quantities of non‐sea‐salt sulphate associated with sea‐salt particles, as observed during ASTEX/MAGE. The main conclusions are that most sulphate associated with sea‐salt particles is formed in cloud droplets that subsequently evaporate and that only a small amount is formed in deliquesced aerosol particles. The model results are sensitive to changes in the assumed sea‐salt emission rate and the overall aerosol size distribution. The latter indicates that a shift in the sea‐salt aerosol distribution toward the smaller particle sizes might explain the observed amount of sulphate associated with sea‐salt particles.

Large-Eddy Simulation Of A Nocturnal Stratocumulus-Topped Marine Atmospheric Boundary Layer: An Uncertainty Analysis

A large-eddy simulation (LES) model has been used to study a nocturnalstratocumulus-topped marine atmospheric boundary layer. The main objectivesof our study have been first to investigate the statistical significance of LES-derived data products. Second, to test the sensitivity of our LES results with respect to the representation of subgrid-scale mixing and microphysical processes, and third to evaluate and to quantify the parametric uncertainty arising from the incomplete knowledge of the environmental parameters that are required to specify the initial and boundary conditions of a particular case study. Model simulations were compared with observations obtained in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). Based on these simulations the following conclusions could be drawn. Resolution(50 × 50 × 25 m3) and domain size (3.2 × 3.2 × 1.5 km3) of the LES calculations were adequate from a numerical point of view to represent the essential features of the stratocumulus-topped boundary layer. However, the ensemble runs performed in our study to investigate the statistical significance of LES-derived data products demonstrate that the area-time averaging procedure for the second-order moments produces only a low degree of statistical reliability in the model results. This illustratesthe necessity of having LES model results that are not only of adequate resolution but also of sufficiently large domain. The impact of different subgrid schemes was small, but the primary effects of drizzle were found to influence the boundary-layer structure in a climatologically significant way. The parametric uncertainty analysis revealed that the largest contribution to the variance of the LES-derived data products is due to theuncertainties in the cloud-top jump of total water mixing ratio and the net radiative forcing. The differences between the model and measurements for most of the simulated quantities were within the modelling uncertainties, but the calculated precipitation rate was found to differ significantly from that derived in the observations.

The Sensitivity Of A Stratocumulus Transition: Model Simulations Of The Astex First Lagrangian

In this study, a one-dimensional ensemble-average model is used to simulatethe Atlantic Stratocumulus Transition Experiment firstLagrangian, where the same airmass was followed from the subtropical high pressure region en route towards the trade wind region. The airmass experiences increasing sea-surface temperature and achange from subsidence to weak ascent on its way south. Thiscauses the marine boundary layer (MBL)to grow and the cloud deck to change from a solid stratocumulus deck tomore broken stratocumulus clouds with cumulus cloudsdeveloping beneath, and reaching up into the stratocumulus clouds. A control run is analyzed and compared in detail with theobservations. Both a statistical evaluation and a more subjective evaluation are performed, where both establish confidencein the model performance. The model captures the MBL growth and the cloudliquid water, as well as the drizzle flux, is well predicted by the model.A sensitivity study was performed with the objective of examining theMBL and the cloud response to external and internal 'forces'.The results show that, if drizzle formation is not allowed,unrealistically high cloud liquid water mixing ratios are predicted. Even though the drizzle flux is very small, it is still important for the water budget of the MBL and for the boundary-layer dynamics.We also found that the sea-surface temperature increase is more important for the increasing cloud top height than the synoptic-scale divergence fields. However, the synoptic-scale subsidence is crucial during the first day, when the sea-surface temperature was constant, in keepingthe cloud top at a constant height. Drizzle evaporation below the cloud base seems to be important for below-cloud condensation. The drizzle predictions are significantly altered when the prescribed cloud droplet and/or drizzle drop numbers are altered.