Low Liquid Water Path Research Articles

Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign

AbstractThis study investigates the response of marine boundary layer (MBL) cloud properties to aerosol loading by accounting for the contributions of large-scale dynamic and thermodynamic conditions and quantifies the first indirect effect (FIE). It makes use of 19-month measurements of aerosols, clouds, and meteorology acquired during the Atmospheric Radiation Measurement Mobile Facility field campaign over the Azores. Cloud droplet number concentrations and cloud optical depth (COD) significantly increased with increasing aerosol number concentration . Cloud droplet effective radius (DER) significantly decreased with increasing . The correlations between cloud microphysical properties [, liquid water path (LWP), and DER] and were stronger under more stable conditions. The correlations between , LWP, DER, and were stronger under ascending-motion conditions, while the correlation between COD and was stronger under descending-motion conditions. The magnitude and corresponding uncertainty of the FIE ranged from 0.060 ± 0.022 to 0.101 ± 0.006 depending on the different LWP values. Under more stable conditions, cloud-base heights were generally lower than those under less stable conditions. This enabled a more effective interaction with aerosols, resulting in a larger value for the FIE. However, the dependence of the response of cloud properties to aerosol perturbations on stability varied according to whether ground- or satellite-based DER retrievals were used. The magnitude of the FIE had a larger variation with changing LWP under ascending-motion conditions and tended to be higher under ascending-motion conditions for clouds with low LWP and under descending-motion conditions for clouds with high LWP. A contrasting dependence of FIE on atmospheric stability estimated from the surface and satellite cloud properties retrievals reported in this study underscores the importance of assessing all-level properties of clouds in aerosol–cloud interaction studies.

Meteorological and Aerosol Effects on Marine Stratocumulus

Abstract This study investigates the effects of meteorological conditions and aerosols on marine stratocumulus in the southeastern Pacific using the Weather Research and Forecasting (WRF) Model. Two regimes with different temperature and moisture conditions in the finest model domain are investigated. The western regime is around 87°–79°W, while the eastern regime is around 79°–71°W. In both regimes, cloud fraction, liquid water path (LWP), cloud thickness, and precipitation show significant diurnal cycles. Cloud fraction can be 0.83 during the night and down to 0.29 during the day in the western regime. The diurnal cycles in the eastern regime have smaller amplitudes but are still very strong. Stratocumulus properties also differ in the two regimes. Compared to the western regime, the eastern regime has lower temperature, higher relative humidity, and a more coupled boundary layer, leading to higher cloud fraction (by 0.11) and lower cloud-base height. The eastern regime also has lower inversion height that causes lower cloud-top height and thinner clouds and, hence, lower LWP and less precipitation. Cloud microphysical properties are very sensitive to aerosols in both regimes. Increasing aerosols greatly increase cloud number concentration, decrease cloud effective radius, and suppress precipitation. Cloud macrophysical properties (cloud fraction, LWP) are not sensitive to aerosols in either regime, most notably in the eastern regime where precipitation amount is less. The changes in cloud fraction and LWP caused by changes in aerosol concentrations are smaller than the changes in the diurnal cycle and the spatial variability between the two regimes.

Effects of Convective Microphysics Parameterization on Large-Scale Cloud Hydrological Cycle and Radiative Budget in Tropical and Midlatitude Convective Regions

Abstract A two-moment microphysics scheme for deep convection was previously implemented in the NCAR Community Atmosphere Model version 5 (CAM5) by Song et al. The new scheme improved hydrometeor profiles in deep convective clouds and increased deep convective detrainment, reducing the negative biases in low and midlevel cloud fraction and liquid water path compared to observations. Here, the authors examine in more detail the impacts of this improved microphysical representation on regional-scale water and radiation budgets. As a primary source of cloud water for stratiform clouds is detrainment from deep and shallow convection, the enhanced detrainment leads to larger stratiform cloud fractions, higher cloud water content, and more stratiform precipitation over the ocean, particularly in the subtropics where convective frequency is also increased. This leads to increased net cloud radiative forcing. Over land regions, cloud amounts are reduced as a result of lower relative humidity, leading to weaker cloud forcing and increased OLR. Comparing the water budgets to cloud-resolving model simulations shows improvement in the partitioning between convective and stratiform precipitation, though the deep convection is still too active in the GCM. The addition of convective microphysics leads to an overall improvement in the regional cloud water budgets.

Satellite estimates of precipitation susceptibility in low‐level marine stratiform clouds

AbstractQuantifying the sensitivity of warm rain to aerosols is important for constraining climate model estimates of aerosol indirect effects. In this study, the precipitation sensitivity to cloud droplet number concentration (Nd) in satellite retrievals is quantified by applying the precipitation susceptibility metric to a combined CloudSat/Moderate Resolution Imaging Spectroradiometer data set of stratus and stratocumulus clouds that cover the tropical and subtropical Pacific Ocean and Gulf of Mexico. Consistent with previous observational studies of marine stratocumulus, precipitation susceptibility decreases with increasing liquid water path (LWP), and the susceptibility of the mean precipitation rate R is nearly equal to the sum of the susceptibilities of precipitation intensity and of probability of precipitation. Consistent with previous modeling studies, the satellite retrievals reveal that precipitation susceptibility varies not only with LWP but also with Nd. Puzzlingly, negative values of precipitation susceptibility are found at low LWP and high Nd. There is marked regional variation in precipitation susceptibility values that cannot simply be explained by regional variations in LWP and Nd. This suggests other controls on precipitation apart from LWP and Nd and that precipitation susceptibility will need to be quantified and understood at the regional scale when relating to its role in controlling possible aerosol‐induced cloud lifetime effects.

On transition-zone water clouds

Abstract. A recent field campaign was conducted to measure the properties of thin, warm convective clouds forming under conditions of weak updrafts. During the campaign, short-lived clouds (on the order of minutes) with droplets' effective radius of 1–2 μm and low liquid water path (~ 500 mg m–2) were measured. These low values are puzzling, since in most studies an effective radius of 4 μm is reported to serve as the lower bound for clouds. A theoretical cloud model designed to resolve the droplet-activation process suggested conditions that favor the formation of such clouds. Here we show that these clouds, which mark the transition from haze to cloud, are highly sensitive to the magnitude of the initial perturbation that initiated them. We define these clouds as "transition-zone clouds". The existence of such clouds poses a key challenge for the analysis of atmospheric observations and models, since they "further smooth" the transition from dry aerosol through haze pockets to cumulus clouds.

The formation of ice in a long‐lived supercooled layer cloud

AbstractThis article focuses on the characteristics of persistent thin single‐layer mixed‐phase clouds. We seek to answer two important questions: (i) how does ice continually nucleate and precipitate from these clouds, without the available ice nuclei becoming depleted? (ii) how do the supercooled liquid droplets persist in spite of the net flux of water vapour to the growing ice crystals? These questions are answered quantitatively using in situ and radar observations of a long‐lived mixed‐phase cloud layer over the Chilbolton Observatory.Doppler radar measurements show that the top 500 m of cloud (the top 250 m of which is mixed‐phase, with ice virga beneath) is turbulent and well‐mixed, and the liquid water content is adiabatic. This well‐mixed layer is bounded above and below by stable layers. This inhibits entrainment of fresh ice nuclei into the cloud layer, yet our in situ and radar observations show that a steady flux of ≈︁100 m−2s−1 ice crystals fell from the cloud over the course of ∼1 day. Comparing this flux to the concentration of conventional ice nuclei expected to be present within the well‐mixed layer, we find that these nuclei would be depleted within less than 1 h. We therefore argue that nucleation in these persistent supercooled clouds is strongly time‐dependent in nature, with droplets freezing slowly over many hours, significantly longer than the few seconds residence time of an ice nucleus counter.Once nucleated, the ice crystals are observed to grow primarily by vapour deposition, because of the low liquid water path (21 g m−2) yet vapour‐rich environment. Evidence for this comes from high differential reflectivity in the radar observations, and in situ imaging of the crystals. The flux of vapour from liquid to ice is quantified from in situ measurements, and we show that this modest flux (3.3 g m−2h−1) can be readily offset by slow radiative cooling of the layer to space.

Correction to “On the dependence of albedo on cloud microphysics over marine stratocumulus clouds regimes determined from Clouds and the Earth's Radiant Energy System (CERES) data”

[1] The dependence of the top-of-the-atmosphere (TOA) albedo A on cloud microphysical properties was investigated for the three largest maritime stratocumulus clouds regimes: off California, Southeast Pacific (Chile-Peru), and southwest Africa (Namibia-Angola). Absolute S and relative SR albedo susceptibilities to perturbations in cloud droplet number concentrations Nd, defined as dA/dNd and dA/dln(Nd) respectively, were calculated for the season having maximum cloud cover during the period 2006–2010. Satellite-based susceptibilities were computed by combining an adiabatically based Nd estimate and liquid water path (LWP) derived from Terra Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals matched with TOA A from the Clouds and the Earth's Radiant Energy System. Empirical susceptibility maps were calculated for three constant LWP intervals at 25, 50, and 90 g−2. It was found that S increases with LWP, with small and spatially homogeneous values for low LWP, and a contrasting increase far offshore for larger LWP values. An overall increase of SR with LWP was also observed, with larger values near the coast for LWP = 25 and 50 g−2. A relatively homogeneous spatial pattern of maximum SR values covered most of each regime's domain for a LWP of 90 g−2. These results highlight the importance of LWP in modulating the albedo susceptibility. The dependencies of S and SR on LWP are mostly explained by variations in the mean Nd and cloud optical thickness (τ), with an increase of S with LWP linked to a decrease in Nd, whereas SR increased with τ and A, until reaching a maximum for A and τ near 0.36–0.4 and 12–14 respectively, and decreasing thereafter, consistent with expectations based on two-stream estimates. Larger SR values in the Southeast Pacific are thought to be the consequence of a drier and more pristine atmosphere. Radiative transfer simulations with realistic values of above-cloud water vapor path and aerosol optical thickness showed that differing atmospheric compositions could explain why the Chile-Peru regime was the marine stratocumulus cloud deck most susceptible to change its TOA albedo due to fractional changes in Nd.

Radiative–Dynamical Feedbacks in Low Liquid Water Path Stratiform Clouds

Abstract When stratiform-cloud-integrated radiative flux divergence (heating) is dependent on liquid water path (LWP) and droplet concentration Nd, feedbacks between cloud dynamics and this heating can exist. These feedbacks can be particularly strong for low LWP stratiform clouds, in which cloud-integrated longwave cooling is sensitive to LWP and Nd. Large-eddy simulations reveal that these radiative–dynamical feedbacks can substantially modify low LWP stratiform cloud evolution when Nd is perturbed. At night, more rapid initial evaporation of the cloud layer occurs when Nd is high, leading to more cloud breaks and lower LWP values that both result in less total cloud longwave cooling. Weakened circulations result from this reduced longwave cooling and entrainment drying is able to counteract cloud growth. When Nd is low, the cloud layer is better maintained because cloud longwave cooling is still relatively strong. During the day, the addition of shortwave warming leads to reduced LWP for all values of Nd and, consequently, further reduced longwave cooling and weakened circulations. For high Nd, these reductions are such that the cloud layer cannot be maintained. For lower Nd, the reductions are smaller and the cloud layer thins but does not dissipate. These results suggest that low LWP cloud layers are more tenuous when Nd is high and are more prone to dissipating during the day. Comparison with other studies suggests the modeled low LWP cloud response may be sensitive to the initial thermodynamic profile and model configuration.

Manipulating marine stratocumulus cloud amount and albedo: a process-modelling study of aerosol-cloud-precipitation interactions in response to injection of cloud condensation nuclei

Abstract. We use a cloud-system-resolving model to study marine-cloud brightening. We examine how injected aerosol particles that act as cloud condensation nuclei (CCN) are transported within the marine boundary layer and how the additional particles in clouds impact cloud microphysical processes, and feedback on dynamics. Results show that the effectiveness of cloud brightening depends strongly on meteorological and background aerosol conditions. Cloud albedo enhancement is very effective in a weakly precipitating boundary layer and in CCN-limited conditions preceded by heavy and/or persistent precipitation. The additional CCN help sustain cloud water by weakening the precipitation substantially in the former case and preventing the boundary layer from collapse in the latter. For a given amount of injected CCN, the injection method (i.e., number and distribution of sprayers) is critical to the spatial distribution of these CCN. Both the areal coverage and the number concentration of injected particles are key players but neither one always emerges as more important than the other. The same amount of injected material is much less effective in either strongly precipitating clouds or polluted clouds, and it is ineffective in a relatively dry boundary layer that supports clouds of low liquid water path. In the polluted case and "dry" case, the CCN injection increases drop number concentration but lowers supersaturation and liquid water path. As a result, the cloud experiences very weak albedo enhancement, regardless of the injection method.

Comparison of CloudSat cloud liquid water paths in arctic summer using ground-based microwave radiometer

Arctic clouds strongly influence the regional radiation balance, temperature, melting of sea ice, and freezing of sea water. Despite their importance, there is a lack of systematic and reliable observations of Arctic clouds. The CloudSat satellite launched in 2006 with a 94 GHz Cloud Profiling Radar (CPR) may contribute to close this gap. Here we compare one of the key parameters, the cloud liquid water path (LWP) retrieved from CloudSat observations and from microwave radiometer (MWR) data taken during the ASCOS (Arctic Summer Cloud Ocean Study) cruise of the research vessel Oden from August to September 2008. Over the 45 days of the ASCOS cruise, collocations closer than 3 h and 100 km were found in only 9 d, and collocations closer than 1 h and 30 km in only 2 d. The poor correlations in the scatter plots of the two LWP retrievals can be explained by the patchiness of the cloud cover in these two days (August 5th and September 7th), as confirmed by coincident MODIS (Moderate-resolution Imaging Spectroradiometer) images. The averages of Oden-observed LWP values are systematically higher (40–70 g m−2) than the corresponding CloudSat observations (0–50 g m−2). These are cases of generally low LWP with presumably small droplets, and may be explained by the little sensitivity of the CPR to small droplets or by the surface clutter.

Sensitivity of tropical climate to low-level clouds in the NCEP climate forecast system

In this work, we examine the sensitivity of tropical mean climate and seasonal cycle to low clouds and cloud liquid water path (CLWP) by prescribing them in the NCEP climate forecast system (CFS). It is found that the change of low cloud cover alone has a minor influence on the amount of net shortwave radiation reaching the surface and on the warm biases in the southeastern Atlantic. In experiments where CLWP is prescribed using observations, the mean climate in the tropics is improved significantly, implying that shortwave radiation absorption by CLWP is mainly responsible for reducing the excessive surface net shortwave radiation over the southern oceans in the CFS. Corresponding to large CLWP values in the southeastern oceans, the model generates large low cloud amounts. That results in a reduction of net shortwave radiation at the ocean surface and the warm biases in the sea surface temperature in the southeastern oceans. Meanwhile, the cold tongue and associated surface wind stress in the eastern oceans become stronger and more realistic. As a consequence of the overall improvement of the tropical mean climate, the seasonal cycle in the tropical Atlantic is also improved. Based on the results from these sensitivity experiments, we propose a model bias correction approach, in which CLWP is prescribed only in the southeastern Atlantic by using observed annual mean climatology of CLWP. It is shown that the warm biases in the southeastern Atlantic are largely eliminated, and the seasonal cycle in the tropical Atlantic Ocean is significantly improved. Prescribing CLWP in the CFS is then an effective interim technique to reduce model biases and to improve the simulation of seasonal cycle in the tropics.

Understanding the Importance of Microphysics and Macrophysics for Warm Rain in Marine Low Clouds. Part II: Heuristic Models of Rain Formation

Abstract Two simple heuristic model formulations for warm rain formation are introduced and their behavior explored. The first, which is primarily aimed at representing warm rain formation in shallow convective clouds, is a continuous collection model that uses an assumed cloud droplet size distribution consistent with observations as the source of embryonic drizzle drops that are then allowed to fall through a fixed cloud, accreting cloud droplets. The second, which is applicable to steady-state precipitation formation in stratocumulus, is a simple two-moment bulk autoconversion and accretion model in which cloud liquid water is removed by drizzle formation and replenished on a externally specified time scale that reflects the efficacy of turbulent overturning that characterizes stratocumulus. The models’ behavior is shown to be broadly consistent with observations from the A-Train constellation of satellites, allowing the authors to explore reasons for changing model sensitivity to microphysical and macrophysical cloud properties. The models are consistent with one another, and with the observations, in that they demonstrate that the sensitivity of rain rate to cloud droplet concentration Nd (which here represents microphysical influence) is greatest for weakly precipitating clouds (i.e., for low cloud liquid water path and/or high Nd). For the steady-state model, microphysical sensitivity is shown to strongly decrease with the ratio of replenishment to drizzle time scales. Thus, rain from strongly drizzling and/or weakly replenished clouds shows low sensitivity to microphysics. This is essentially because most precipitation in these clouds is forming via accretion rather than autoconversion. For the continuous-collection model, as cloud liquid water content increases, the precipitation rate becomes more strongly controlled by the availability of cloud liquid water than by the initial embryo size or by the cloud droplet size. The models help to explain why warm rain in marine stratocumulus clouds is sensitive to Nd but why precipitation from thicker cumulus clouds appears to be less so.

On the precipitation susceptibility of clouds to aerosol perturbations

Atmospheric aerosol particles act as cloud condensation nuclei, affording them the ability to influence cloud microphysics, planetary albedo, and precipitation. Models of varying complexity and satellite observations from NASA's A‐Train constellation of satellites are used to determine what controls the precipitation susceptibility of warm clouds to aerosol perturbations. Three susceptibility regimes are identified: (i) clouds with low liquid water path (LWP) generate very little rain and are least susceptible to aerosol; (ii) clouds with intermediate LWP where aerosol most effectively suppress precipitation; and (iii) clouds with high LWP, where the susceptibility begins to decrease because the precipitation process is efficient owing to abundant liquid water. Remarkable qualitative agreement between remote sensing observations and model predictions provides the first suggestions that certain regions of the Earth might be more vulnerable to pollution aerosol. Targeted pollution control strategies in such regions would most benefit water availability via precipitation.

Aerosol effects on the inter‐cloud region of a small cumulus cloud field

In a recent paper, we used observations to show that the inter‐cloud “cloud‐free regime” (or “twilight zone”) is not free of clouds and comprises small fragments of clouds, hydrated aerosol, as well as incipient, hesitant, and decaying clouds. In this paper we characterize the properties of, and investigate the aerosol affect on, the twilight zone using high resolution large eddy simulations of clean and polluted shallow cumulus. We analyze the occurrence of these clouds with weak signature and show that cloud fields forming in polluted environments are characterized by a more rapid decrease in liquid water path with increasing distance from cloud than their cleaner counterparts, and that the inter‐cloud zone contains significantly fewer low liquid water path pockets. This reduction in liquid water content in the area considered cloud‐free in the polluted cases is countered by more aerosol uptake of water vapor.

A Refined Two-Channel Microwave Radiometer Liquid Water Path Retrieval for Cold Regions by Using Multiple-Sensor Measurements

Traditional two-channel microwave radiometers (MWRs) are widely used to measure cloud liquid water path (LWP); however, the retrieved LWPs are subject to relatively large uncertainties, particularly for low LWP clouds. By reformulating the statistical retrieval method with clear-sky measurements as a reference, a simple method is presented to significantly reduce uncertainties in the LWP retrieval due to errors in MWR calibration, uncertainties in the absorption coefficients of atmospheric gases, and variations in the vertical profiles of temperature and pressure. The improvement is illustrated by comparing the statistics of the erroneous clear-sky LWP for the Department of Energy's Atmospheric Radiation Measurement Program Climate Research Facility observations at the North Slope of Alaska site and by comparing LWP retrieved with a multiple-sensor algorithm and LWP retrieved based mainly on MWR measurements. This letter also demonstrates the importance of using correct water cloud temperature and temperature-dependent water absorption coefficients for MWR LWP retrieval over cold regions. This approach can be easily implemented for combined MWR, ceilometer, and surface meteorological measurements.

Accurate Liquid Water Path Retrieval from Low-Cost Microwave Radiometers Using Additional Information from a Lidar Ceilometer and Operational Forecast Models

Abstract Water clouds have an important impact on the radiative balance of the earth. The use of ground-based dual-frequency microwave radiometers to derive both liquid water path (LWP) and water vapor path (WVP) is well established, but uncertainties over the dry, water vapor, and liquid water absorption coefficients and the radiometric calibration can lead to errors in the retrieved LWP. A method in which additional information from a lidar ceilometer is used to identify the presence of liquid water clouds and their altitude is described. When such clouds are absent, the radiometric calibrations of the two frequencies are optimally adjusted so that the retrieved LWP is forced to zero; when they are present the calibrations are interpolated from the nearest clear-sky periods before and after, and the temperature of the cloud is used to refine the liquid water absorption coefficient (with the temperature profile taken from a forecast model). This procedure is insensitive to the choice of absorption model, removes the troublesome negative values of LWP that can be retrieved, and provides more accurate values of low LWP in thin clouds. Analysis shows that LWP as low as 10 g m−2 can be reliably retrieved, 90% of the time the error being less than 50%, and for LWP greater than 20 g m−2 the error is less than 10%. An additional advantage is that the retrieval can tolerate uncertainties in the various absorption coefficients and is unaffected by slow drifts in brightness temperature errors of up to 5 K. Previous techniques have required that these temperatures be accurate to 0.5 K or better, which entails careful calibration and can be quite difficult to achieve.

The Role of Eddy Diffusivity Profiles on Stratocumulus Liquid Water Path Biases

Abstract Results from simulations of the stratocumulus-topped boundary layer with one-dimensional versions of general simulation models typically exhibit a wide range of spread in the modeled liquid water path (LWP). These discrepancies are often attributed to differences in the modeled entrainment rate. Results from a large eddy simulation of the First International Satellite Cloud Climatology Project Regional Experiment I stratocumulus case are analyzed. The diagnosed eddy diffusivities for heat and moisture are found to differ by about a factor of 3. Moreover, both have a much larger magnitude than the ones typically applied in boundary layer parameterization schemes. Motivated by these results mean state solutions are analyzed for the specific case in which the vertical fluxes of heat and moisture are prescribed, whereas eddy diffusivity profiles are systematically varied by multiplication with a constant factor. The solutions demonstrate that any value, ranging from zero to a maximum adiabatic value, can be obtained for the LWP. In the subtropical parts over the ocean where horizontally extended stratocumulus fields persist, the surface sensible heat flux is typically small, whereas surface evaporation and entrainment of relatively dry air from above the surface can result in significant moisture fluxes. If the eddy diffusivity values are small, then the mean specific humidity will tend to decrease quite rapidly with height in order to support the humidity flux. This results in erroneous low humidity values in the upper part of the boundary layers causing low LWP values.

Observed Relationships between Arctic Longwave Cloud Forcing and Cloud Parameters Using a Neural Network

Abstract A neural network technique is used to quantify relationships involved in cloud–radiation feedbacks based on observations from the Surface Heat Budget of the Arctic (SHEBA) project. Sensitivities of longwave cloud forcing (CFL) to cloud parameters indicate that a bimodal distribution pattern dominates the histogram of each sensitivity. Although the mean states of the relationships agree well with those derived in a previous study, they do not often exist in reality. The sensitivity of CFL to cloud cover increases as the cloudiness increases with a range of 0.1–0.9 W m−2 %−1. There is a saturation effect of liquid water path (LWP) on CFL. The highest sensitivity of CFL to LWP corresponds to clouds with low LWP, and sensitivity decreases as LWP increases. The sensitivity of CFL to cloud-base height (CBH) depends on whether the clouds are below or above an inversion layer. The relationship is negative for clouds higher than 0.8 km at the SHEBA site. The strongest positive relationship corresponds to clouds with low CBH. The dominant mode of the sensitivity of CFL to cloud-base temperature (CBT) is near zero and corresponds to warm clouds with base temperatures higher than −9°C. The low and high sensitivity regimes correspond to the summer and winter seasons, respectively, especially for LWP and CBT. Overall, the neural network technique is able to separate two distinct regimes of clouds that correspond to different sensitivities; that is, it captures the nonlinear behavior in the relationships. This study demonstrates a new method for evaluating nonlinear relationships between climate variables. It could also be used as an effective tool for evaluating feedback processes in climate models.

Model predicted low-level cloud parameters: Part I: Comparison with observations from the BALTEX Bridge Campaigns

The BALTEX Bridge Campaigns (BBC), which were held in the Netherlands in 2001 and 2003 around the Cabauw Experimental Site for Atmospheric Research (CESAR), have provided detailed information on clouds. This paper is an illustration of how these measurements can be used to investigate whether ‘state-of-the-art’ atmospheric models are capable of adequately representing clouds. Here, we focus on shallow low-level clouds with a substantial amount of liquid water. In situ, ground-based and satellite remote sensing measurements were compared with the output of three non-hydrostatic regional models (Lokal-Modell, LM; Méso-NH; fifth-generation Mesoscale Model, MM5) and two hydrostatic regional climate models (Regional Atmospheric Climate Model version 2, RACMO2; Rossby Centre Atmospheric Model, RCA). For the two selected days, Méso-NH and MM5 reproduce the measured vertical extent of the shallow clouds, but the liquid water content of the clouds is generally overestimated. In LM and the climate models the inversion is too weak and located at a level too close to the surface resulting in an overestimation of the vertical extent of the clouds. A sensitivity integration with RACMO2 shows that the correspondence between model output and measurements can be improved by a doubling of the vertical resolution; this induces an increase in the modelled inversion strength and cloud top pressure. LM and Méso-NH underestimate the lifetime of clouds. A comparison between model output and cloud cover derived from the Moderate Resolution Imaging Spectrometer (MODIS) indicates that this deficiency is not due to advection of too small cloud systems; it is rather due to an overestimation of the variability in the vertical velocity. All models overestimate the specific humidity near the surface and underestimate it at higher atmospheric levels, indicating that the models underestimate the mixing of moisture in the boundary layer. This deficiency is slightly reduced by inclusion of parameterised shallow convection in the non-hydrostatic models, which enhances the mixing of heat and moisture in the boundary layer. Consequently, the explicitly resolved updrafts weaken resulting in reduced condensation rates and lower liquid water path. The temporal variability of cloud occurrence is hardly affected by inclusion of parameterised shallow convection.

Characteristics of mixed‐phase clouds. I: Lidar, radar and aircraft observations from CLARE'98

AbstractResults are presented from two case‐studies during the 1998 Cloud Lidar And Radar Experiment (CLARE'98) in which mixed‐phase clouds were observed by a multitude of ground‐based and airborne instruments. In both cases supercooled liquid water was present in the form of highly reflective layers in lidar imagery, while the radar echo was dominated by the contribution from the much larger ice particles. In the first case‐study, four individual liquid‐water layers were observed by an airborne nadir‐pointing polarimetric lidar at temperatures between −7°C and −15°C, embedded within a warm‐frontal ice cloud. Their phase was confirmed by the in situ measurements and by their very low depolarization of the lidar signal. The effective droplet radius ranged from 2 to 5 µm. Simultaneous temperature and vertical‐wind measurements by the aircraft demonstrated that they were generated by a gravity wave with a wavelength of around 15 km. Thin sector plates grew rapidly in the high‐supersaturation conditions and were responsible for the high values of differential reflectivity measured by the ground‐based radar in the vicinity of the layers. In the second case‐study a liquid‐water altocumulus layer was observed at −23°C, which was slowly glaciating. Profiles of liquid and ice extinction coefficient, water content and effective radius were derived from the remote measurements taken in both cases, using radar–lidar and dual‐wavelength radar techniques to size the ice particles; where in situ validation was available, agreement was good. Radiative‐transfer calculations were then performed on these profiles to ascertain the radiative effect of the supercooled water. It was found that, despite their low liquid‐water path (generally less than 10 – 20 g m−2), these clouds caused a significant increase in the reflection of solar radiation to space, even when cirrus was present, above which the long‐wave signal dominated. In the cases considered, their capacity to decrease the net absorbed radiation was at least twice as large as that of the ice. The layers were typically 100–200 m thick, suggesting that they are unlikely to be adequately represented by the resolutions of current forecast and climate models. These results suggest that a spaceborne lidar and radar would be ideally suited to characterizing the occurrence and climatological importance of mixed‐phase clouds on a global scale. © Royal Meteorological Society, 2003. P. N. Francis's contribution is Crown copyright.