Marine Stratocumulus Clouds Research Articles

Radiative Horizontal Transport and Absorption in Stratocumulus Clouds

The equation of radiative energy balance in a homogeneous plane-parallel cloud accounts for the photon transport in the vertical direction only. From this equation, the absorption can be evaluated as a difference between the net radiative fluxes measured on the cloud top and bottom. Real clouds exhibit extreme horizontal variability of their optical and, hence, radiative properties. So the radiative energy balance equation for inhomogeneous clouds includes an extra term to describe the net energy transport in the horizontal directions (radiative horizontal transport). This study investigates the horizontal transport in nonhomogeneous stratocumulus clouds and its influence on the accuracy of absorption estimates. Using a realistic fractal model of marine stratocumulus clouds and Monte Carlo method, it is shown that the magnitude of horizontal transport 1) is comparable with those of albedo, transmittance, and absorptance and 2) depends strongly on the fractal dimension of cloud optical depth. Because of the horizontal transport, there is no unique dependence between optical and radiative properties of given pixel. The neglect of the horizontal transport is a major source of uncertainty in absorption estimates. Two possible ways of improving the inhomogeneous cloud absorption estimates from field measurements are discussed.

Radiative effects of sub‐mean free path liquid water variability observed in stratiform clouds

A unique 4‐cm‐resolution data set of stratocumulus cloud liquid water content (LWC) made possible, for the first time, the generation of credible models of cloud LWC down to centimeter‐scales, well below the photon mean free path (mfp) which is typically tens of meters. These models are exploited to study the errors in the usual assumption of LWC homogeneity within three‐dimensional (3D) radiative transfer grid boxes, by doing Monte Carlo computations for 30‐m cubes resolved down to 10‐cm scales. Cloud models with and without vertical variability are compared. We found that vertically homogeneous but horizontally variable cloud structure produces cloud radiation biases of 3–5% relative to the assumption of complete homogeneity. The addition of vertical variability similar to the horizontal variability reduces these biases well below 1%. We therefore conclude that 3D cloud models resolved down to the mfp scale (20–30 m for marine stratocumulus clouds), with an incorrect assumption of homogeneity below that scale are sufficient for modeling radiative properties averaged over mfp and larger scales in both visible and near‐infrared spectral regions. This conclusion is restricted to only the overcast cases.

The Energy Budget of the NCAR Community Climate Model: CCM3*

The energy budget of the latest version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3) is described. The energy budget at the top of the atmosphere and at the earth’s surface is compared to observational estimates. The annual mean, seasonal mean, and seasonal cycle of the energy budget are evaluated in comparison with earth radiation budget data at the top of the atmosphere and with the NCAR Ocean Model (NCOM) forcing data at the ocean’s surface. Individual terms in the energy budget are discussed. The transient response of the top-of-atmosphere radiative budget to anomalies in tropical sea surface temperature is also presented. In general, the CCM3 is in excellent agreement with ERBE data in terms of annual and seasonal means. The seasonal cycle of the top-of-atmosphere radiation budget is also in good (<10 W m−2) agreement with ERBE data. At the surface, the model shortwave flux over the oceans is too large compared to data obtained by W. G. Large and colleagues (∼20–30 W m−2). It is argued that this bias is related to a model underestimate of shortwave cloud absorption. The major biases in the model are related to the position of deep convection in the tropical Pacific, summertime convective activity over land regions, and the model’s inability to realistically represent marine stratus and stratocumulus clouds. Despite these deficiencies, the model’s implied ocean heat transport is in very good agreement with the explicit ocean heat transport of the NCOM uncoupled simulations. This result is a major reason for the success of the NCAR Climate System Model.

On the validity of the independent pixel approximation for boundary layer clouds observed during ASTEX

The two‐dimensional radiative transfer behavior of nine marine stratocumulus clouds observed by cloud radar during the Atlantic Stratocumulus Transition Experiment is examined. The cloud radar resolves the vertical structure to 37.5 m. The method of [Frisch et al., 1995] is used to convert radar reflectivities to extinction fields. Three constructions of the same cloud field help elucidate underlying causes of variability: one is fully two‐dimensional, while the other two have vertically uniform extinction fields but possess either a flat cloud top or the original cloud top topography. Two‐dimensional solar radiative transfer results are compared with the independent pixel approximation (IPA) result.At the scale of the domain (≈ 7km) the IPA albedo bias is small, even after including vertical structure. Locally, in contrast, the direct solar beam interaction with cloud top geometry competes with radiative smoothing as the dominant radiative process. Power spectral analysis of nadir reflectances is dominated by radiative smoothing for overhead Sun, and side illumination/shadowing of cloud top bumps for low Sun. A method that incorporates direct beam interactions with the cloud geometry, in addition to radiative smoothing, significantly improves correlations of a smoothed IPA radiance field with the 2‐D reflectances. In a remote sensing application, optical depth and albedo retrieval biases from plane‐parallel theory depend on the spatial scale chosen to emulate a satellite pixel size. For scales less than a few kilometers and with low Sun, cloud top topography can cause large positive optical depth biases even when averaged over the entire domain. A larger spatial scales the negative IPA bias always dominates. Domain‐averaged monochromatic albedo retrieval errors remain below 0.005, a relative error of less then 1%.

Estimation of water cloud properties from satellite microwave, infrared and visible measurements in oceanic environments: 1. Microwave brightness temperature simulations

Theoretical calculations are used to examine the spectral characteristics of SSM/I (special sensor microwave/imager) brightness temperature (Tb) values for non‐precipitating clouds over oceans. It was found that liquid water path (LWP) and the cloud water temperature (Tw) could be derived simultaneously with a technique using the SSM/I 37‐GHz and 85‐GHz brightness temperatures. Uncertainties in column water vapor (CWV) are the most important error sources in the estimation of LWP and Tw, while ice particles smaller than 100 μm in nonprecipitating clouds have a very weak effect (&lt;1 K) on the Tb values at the relevant SSM/I frequencies. When all SSM/I instrument noise and error sources associated with sea surface temperature, wind speed, and CWV are considered, the biases in LWP from current microwave methods are very small (≤0.01 mm) and the standard deviations vary from 0.02 to 0.04 mm. The Tw bias and standard deviation decrease with increasing LWP from about 6 and 8 K, respectively, for clouds with low LWP to &lt;1 K for LWP &gt;0.4 mm. For most marine stratocumulus clouds (LWP ∼0.1 to 0.2 mm) the Tw bias and standard deviation are about 2 and 4 K, respectively, resulting in cloud height errors of ∼1 to 2 km. The method should yield an improvement in the accuracy of retrieved LWP because it more closely approximates cloud temperature than previous techniques. To use the radiative transfer results, it is necessary to normalize or calibrate them to the observations. This relative calibration using 22‐GHz brightness temperatures reveals differences of 2.86 K and −1.93 K for the 37‐GHz horizontal and 85‐GHz vertical channels, respectively, between the SSM/I observations and the model simulations. In multilayered cloud conditions, this new microwave analysis method, when combined with infrared data, should make it possible to determine cloud temperature for an upperlevel ice cloud from the infrared brightness temperatures while simultaneously deriving Tw and LWP for the lower liquid water cloud with the microwave data.

Nonlocal independent pixel approximation: direct and inverse problems

The independent pixel approximation (IPA), which treats radiative properties of each pixel independently by using standard plane-parallel calculations preserves scale-invariance found in the analyses of the horizontal variability of liquid water in marine stratocumulus clouds. Several studies, however, report a violation of scale-invariance in LANDSAT cloud radiance fields that are much smoother than cloud structure on small scales. This shows a limitation of IPA on small scales: it is unable to simulate the smooth small-scale behavior that is due to the horizontal photon transport. This paper introduces a nonlocal independent pixel approximation (NIPA) that extends the IPA by incorporating empirically the smoothing effects of horizontal interpixel fluxes through a convolution product of the IPA and an approximate Green function for radiative transfer. The authors also address the inverse problem of cloud optical depth retrieval from satellite data, showing how NIPA can be used to overcome the limitations of current IPA-based methods at small scales.

What Controls Stratocumulus Radiative Properties? Lagrangian Observations of Cloud Evolution

Marine stratocumulus clouds have a large impact on the earth’s radiation budget. Their optical properties vary on two distinct timescales, one associated with the diurnal cycle of solar insolation and another with the downstream transition to trade cumulus. Hypotheses regarding the control of cloud radiative properties fall broadly into two groups: those focused on the effects of precipitation, and those concerned with the environment in which the clouds evolve. Reconciling model results and observations in an effort to develop parameterizations of cloud optical properties is difficult because marine boundary layer clouds are not in equilibrium with their local environment. The authors describe a new technique for the observation of boundary layer cloud evolution in a moving or Lagrangian frame of reference. Blending satellite imagery and gridded environmental information, the method provides a time series of the environmental conditions to which the boundary layer is subject and the properties of clouds as they respond to external forcings. The technique is combined with in situ observations of precipitation off the coast of California and compared with the downstream evolution of cloud fraction in five cases that were observed to be precipitating with three cases that were not. In this small dataset cloud fraction remains almost uniformly high, and there is no relationship between the presence of precipitation and the evolution of cloud fraction on 1- and 2-day timescales. Analysis of a large number of examples shows that clouds in this region have a typical pattern of diurnal evolution such that clouds that are optically thicker than about 10 during the morning are unlikely to break up over the course of the day but will instead show a large diurnal cycle in optical depth. Morning cloud optical thickness and the resultant susceptibility to breakup have a much larger impact on diurnally averaged cloud radiative forcing than do diurnal variations in cloud properties. Cloud response is significantly correlated with lower tropospheric temperature stratification at all times, though the best correlation exists when cloud response lags stability by at least 16 h. Sea surface temperature is also correlated with cloud properties during the period in which cloud response is measured and the 12 h prior. The authors suggest that sea surface temperature plays two competing roles in determining boundary layer cloudiness, with rapid changes in SST promoting cloudiness on short timescales but tending to lead to a more rapid transition to the trade cumulus regime.

A modeling study of the effect of drizzle on cloud optical depth and susceptibility

This paper examines the impact of drop spectral broadening, generated by the collection process, on the optical depth, cloud albedo, and susceptibility of marine stratocumulus clouds. The results are arrived at using (1) the output from a simple box model calculation of collection and (2) the output from an eddy‐resolving model of stratocumulus clouds that explicitly represents the size distribution of the drops. It is shown that commonly used relationships for cloud optical properties developed for narrow spectra do not generally apply to spectra undergoing spectral broadening. The optical depth dependence on the drop number concentration to the one‐third power is shown to be an overestimate of the optical depth when spectra broaden through collection. In addition, the cloud susceptibility dependence on drop number is shown to be larger for spectra experiencing broadening than for narrow spectra.

Aerosols from biomass burning over the tropical South Atlantic region: Distributions and impacts

The NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) expedition was conducted September 21 through October 26, 1992, to investigate factors responsible for creating the seasonal South Atlantic tropospheric ozone maximum. During these flights, fine aerosol (0.1–3.0 μm) number densities were observed to be enhanced roughly tenfold over remote regions of the tropical South Atlantic and greater over adjacent continental areas, relative to northern hemisphere observations and to measurements recorded in the same area during the wet season. Chemical and meteorological analyses as well as visual observations indicate that the primary source of these enhancements was biomass burning occurring within grassland regions of north central Brazil and southeastern Africa. These fires exhibited fine aerosol (N) emission ratios relative to CO (dN/dCO) of 22.5 ± 9.7 and 23.6 ± 15.1 cm−3parts per billion by volume (ppbv)−1over Brazil and Africa, respectively. Convection coupled with counterclockwise flow around the South Atlantic subtropical anticyclone subsequently distributed these aerosols throughout the remote South Atlantic troposphere. We calculate that dilute smoke from biomass burning produced an average tenfold enhancement in optical depth over the continental regions as well as a 50% increase in this parameter over the middle South Atlantic Ocean; these changes correspond to an estimated net cooling of up to 25 W m−2and 2.4 W m−2during clear‐sky conditions over savannas and ocean respectively. Over the ocean our analyses suggest that modification of CCN concentrations within the persistent eastern Atlantic marine stratocumulus clouds by entrainment of subsiding haze layers could significantly increase cloud albedo resulting in an additional surface radiative cooling potentially greater in magnitude than that caused by direct extinction of solar radiation by the aerosol particles themselves.

Evaluation of sulfate aerosols indirect effect in marine stratocumulus clouds using observation‐derived cloud climatology

The indirect shortwave effect of anthropogenic sulfate aerosol augmentation in marine stratocumulus clouds has been evaluated using global cloud climatology, sulfate aerosol data from a chemical model, and cloud albedo augmentation obtained from a large‐eddy simulation (LES) cloud model with explicit microphysics. It is found that the annually and globally (over the oceans) averaged indirect shortwave forcing is −1.1 Wm−2 with a hemispheric difference of 0.4 Wm−2. Given the many uncertainties in the aerosol‐CCN‐cloud microstructure links, this estimate, most likely, should be considered as an upper limit of the aerosol indirect forcing. The hemispheric forcing has a strong seasonal cycle with the Northern Hemisphere (NH) forcing exceeding the Southern Hemisphere (SH) forcing during the NH spring and summer and the SH forcing prevailing during the SH spring and summer. We also estimate that the single layer St/Sc contributes most to the aerosol indirect forcing (60%) with an additional 25% from the St/Sc overlapped with Ci/Cs/Cc, and 15% from St/Sc overlapped with As/Ac.

Microphysical and short-wave radiative structure of wintertime stratocumulus clouds over the Southern Ocean

Results are presented from an aircraft measurement campaign carried out over the Southern Ocean near the north-west coast of Tasmania, Australia. The microphysical and radiative characteristics of marine stratocumulus cloud sheets were sampled on four days, three of which are considered baseline days when air parcels had traversed long distances over the ocean without having been exposed to anthropogenic sources of pollution. Clouds were depleted by drizzle, with cloud liquid-water often smaller than 50% of the expected adiabatic value. Horizontal variability in liquid-water content associated with non-drizzle droplets was primarily caused by variations in the droplet number concentration, which was found to be among the lowest ever recorded (10 to 40 cm −3 for clouds of up to 300 m deep). Cloud albedos integrated over the solar spectrum varied from 40 to 60%, which roughly agreed with radiative-transfer computations. Data from one non-baseline day were also obtained and compared with the data obtained during baseline conditions. The functional dependence of cloud optical depth on liquid-water path was found to be much stronger on this day than the others, due to the smaller size of the cloud droplets present. Calculations show that on the days when drizzle was intense, the cloud optical depth could have been reduced by as much as 50% by shifts to larger values of the cloud droplet effective radius.

Microphysical and short‐wave radiative structure of wintertime stratocumulus clouds over the Southern Ocean

AbstractResults are presented from an aircraft measurement campaign carried out over the Southern Ocean near the north‐west coast of Tasmania, Australia. The microphysical and radiative characteristics of marine stratocumulus cloud sheets were sampled on four days, three of which are considered baseline days when air parcels had traversed long distances over the ocean without having been exposed to anthropogenic sources of pollution. Clouds were depleted by drizzle, with cloud liquid‐water often smaller than 50% of the expected adiabatic value. Horizontal variability in liquid‐water content associated with non‐drizzle droplets was primarily caused by variations in the droplet number concentration, which was found to be among the lowest ever recorded (10 to 40 cm −3 for clouds of up to 300 m deep). Cloud albedos integrated over the solar spectrum varied from 40 to 60%, which roughly agreed with radiative‐transfer computations. Data from one non‐baseline day were also obtained and compared with the data obtained during baseline conditions. The functional dependence of cloud optical depth on liquid‐water path was found to be much stronger on this day than the others, due to the smaller size of the cloud droplets present. Calculations show that on the days when drizzle was intense, the cloud optical depth could have been reduced by as much as 50% by shifts to larger values of the cloud droplet effective radius.

Microphysics of Marine Stratocumulus Clouds with Two Drizzle Modes

Abstract This observational study looks at the distribution of some cloud microphysical properties measured from the University of Washington's aircraft in mostly unbroken stratocumulus (Sc) clouds in the vicinity of the Azores Islands during ASTEX. The average behavior of the Sc includes the presence of two drizzle modes. In 85% of the Sc the amount of drizzle LWC (liquid water content in droplets with radius r > 20 µm) is less than 0.01 g m−3, while in the rest, drizzle LWC is much larger than 0.01 g m−3. The microphysics of light-drizzle Sc approach classical conditions, because measured and adiabatic LWC profiles are similar, and droplet spectral dispersions decrease with height. The lognormal function fits approximately measured droplet spectra of light-drizzle Sc. Standard deviations of these spectra remain about constant with height, vary over a small range, and average 1.74-µm radius. Drizzle LWC > 0.01 g m−3 is found on the average in Sc with spectra that have effective radii re &ge 16 µm. This h...

Unrealistic desiccation of marine stratocumulus clouds by enhanced solar absorption

THE absorption of solar radiation by clouds affects the distribution of heat that drives atmospheric and ocean circulations. Many investigators have measured cloud solar absorption values exceeding theoretical estimates, although the discrepancies have been regarded as inconclusive1. But more recent measurements indicate that clouds absorb up to three times as much solar energy than conventional theory predicts2,3. Marine stratocumulus clouds are particularly sensitive to solar absorption because marine boundary-layer mixing is typically driven by cloud radiative processes. Here we present model simulations of a stratocumulus-topped marine boundary layer, incorporating different levels of solar absorption. With conventional absorption, the simulations reproduce observed cloud behaviour. But those with artificially enhanced absorption result in an unrealistic daytime depletion of cloud water, because a reduction in cloud radiative cooling results in decreased boundary-layer mixing. Moreover, we show that it is unlikely that liquid water or any plausible dissolved material can absorb the energy required by the recent measurements of solar absorption. Our results therefore indicate that either enhanced solar absorption occurs only in clouds other than marine stratocumulus, or the enhancement of cloud solar absorption indicated by recent measurements2,3 is overestimated.

Wide-Area Determination of Cloud Microphysical Properties from NOAA AVHRR Measurements for FIRE and ASTEX Regions

A method for satellite remote sensing of cloud optical thickness and effective particle radius has been developed to apply to NOAA AVHRR multispectral radiance data. Undesirable radiation components such as ground-reflected solar radiation and thermal radiation are guessed from satellite-received radiances in channels 1, 3, and 4 of AVHRR and subtracted from radiances in channels 1 and 3 to derive the reflected solar radiation of a cloud layer that includes information about cloud microphysical properties. This method can be applied to a broad range of water clouds from semitransparent to thick clouds. This method was applied to AVHRR data acquired over oceans during the First ISCCP Regional Experiment and the Atlantic Stratocumulus Transition Experiment. The authors found good agreement between satellite-derived and in situ microphysical quantities. The presence of drizzle droplets in optically thin clouds was also confirmed from the satellite observation. Furthermore, the results show that marine stratocumulus clouds were drastically modified by ship track effluents and dust-contaminated aifflow from the continent.

Using coincident multispectral satellite data to assess the accuracy of special sensor microwave imager liquid water path measurements

We present techniques to identify and reduce the sources of error in the liquid water path (LWP) of marine stratocumulus clouds retrieved from the special sensor microwave imager (SSMI), using coincident high‐resolution visible and infrared data provided by the operational linescan system (OLS). The OLS data are used to identify cloud‐free areas in order to determine the degree of error in the estimated clear sky polarization difference, an important parameter in the SSMI retrieval algorithm. The retrieval algorithm is refined by including the cloud top temperature derived from the OLS infrared data. Errors in the LWP measurements owing to broken or inhomogeneous clouds are difficult to quantify, but a subpixel analysis of cloud type and spatial variability reveals which measurements are more likely to be biased. The application of these techniques on a set of sample images confirms that the relationships between the liquid water path and other cloud parameters are most robust in overcast conditions.

Constraints on droplet growth in radiatively cooled stratocumulus clouds

Radiative cooling near the top of a layer cloud plays a dominant role in droplet condensation growth. The impact of this cooling on the evolution of small droplets and the formation of precipitation‐sized drops is calculated using a microphysical model that includes radiatively driven condensation and coalescence. The cloud top radiative environment used for these calculations is determined using a mixed‐layer model of a marine stratocumulus cloud with a subsiding, radiatively cooled inversion. Calculations of the radiatively driven equilibrium supersaturation show that net long wave emission by cloud droplets produces supersaturations below 0.04% for typical nocturnal conditions. While supersaturations as low as this will force evaporation for droplets smaller than ≈ 5 μm, radiatively enhanced growth for larger droplets can reduce the time required to produce precipitation‐sized particles by a factor of 2–4, compared with droplets in a quiescent cloud without flux divergence. The impact of this radiative enhancement on the acceleration of coalescence is equivalent to that produced in updrafts of 0.1–0.5 ms−1, and varies linearly with the total emitted flux (the “radiative exchange”).

On the derivation of tropospheric column ozone from radiances measured by the total ozone mapping spectrometer

We have developed a new algorithm to retrieve tropospheric column ozone on a daily basis directly from the measured total ozone mapping spectrometer (TOMS) albedos at a spatial resolution of about 50 km. This new algorithm is applied to the retrieval of tropospheric ozone over the region bounded by 20°W and 60°E longitude and 20°S and 0°S latitude during the 1989 biomass burning season when tropospheric ozone approaches 20% of total ozone. We find that for the conditions of high tropospheric ozone during biomass burning, the archived TOMS data underestimates total ozone over cloud free regions and overestimates ozone over the marine stratocumulus cloud off the west coast of Africa. This error can be as high as 15 Dobson units. Thus previous methods which derive tropospheric ozone from the TOMS archived ozone data are subject to error. Details of the algorithm and an analysis of the expected error are presented.

A Model for Particle Microphysics, Turbulent Mixing, and Radiative Transfer in the Stratocumulus-Topped Marine Boundary Layer and Comparisons with Measurements

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics module that resolves the size distributions of aerosols and cloud droplets, a turbulence module that treats vertical mixing between layers, and a multiple wavelength radiative transfer module that calculates radiative heating rates and cloud optical properties. The results of a 12-h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an ∼500-m thick, summertime marine stratocumulus cloud layer by Nicholls. However, in this case, the model predictions of turbulent fluxes between the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine stratus layer made under gale conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties are generally reproduced by the model, although it underpredicts the entrainment of overlying air at cloud top under gale conditions. Sensitivities of the model results are explored. The vertical profile of cloud droplet concentration is sensitive to the lower size cutoff of the droplet size distribution due to the presence of unactivated haze particles in the lower region of the modeled cloud. Increases in total droplet concentrations do not always produce less drizzle and more cloud water in the model. The radius of the mean droplet volume does not correlate consistently with drizzle, but the effective droplet radius does. The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input condensation nuclei and by omitting the effect of cloud-top radiative cooling on the condensational growth of cloud droplets. The omission of infrared scattering produces noticeable changes in cloud properties. The collection efficiencies for droplets <30-µm radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizontal wind also has a significant effect on a 12-h model simulation of cloud structure. Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplification is that the model assumes horizontal homogeneity, and, therefore, does not resolve updrafts and downdrafts. Likely consequences of this simplification include overprediction of the growth of droplets by condensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and underprediction of peak supersaturations.

An Observational Study of the Role of Penetrating Cumulus in a Marine Stratocumulus-Topped Boundary Layer

Abstract Isolated cumuli penetrating through marine stratocumulus clouds were documented during the Atlantic Stratocumulus Transition Experiment. This paper aims at understanding the role of the penetrating cumulus in regulating stratocumulus and boundary-layer structure through analysis of data from the NCAR Electra aircraft. When penetrating cumulus clouds are present, the boundary layer is generally decoupled from the near-surface air except in the cumulus region. Therefore, air in the cumulus region includes air entrained at the cloud top, as well as air modified by surface processes. In the stratocumulus region, however, entrained inversion air and moist surface air are confined to separate layers. As a result, large horizontal variations are found in scalars, such as ozone and water vapor. Turbulence statistics and conditional sampling of entrainment events in the cumulus and stratocumulus regions indicate that stronger entrainment may occur at the cumulus top compared to the surrounding stratocumul...