Distribution Of Liquid Water Path Research Articles

The Spiderweb Structure of Stratocumulus Clouds

Stratocumulus clouds have a distinctive structure composed of a combination of lumpy cellular structures and thin elongated regions, resembling canyons or slits. The elongated slits are referred to as “spiderweb” structure to emphasize their interconnected nature. Using very high resolution large-eddy simulations (LES), it is shown that the spiderweb structure is generated by cloud-top evaporative cooling. Analysis of liquid water path (LWP) and cloud liquid water content shows that cloud-top evaporative cooling generates relatively shallow slits near the cloud top. Most of liquid water mass is concentrated near the cloud top, thus cloud-top slits of clear air have a large impact on the entire-column LWP. When evaporative cooling is suppressed in the LES, LWP exhibits cellular lumpy structure without the elongated low-LWP regions. Even though the spiderweb signature on the LWP distribution is negligible, the cloud-top evaporative cooling process significantly affects integral boundary layer quantities, such as the vertically integrated turbulent kinetic energy, mean liquid water path, and entrainment rate. In a pair of simulations driven only by cloud-top radiative cooling, evaporative cooling nearly doubles the entrainment rate.

How finely do we need to represent the stratocumulus radiative effect?

Cloud radiative cooling at the top of stratocumulus is crucial to power turbulence in the well‐mixed, stratocumulus‐topped atmospheric boundary layer (ABL). The present work uses a large‐eddy simulation model to understand how finely the spatial structure of the cloud radiative effect (CRE) has to be represented in order to sustain a stratocumulus‐topped ABL in equilibrium. We first explore whether the vertical structure of the CRE can be simplified, and it appears that the cloud radiative cooling in the upper part of the cloud is essential to the existence of the equilibrium stratocumulus, and that the CRE has to be represented in some detail around the cloud top to sustain this equilibrium. Second, we investigate how the horizontal heterogeneities of CRE impact the stratocumulus equilibrium. Horizontal homogenization of the radiative cooling has a small thinning impact on the stratocumulus‐topped ABL. Because of the nonlinear dependence of the cloud radiative cooling on the liquid water path (LWP), computing the CRE from the mean liquid water profile overestimates the vertically integrated CRE and yields an unstable, ever‐growing ABL. Assuming a simple sub‐domain distribution of LWP and adjusting the cloud radiative cooling accordingly is sufficient to correct for this overestimation and sustain a stratocumulus equilibrium. Finally, we check that vertical and horizontal simplifications of the CRE are compatible, which opens perspectives for its representation in bulk models and larger‐scale models.

Characteristics of cloud liquid water path from SEVIRI onboard the Meteosat Second Generation 2 satellite for several cloud types

Abstract. In this study the temporal and spatial characteristics of the liquid water path (LWP) of low, middle and high level clouds are analysed using space-based observations from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument onboard the Meteosat Second Generation 2 (MSG 2) satellite. Both geophysical quantities are part of the CLAAS (CLoud property dAtAset using SEVIRI) data set and are generated by EUMETSAT's Satellite Application Facility on Climate Monitoring (CM SAF). In this article we focus on the statistical properties of LWP, retrieved during daylight conditions, associated with individual cloud types. We analysed the intrinsic variability of LWP, that is, the variability in only cloudy regions and the variations driven by cloud amount. The relative amplitude of the intrinsic diurnal cycle exceeded the cloud amount driven amplitude in our analysed cases. Our results reveal that each cloud type possesses a characteristic intrinsic LWP distribution. These frequency distributions are constant with time in the entire SEVIRI field of view, but vary for smaller regions like Central Europe. Generally the average LWP is higher over land than over sea; in the case of low clouds this amounts to 15–27% in 2009. The variance of the frequency distributions is enhanced as well. Also, the average diurnal cycle of LWP is related to cloud type with the most pronounced relative diurnal variations being detected for low and middle level clouds. Maps of the relative amplitude and the local time of maximum LWP show the variation throughout the SEVIRI field of view.

The Impact of Low Clouds on Surface Shortwave Radiation in the ECMWF Model

AbstractThe long-term measurement records from the Atmospheric Radiation Measurement site on the Southern Great Plains show evidence of a bias in the ECMWF model’s surface irradiance. Based on previous studies, which have suggested that summertime shallow clouds may contribute to the bias, an evaluation of 146 days with observed nonprecipitating fair-weather cumulus clouds is performed. In-cloud liquid water path and effective radius are both overestimated in the model with liquid water path dominating to produce clouds that are too reflective. These are compensated by occasional cloud-free days in the model such that the fair-weather cumulus regime overall does not contribute significantly to the multiyear daytime mean surface irradiance bias of 23 W m−2. To further explore the origin of the bias, observed and modeled cloud fraction profiles over 6 years are classified and sorted based on the surface irradiance bias associated with each sample pair. Overcast low cloud conditions during the spring and fall seasons are identified as a major contributor. For samples with low cloud present in both observations and model, opposing surface irradiance biases are found for overcast and broken cloud cover conditions. A reduction of cloud liquid to a third for broken low clouds and an increase by a factor of 1.5 in overcast situations improves agreement with the observed liquid water path distribution. This approach of combining the model shortwave bias with a cloud classification helps to identify compensating errors in the model, providing guidance for a targeted improvement of cloud parameterizations.

Estimation of Liquid Cloud Properties that Conserve Total-Scene Reflectance Using Satellite Measurements

Abstract A new method of deriving statistical moments related to the distribution of liquid water path over partially cloudy scenes is tested using a satellite cloud climatology. The method improves the ability to reconstruct total-scene visible reflectance when compared with an approach that relies on valid liquid water path retrievals, and thus it maintains physical consistency with the primary satellite observations when deriving cloud climatologies. A global application of the new method finds a mean bias of −0.008 ± 0.017 when reconstructing total-scene reflectance from liquid water path distributions, as compared with a bias of 0.05 ± 0.047 when using a conventional approach. Application of the method to a multidecadal cloud climatology suggests that this may provide a means of identifying data artifacts that could affect long-term cloud property trends. The conservation of reflectance plus the ease of applicability to various satellite datasets makes this method a valuable tool for model validation and comparison of satellite climatologies. Gaussian and gamma functions are used to approximate the distribution of horizontal subgrid-scale liquid water path for 1° × 1° scenes, and while both functions perform well for the majority of atmospheric conditions, it is found that the Gaussian distribution generates a negative bias for cases in which visible reflectance is very high and that neither function is able to represent liquid water path well in the few cases in which the observed distribution is bi- or multimodal.

The effect of temperature and humidity fluctuations on the liquid water path of non‐precipitating closed‐cell stratocumulus clouds

AbstractThe spatial variability of the liquid water path (LWP) is analyzed from a large‐eddy simulation of the diurnal cycle of stratocumulus clouds as observed during FIRE I. In stratocumulus clouds, the temperature and the total specific humidity cannot fluctuate independently, but are tightly connected to fluctuations in the liquid water potential temperature. If the latter are relatively small, a strong positive correlation between the temperature and the total specific humidity can be expected. The effect of temperature fluctuations on the magnitude of liquid water fluctuations must be considered to properly compute the LWP distribution in stratocumulus clouds.The simulated stratocumulus cloud fields are used to compute the albedo inhomogeneity factor χ according to the effective thickness approach. During the day, the mean LWP decreases due to short‐wave radiative warming. Also, the probability density function for the LWP becomes positively skewed due to cumuli that have their base well below the mean stratocumulus cloud‐base height. For this situation, the inhomogeneity factor χ displays a minimum value of about 0.85. For solid (unbroken) cloud with an assumed Gaussian distribution for the optical depth, we find that the minimum inhomogeneity factor will be about 0.8. This minimum value for the inhomogeneity factor, and that from the LES results, are larger than reported from FIRE I observations. Copyright © 2008 Royal Meteorological Society

An analysis of statistical characteristics of stratus and stratocumulus over eastern Pacific

Observational measurements of cloud base, top, thickness, fraction, and liquid water path (LWP) are analyzed to study the macro‐ and micro‐physical properties of stratus and stratocumulus over the eastern Pacific. It is found that the increase in liquid water content with height in cloud is significantly less than the adiabatic rate. The variations of cloud base and top both contribute to the variation of cloud thickness that approximately fits a normal distribution better than either cloud base or top. Because the distribution of LWP depends on the intervals of cloud fraction (or other environmental variables, for example, relative humidity), the determination of cloud fraction from the mean LWP alone would introduce large uncertainties particularly when LWP is large.

Recent improvements to LWP retrievals from AVHRR

In the framework of the Cloud Liquid Water Network (CLIWA-NET) project, an attempt is made to reconstruct the spatial distribution of liquid water path (LWP) by combining data from a network of ground-based microwave radiometers with imagery of the Advanced Very High Resolution Radiometer (AVHRR). In order to provide atmospheric modelers sufficient high-quality reference data for model evaluation and improvement, measurements from three intensive measurement campaigns are analyzed. This paper describes the AVHRR quantitative cloud analysis environment that was developed and tested at the Royal Netherlands Meteorological Institute (KNMI) during the project. The environment is named KLAROS (KNMI Local Implementation of APOLLO in an Operational System). KLAROS includes retrieval of a suite of physical cloud parameters. In order to evaluate the liquid water path retrieval, ground-based radiometer measurements are used. A new technique, called the Russian doll method, is introduced to assess the relation between ground and satellite measurements. This relation depends on the variability in time and space of the observed cloud fields and shows up clearly in the Russian doll diagrams. First, results from the comparisons give confidence that KLAROS gives plausible liquid water path values. We find that AVHHR data from the NOAA-16 platform is much better suited for quantitative cloud analysis than its predecessors. In particular, the availability of a 1.6-μm channel and the good calibration contribute to the improved accuracies. The retrieval method for liquid water path will be implemented in the framework of the Satellite Application Facility of Climate Monitoring (CM-SAF).

Accuracy of cloud liquid water path from ground‐based microwave radiometry 2. Sensor accuracy and synergy

The influence of microwave radiometer accuracy on retrieved cloud liquid water path (LWP) was investigated. Sensor accuracy was assumed to be the sum of the relative (i.e., Gaussian noise) and the absolute accuracies of brightness temperatures. When statistical algorithms are developed the assumed noise should be as close as possible to the real measurements in order to avoid artifacts in the retrieved LWP distribution. Typical offset errors of 1 K in brightness temperatures can produce mean LWP errors of more than 30 g m−2 for a two‐channel radiometer retrieval, although positively correlated brightness temperature offsets in both channels reduce this error to 16 g m−2. Large improvements in LWP retrieval accuracy of about 50% can be achieved by adding a 90‐GHz channel to the two‐channel retrieval. The inclusion of additional measurements, like cloud base height from a lidar ceilometer and cloud base temperature from an infrared radiometer, is invaluable in detecting cloud free scenes allowing an indirect evaluation of LWP accuracy in clear sky cases. This method was used to evaluate LWP retrieval algorithms based on different gas absorption models. Using two months of measurements, the Liebe 93 model provided the best results when the 90‐GHz channel was incorporated into the standard two‐channel retrievals.

Influence of anthropogenic aerosol on cloud optical depth and albedo shown by satellite measurements and chemical transport modeling.

The Twomey effect of enhanced cloud droplet concentration, optical depth, and albedo caused by anthropogenic aerosols is thought to contribute substantially to radiative forcing of climate change over the industrial period. However, present model-based estimates of this indirect forcing are highly uncertain. Satellite-based measurements would provide global or near-global coverage of this effect, but previous efforts to identify and quantify enhancement of cloud albedo caused by anthropogenic aerosols in satellite observations have been limited, largely because of strong dependence of albedo on cloud liquid water path (LWP), which is inherently highly variable. Here we examine satellite-derived cloud radiative properties over two 1-week episodes for which a chemical transport and transformation model indicates substantial influx of sulfate aerosol from industrial regions of Europe or North America to remote areas of the North Atlantic. Despite absence of discernible dependence of optical depth or albedo on modeled sulfate loading, examination of the dependence of these quantities on LWP readily permits detection and quantification of increases correlated with sulfate loading, which are otherwise masked by variability of LWP, demonstrating brightening of clouds because of the Twomey effect on a synoptic scale. Median cloud-top spherical albedo was enhanced over these episodes, relative to the unperturbed base case for the same LWP distribution, by 0.02 to 0.15.

Bounded cascade clouds: albedo and effective thickness

Abstract. If climate models produced clouds having liquid water amounts close to those observed, they would compute a mean albedo that is often much too large, due to the treatment of clouds as plane-parallel. An approximate lower-bound for this "plane-parallel albedo bias" may be obtained from a fractal model having a range of optical thicknesses similar to those observed in marine stratocumulus, since they are more nearly plane-parallel than most other cloud types. We review and extend results from a model which produces a distribution of liquid water path having a lognormal-like probability density and a power-law wavenumber spectrum, with parameters determined by stratocumulus observations. As the spectral exponent approaches -1, the simulated cloud approaches a well-known multifractal, referred to as the "singular model", but when the exponent is -5/3, similar to what is observed, the cloud exhibits qualitatively different scaling properties, the socalled "bounded model". The mean albedo for bounded cascade clouds is a function of a fractal parameter, 0 << 1, as well as the usual plane-parallel parameters such as single scattering albedo, asymmetry, solar zenith angle, and mean vertical optical thickness. A simple expression is derived to determine from the variance of the logarithm of the vertically-integrated liquid water. The albedo is shown to be approximated well by the plane-parallel albedo of a cloud having an "effective" vertical optical thickness, smaller than the mean thickness by a factor χ(f), which is given as an analytic function of f. California stratocumulus have a mean fractal parameter (f) ≈ 0.5, relative albedo bias of 15%, and an effective thickness 30% smaller than the mean thickness (χ ≈ 0.7). For typical observed values of mean liquid water and (f), the effective thickness approximation gives a plane-parallel albedo within 3% of the mean albedo.