Marine Stratus Clouds Research Articles

Large contribution of organic aerosols to cloud-condensation-nuclei concentrations

THE albedo and radiative properties of marine stratus clouds are determined largely by the number density of cloud condensation nuclei (CCN) over the oceans. Modelling studies have suggested that most of these nuclei are sulphate aerosols derived from both anthropogenic and natural sources1. Here we present evidence that organic aerosols also play a key role in cloud nucleation. We determine the relative contributions of sulphate and organic aerosols to CCN concentrations at a marine site known to be influenced by anthropogenic emissions, and find that organic aerosols account for the major part of both the total aerosol number concentration and the CCN fraction. Thus, in regions that are affected by anthropogenic pollutants, organic aerosols may play at least as important a role as sulphate aerosols in determining the climate effect of clouds.

Boundary layer stratus clouds: Inferred from satellite infrared spectral measurements over oceans

A method to remotely sense marine stratus clouds, that are underneath the boundary layer inversion produced by strong subsidence, is developed with the help of the measurements made by the Infrared Interferometer Spectrometer (IRIS) on board the Nimbus 4 satellite. These measurements, which had a spectral resolution of 2.8 cm -1 and a field view of ≈ 100 km in the region 870–980 cm -1, allow one to compare the weak absorption due to water vapor lines with that of the weak ν 3-ν 1 band of CO 2. From such a comparison it is possible to sense the presence of these boundary layer stratus clouds. Maps of these clouds over subsidence regions of the tropical and subtropical oceans deduced from IRIS data show the usefulness of the technique to climatological studies.

Vertical Mixing in Clear Air and Clouds

The realization that clouds are diluted by the entrainment of dry air from outside the cloud boundaries began more than 40 years ago. Cloud-top entrainment was first clearly recognized by Squires in 1958. Warner and Squires showed that the dilution increased towards cloud top and was uniform across the cloud perpendicular to the shear. Not until the mid 1970s, however, did more evidence begin to show that cumulus clouds have a structure from upshear to downshear where rapid dilution by the mixing of dry air from above cloud top eventually totally evaporates the cloud parcels. This slow development was partially caused by the lack of appropriate thermodynamic diagrams for cloud microphysics analysis. In this paper, an easy approach to thermodynamic analysis is introduced for studying cloud mixing and entrainment structure. The advantages of using the temperature and wet-bulb temperature of air parcels, after compression to 1000 mb, as the coordinates of a general thermodynamic diagram are discussed and illustrated with examples. These variables are easily calculated on a small computer and then directly plotted. No further graphical calculations are needed for parcel mixing or for comparing densities. Total water mixing ratios and condensation pressures are directly readable for each composition.more » An important point is that these displays can be automatically produced without including any significant approximations. Thus, the details of how to process the data, or plot points and draw lines, is no longer a consideration in the use of such analysis, and no analog calculations are needed to produce the graphs. Observational examples are discussed, and the use of the computer program raises new questions about radiative cooling in and above cloud top and about the penetration by turbules carrying heat and moisture of marine stratus clouds and subcloud air. 16 refs., 10 figs.« less

Theoretical development and sensitivity tests of a stratus cloud droplet size retrieval method for AVHRR-K/L/M

A modified AVHRR multispectral imaging instrument is to be flown on the NOAA polar-orbiting satellites beginning with NOAA-K, and is designated AVHRR-K/L/M. The instrument will include a narrowed near-visible Channel 2 centered at 0.85 μm, and a new near-infrared Channel 3A centered at 1.6 μm. In this study, we describe a retrieval method for marine stratus cloud droplet size which utilizes the radiance measurements in these channels. Factors which influence the sensitivity and accuracy of the estimation method are discussed, and results of model simulations designed to test the method are presented. Potential retrieval errors due to haze within the boundary layer are identified. It is concluded that the effective radius of the cloud droplet size distribution can be retrieved for clouds of optical depth seven or greater which fill the AVHRR radiometer field of view.

Heterogeneous production of cloud condensation nuclei in the marine atmosphere

Model calculations are presented which indicate that newly created particles in the marine atmosphere will commonly only be able to achieve sizes, by means of gas‐phase processes, sufficient to activate in cumuliform clouds. Aqueous sulfate production in such clouds will be generally necessary to grow them large enough to activate in marine stratus clouds.

Comparison of cloud forcing derived from the Earth Radiation Budget Experiment with that simulated by the NCAR community climate model

A comprehensive comparison of top of atmosphere radiative quantities from the Earth Radiation Budget Experiment (ERBE) with the same quantities from the latest version of the National Center for Atmospheric Research Community Climate Model (CCM) is presented. The ERBE data set offers a unique collection of top of atmosphere radiation fields for it includes clear‐sky radiation fields that have in the past not been available for such comparisons. Comparison between ERBE clear‐sky longwave fluxes and CCMl indicates larger outgoing flux in the model than measured by ERBE. This overestimation is ascribed to the prevalent dryness of the model. Comparison of clear sky albedo indicates good agreement over oceans and land. Total outgoing longwave flux also reflects the moisture deficiency of the model, but differences due to an underprediction of “effective” high cloud are also apparent. The clear‐sky and total fluxes are combined to form the cloud radiative forcing from the ERBE data and the CCM. Comparison of shortwave cloud radiative forcing indicates deficiencies in the model where marine stratus clouds are absent. Large longwave cloud forcing over the tropical deep convective regions in Indonesia and South America are present in the model but are underestimated compared with the ERBE results. Three regions, located over Indonesia, the equatorial Pacific, and the North Atlantic, are considered in detail. For the Indonesian region where deep convection is present, we consider the statistical correlation between the longwave cloud forcing and the shortwave cloud forcing from the ERBE data and CCM. Results indicate a near cancellation between the SWCF and LWCF for these regions, whereas the model predicts a net cooling. Another major area of discrepancy is over the North Atlantic and Pacific oceans where ERBE shows that clouds significantly reduce the solar heating of the oceans. While the model simulates this cooling, the magnitude is underpredicted by more than a factor of 2.

Bistability of CCN concentrations and thermodynamics in the cloud-topped boundary layer

RECENT discussions of the effect of cloud condensation nuclei (CCN) in controlling the albedo of marine stratus cloud and hence global climate1–3have raised questions concerning the factors that control CCN populations and how (or whether) such factors should be included in climate models4. The observed near-constancy5 of CCN concentrations in remote marine air is remarkable in view of the wide variety of possible conditions, the range over which they might be expected to vary, and the considerable difference between marine and continental CCN concentrations. Here we explain this relative lack of variability, thought to be due to a balance of source and sink processes6,7, in a simple model of the marine cloud-topped boundary layer. Solutions to this model give two stable CCN concentration regimes—one corresponding to the low concentration observed over the ocean, the other corresponding to the higher concentration observed over land—each dominated by a different sink mechanism. The two regimes have very different optical properties, and thus may be of climatic significance.

Numerical study of droplet size dependent chemistry in oceanic, wintertime stratus cloud at southern mid-latitudes

Cloud droplet chemistry is modelled for the first 150 m of rise in a wintertime, mid-latitude, marine stratus cloud using observations made at and near the Cape Grim Baseline Station as a source of input parameters. The emphasis in this work was to study the variation in droplet chemistry as a function of both droplet size and nucleus composition, with a particular focus on the way in which oxidation of dissolved sulfur dioxide varied. At 150 m above the condensation level, solute concentration as a function of droplet size was found to increase by as much as 2 to 3 orders of magnitude for only a factor of 2 increase in droplet radius, primarily as a consequence of the 1/r dependence in the droplet growth equation. This type of size dependence exists at all levels in the model cloud, and has a significant influence on oxidation rate of sulfur dioxide in droplets growing on ‘sulfate’ nuclei, oxidation by ozone being favoured in the smallest droplets, but oxidation by hydrogen peroxide being favoured in the larger droplets. Oxidation by ozone is favoured at all sizes in droplets formed on sea-salt nuclei as a result of the initially high alkalinity of these droplets, and in the cloud overall is calculated to be the more important oxidation pathway. Although based on a simplified chemical scheme, these results suggest that both size-dependent and nucleus-dependent chemistry of cloud droplets may need to be considered explicitly in cloud modelling work. Volume-weighted mean pH values in the range 5 to 6 were predicted from sensitivity studies in which input variables were varied over reasonable ranges, in agreement with two sets of bulk cloud-water pH data obtained by aircraft near Cape Grim.

The Relationship of Marine Stratus Clouds to Wind and Temperature Advection

We have studied the changes in marine stratocumulus cloud cover observed during the FIRF, Program and how cloud cover related to synoptic conditions. Statistical analyses of the 21 day FIRE period show that marine stratocumulus cloud cover over the eastern Pacific ocean was related to wind direction and temperature advection. Good coorelations were found between the cloud cover fraction observed on satellite imagery and the NMC Global Spectral Model analyses of surface winds and temperature advection. This comparison was made in even locations in the Eastern Pacific. Regional differences were found between the area of FIRE operations several hundred kilometers west of San Diego and the other oceanic areas studied.

Cloud Top Liquid Water from Lidar Observations of Marine Stratocumulus

Marine stratus clouds were simultaneously observed by nadir Nd:YAG lidar measurements and in situ cloud physics measurements. A procedure was applied to derive the two-dimensional vertical cross section of the liquid water from within the cloud top lidar observations. A comparison to direct in-cloud liquid water observations gave good results. The liquid water retrieval was limited to an effective optical depth of 1.5. The true cloud optical thickness was also obtained from the retrieval procedure to a corresponding limit of 3.8. The optical thickness of the observed marine stratus clouds was predominantly below 3.0.

Cloud droplets: Solute concentration is size dependent

An experiment is described in which cloud droplets are sampled as a function of their size using a counterflow virtual impactor, thereby revealing the degree to which the different sized droplets in a cloud are chemically different. The chief result is that the larger droplets sampled from a marine stratus cloud had a solute mass concentration that was a factor of 3 greater than in the smaller droplets.

Climate forcing implications from Vostok ice-core sulphate data

The hypothesis that the number concentration of cloud condensation nuclei (mainly the sulphate particles produced by the oxidation of dimethylsulphide emitted from the ocean1) influences marine stratus cloud albedo, and hence global climate2, is examined using the non-seasalt sulphate (nss-SO2–4) profile that was recently obtained3 along the 160 kyr Vostok (Antarctica) ice core. The deduced 20–46% increase in nss-SO2–4 content in the Antarctic atmosphere during full glacial (compared with interglacial) conditions is consistent with higher dimethylsulphide emissions from marine biota productivity. Similar spectral features and correlation of CO2 and nss-SO2–4 suggest a global significance for these changes, and a possible link between CO2 and dimethylsulphide emissions. Assuming that the global average number population of cloud condensation nuclei is proportional to nss-SO2–4 in the Antarctic atmosphere, a global radiative cooling at the surface of up to 1 K would result, reinforcing the effect of CO2 (˜0.6K; ref. 4) and of total insolation changes5 (0.2 K).

A Theoretical Solution to the Motion of an Atmospheric Spherical Vortex

Abstract Several atmospheric phenomena have been studied in the past in which buoyant fluid convects upward as a compact “blob” of cloudy or smoky air. In such volumes there is a strong updraft in the middle of the blob, and slower rising, or descending air around the outside. Such blobs of cooler diluted air are an important candidate dynamical entity for diluting cumulus and stratus clouds with dry air entrained from above; they have been identified in observations of marine stratus clouds. In the past they have been studied in water tank experiments but no adequate theoretical description has been available. This paper presents a theoretical analysis that will allow formulation of the dynamical equations so they can be developed for use in nonself-similar conditions. The rising vortex, in the theoretical model for the self-similar buoyant blob, spreads at the same angle as observed in previous studies with water tank experiments.

A Study of the Short‐Wave Radiative Properties of Marine Stratus: Aircraft Measurements and Model Comparisons

AbstractAircraft measurements of the short‐wave albedo of a marine stratus cloud are described and compared with results from radiative transfer models of plane‐parallel clouds. the albedos are measured in the visible and near‐infrared parts of the solar spectrum using clear dome and red dome pyranometers. Using all the available liquid water content data and the measured microphysical properties, two model clouds are constructed to represent the maximum and minimum liquid water paths encountered during the experiment. Model albedo calculations based on these clouds are compared with the range of albedos measured just above cloud top. the natural variation of liquid water path at the cloud edge is exploited as a further qualitative assessment of the models' behaviour.

Lidar depolarization from multiple scattering in marine stratus clouds

The generation of ruby lidar depolarization from multiple scattering in marine stratus clouds has been examined systematically from a field site on the southern California coast. Investigated were the effects on the linear depolarization ratio δ of lidar receiver field of view (FOV), elevation angle, and laser alignment error. An approximately linear increase in maximum δ values was observed with increasing receiver FOV, and the importance of accurate transmitter/receiver alignment has been demonstrated. An elevation angle dependence to the δ values was observed as a consequence of the vertical inhomogeneity of water cloud content above cloud base. Time histories of the depolarization characteristics of dissipating stratus clouds revealed significant variability in δ values due to cloud composition variations. Employing a 1-mrad transmiter FOV, maximum δ values of 0.21 and 0.33 were observed with 1- and 3-mrad receiver FOVs, respectively.

Profiles of optical Extinction Coefficients Calculated from Droplet Spectra Observed in Marine Stratus Cloud Layers

Airborne measurements of droplet spectra n(r) where r is droplet radius were made in stratus cloud layers over the ocean 130 km southwest of San Diego. Optical extinction coefficients σe were calculated at selected wavelengths (λ= 0.53, 3.75 and 10.59 μm) from these n(r) and were used to construct average vertical profiles of σe between 200 m above cloud base to 250 m below cloud base. An approximation to σe a power function of the liquid water content w was found to be superior compared with approximations involving linear functions of w or the cross-section area. The horizontal variability of σe in the clouds was examined. The average horizontal scale size of σe was estimated to be 3 km and the horizontal measuring distance required to obtain values of σe with a relative accuracy of 10% was found to be in excess of 30 km.

Droplet spectra observed in marine stratus cloud layers : Noonkester, V.R., 1984. J. atmos. Sci., 41(5): 829–845. Naval Ocean Systems Center, San Diego, Calif. 92152, USA

Droplet spectra observed in marine stratus cloud layers : Noonkester, V.R., 1984. J. atmos. Sci., 41(5): 829–845. Naval Ocean Systems Center, San Diego, Calif. 92152, USA