Aerosol-landscape-cloud interaction: Signatures of topography effect on cloud droplet formation

Aerosol–landscape–cloud interaction: signatures of topography effect on cloud droplet formation

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-7955-2017 ◽

2017 ◽

Vol 17 (12) ◽

pp. 7955-7964 ◽

Cited By ~ 1

Author(s):

Sami Romakkaniemi ◽

Zubair Maalick ◽

Antti Hellsten ◽

Antti Ruuskanen ◽

Olli Väisänen ◽

...

Keyword(s):

Liquid Water ◽

Cloud Condensation Nuclei ◽

Measurement Data ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Liquid Water Content ◽

Droplet Formation ◽

Cloud Base ◽

Topography Effect ◽

The Hill

Abstract. Long-term in situ measurements of aerosol–cloud interactions are usually performed in measurement stations residing on hills, mountains, or high towers. In such conditions, the surface topography of the surrounding area can affect the measured cloud droplet distributions by increasing turbulence or causing orographic flows and thus the observations might not be representative for a larger scale. The objective of this work is to analyse, how the local topography affects the observations at Puijo measurement station, which is located in the 75 m high Puijo tower, which itself stands on a 150 m high hill. The analysis of the measurement data shows that the observed cloud droplet number concentration mainly depends on the cloud condensation nuclei (CCN) concentration. However, when the wind direction aligns with the direction of the steepest slope of the hill, a clear topography effect is observed. This finding was further analysed by simulating 3-D flow fields around the station and by performing trajectory ensemble modelling of aerosol- and wind-dependent cloud droplet formation. The results showed that in typical conditions, with geostrophic winds of about 10 m s−1, the hill can cause updrafts of up to 1 m s−1 in the air parcels arriving at the station. This is enough to produce in-cloud supersaturations (SSs) higher than typically found at the cloud base of ∼  0.2 %), and thus additional cloud droplets may form inside the cloud. In the observations, this is seen in the form of a bimodal cloud droplet size distribution. The effect is strongest with high winds across the steepest slope of the hill and with low liquid water contents, and its relative importance quickly decreases as these conditions are relaxed. We therefore conclude that, after careful screening for wind speed and liquid water content, the observations at Puijo measurement station can be considered representative for clouds in a boreal environment.

A depolarisation lidar based method for the determination of liquid-cloud microphysical properties

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-9917-2014 ◽

2014 ◽

Vol 7 (9) ◽

pp. 9917-9992 ◽

Cited By ~ 1

Author(s):

D. P. Donovan ◽

H. Klein Baltink ◽

J. S. Henzing ◽

S. R. de Roode ◽

A. P. Siebesma

Keyword(s):

Water Content ◽

Multiple Scattering ◽

Liquid Water ◽

Cloud Droplet ◽

Optimal Estimation ◽

Liquid Water Content ◽

Cloud Base ◽

Base Region ◽

Large Eddy Simulation Model ◽

Inversion Procedure

Abstract. The fact that polarisation lidars measure a depolarisation signal in liquid clouds due to the occurrence of multiple-scattering is well-known. The degree of measured depolarisation depends on the lidar characteristics (e.g. wavelength and receiver field-of-view) as well as the cloud macrophysical (e.g. liquid water content) and microphysical (e.g. effective radius) properties. Efforts seeking to use depolarisation information in a quantitative manner to retrieve cloud properties have been undertaken with, arguably, limited practical success. In this work we present a retrieval procedure applicable to clouds with (quasi-)linear liquid water content (LWC) profiles and (quasi-)constant cloud droplet number density in the cloud base region. Thus limiting the applicability of the procedure allows us to reduce the cloud variables to two parameters (namely the derivative of the liquid water content with height and the extinction at a fixed distance above cloud-base). This simplification, in turn, allows us to employ a fast and robust optimal-estimation inversion using pre-computed look-up-tables produced using extensive lidar Monte-Carlo multiple-scattering simulations. In this paper, we describe the theory behind the inversion procedure and successfully apply it to simulated observations based on large-eddy simulation model output. The inversion procedure is then applied to actual depolarisation lidar data corresponding to a range of cases taken from the Cabauw measurement site in the central Netherlands. The lidar results were then used to predict the corresponding cloud-base region radar reflectivities. In non-drizzling condition, it was found that the lidar inversion results can be used to predict the observed radar reflectivities with an accuracy within the radar calibration uncertainty (2–3 dBZ). This result strongly supports the accuracy of the lidar inversion results. Results of a comparison between ground-based aerosol number concentration and lidar-derived cloud droplet number densities are also presented and discussed. The observed relationship between the two quantities is seen to be consistent with the results of previous studies based on aircraft-based in situ measurements.

Stratocumulus Liquid Water Content from Dual-Wavelength Radar

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech1768.1 ◽

2005 ◽

Vol 22 (8) ◽

pp. 1207-1218 ◽

Cited By ~ 61

Author(s):

Robin J. Hogan ◽

Nicolas Gaussiat ◽

Anthony J. Illingworth

Keyword(s):

Water Content ◽

Liquid Water ◽

Wave Attenuation ◽

Droplet Size Distribution ◽

Liquid Water Content ◽

Radar Signal ◽

Cloud Base ◽

Liquid Water Path ◽

Atmospheric Radiation Measurement ◽

The Difference

Abstract A technique is described to retrieve stratocumulus liquid water content (LWC) using the differential attenuation measured by vertically pointing radars at 35 and 94 GHz. Millimeter-wave attenuation is proportional to LWC and increases with frequency, so LWC can be derived without the need to make any assumptions on the nature of the droplet size distribution. There is also no need for the radars to be well calibrated. A significant advantage over many radar techniques in stratocumulus is that the presence of drizzle drops (those with a diameter larger than around 50 μm) does not affect the retrieval, even though such drops may dominate the radar signal. It is important, however, that there are not significant numbers of drops larger than 600 μm, which scatter outside of the Rayleigh regime at 94 GHz. A lidar ceilometer is used to locate the cloud base in the presence of drizzle falling below the cloud. An accuracy of around 0.04 g m−3 is achievable with averaging over 1 min and 150 m (two range gates), but for the previously suggested frequency pair of 10 and 35 GHz, the corresponding accuracy would be considerably worse at 0.34 g m−3. First, the retrieval of LWC is simulated using aircraft-measured size spectra taken from a profile through marine stratocumulus. Results are then presented from two case studies—one using two cloud radars at Chilbolton in southern United Kingdom, and another using the Cloud Profiling Radar System at the Atmospheric Radiation Measurement site in Oklahoma. The liquid water path from the technique was found to be in good agreement with the values that were obtained from microwave radiometers, with the difference between the two being close to the accuracy of the radiometer retrieval. In the case of well-mixed stratocumulus, the profiles were close to adiabatic.

Understanding aerosol–cloud interactions through modeling the development of orographic cumulus congestus during IPHEx

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-1413-2019 ◽

2019 ◽

Vol 19 (3) ◽

pp. 1413-1437 ◽

Cited By ~ 2

Author(s):

Yajuan Duan ◽

Markus D. Petters ◽

Ana P. Barros

Keyword(s):

Water Content ◽

Liquid Water ◽

Cloud Droplet ◽

Liquid Water Content ◽

Cloud Microphysics ◽

Turbulent Dispersion ◽

Cloud Base ◽

Droplet Growth ◽

Aerosol Cloud ◽

Aerosol Cloud Interactions

Abstract. A new cloud parcel model (CPM) including activation, condensation, collision–coalescence, and lateral entrainment processes is used to investigate aerosol–cloud interactions (ACIs) in cumulus development prior to rainfall onset. The CPM was applied with surface aerosol measurements to predict the vertical structure of cloud development at early stages, and the model results were evaluated against airborne observations of cloud microphysics and thermodynamic conditions collected during the Integrated Precipitation and Hydrology Experiment (IPHEx) in the inner region of the southern Appalachian Mountains (SAM). Sensitivity analysis was conducted to examine the model response to variations in key ACI physiochemical parameters and initial conditions. The CPM sensitivities mirror those found in parcel models without entrainment and collision–coalescence, except for the evolution of the droplet spectrum and liquid water content with height. Simulated cloud droplet number concentrations (CDNCs) exhibit high sensitivity to variations in the initial aerosol concentration at cloud base, but weak sensitivity to bulk aerosol hygroscopicity. The condensation coefficient ac plays a governing role in determining the evolution of CDNC, liquid water content (LWC), and cloud droplet spectra (CDS) in time and with height. Lower values of ac lead to higher CDNCs and broader CDS above cloud base, and higher maximum supersaturation near cloud base. Analysis of model simulations reveals that competitive interference among turbulent dispersion, activation, and droplet growth processes modulates spectral width and explains the emergence of bimodal CDS and CDNC heterogeneity in aircraft measurements from different cloud regions and at different heights. Parameterization of nonlinear interactions among entrainment, condensational growth, and collision–coalescence processes is therefore necessary to simulate the vertical structures of CDNCs and CDSs in convective clouds. Comparisons of model predictions with data suggest that the representation of lateral entrainment remains challenging due to the spatial heterogeneity of the convective boundary layer and the intricate 3-D circulations in mountainous regions.

The dual-field-of-view polarization lidar technique: a new concept in monitoring aerosol effects in liquid-water clouds – theoretical framework

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-15247-2020 ◽

2020 ◽

Vol 20 (23) ◽

pp. 15247-15263 ◽

Cited By ~ 1

Author(s):

Cristofer Jimenez ◽

Albert Ansmann ◽

Ronny Engelmann ◽

David Donovan ◽

Aleksey Malinka ◽

...

Keyword(s):

Extinction Coefficient ◽

Liquid Water ◽

Cloud Droplet ◽

Condensation Nucleus ◽

Liquid Water Content ◽

Theoretical Background ◽

Effective Radius ◽

Cloud Condensation Nucleus ◽

Cloud Base ◽

Analysis Scheme

Abstract. In a series of two articles, a novel, robust, and practicable lidar approach is presented that allows us to derive microphysical properties of liquid-water clouds (cloud extinction coefficient, droplet effective radius, liquid-water content, cloud droplet number concentration) at a height of 50–100 m above the cloud base. The temporal resolution of the observations is on the order of 30–120 s. Together with the aerosol information (aerosol extinction coefficients, cloud condensation nucleus concentration) below the cloud layer, obtained with the same lidar, in-depth aerosol–cloud interaction studies can be performed. The theoretical background and the methodology of the new cloud lidar technique is outlined in this article (Part 1), and measurement applications are presented in a companion publication (Part 2) (Jimenez et al., 2020a). The novel cloud retrieval technique is based on lidar observations of the volume linear depolarization ratio at two different receiver fields of view (FOVs). Extensive simulations of lidar returns in the multiple scattering regime were conducted to investigate the capabilities of a dual-FOV polarization lidar to measure cloud properties and to quantify the information content in the measured depolarization features regarding the basic retrieval parameters (cloud extinction coefficient, droplet effective radius). Key simulation results and the overall data analysis scheme developed to obtain the aerosol and cloud products are presented.

Turbulent transport and deposition of fog droplets in the Central Namib

10.5194/egusphere-egu2020-16201 ◽

2020 ◽

Author(s):

Robert Spirig ◽

Christian Feigenwinter ◽

Roland Vogt

Keyword(s):

Water Content ◽

Sea Level ◽

Liquid Water ◽

Turbulent Transport ◽

Water Flux ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Field Measurements ◽

Liquid Water Content ◽

Fog Water

<p>Regular, nocturnal fog is a defining and seasonally varying feature in the Namib desert. Historical observations were limited to the binary measure of fog occurrence and the concurrent fog water input is quantified only since 2014 via the FogNet using Juvik fog collectors. This installation opened new avenues of research such as the efficiency of the transport mechanism, sampling and spatial variation thereof. An eddy covariance setup of a cloud droplet probe and collocated sonic(s) was installed in turns at the two FogNet stations Vogelfederberg (23.10&#176;S, 15.03&#176;E, 515 m above sea level) and Gobabeb (23.56&#176;S, 15.04&#176;E, 406 m above sea level) for 2 years in the frame of the Namib Fog Life Cycle Analysis Field Measurements (NaFoLiCA-F) project. With this setup, we gathered duration, droplet size distribution, droplet concentration, liquid water content, turbulent liquid water flux and the fog water input via the Juvik fog collector with a total of over 150 fog events. We found that fog appears suddenly and front-like as seen by an increase of droplet numbers by several magnitudes and dissolves more gradually towards the morning. All droplet classes of the resolved range of 2 to 50 &#181;m are present, but at the Vogelfederberg with around 2 to 3 times larger fog water input, the mean and median of the distribution are lower due to comparably fewer large droplets. Liquid water fluxes at both sites resulted in a net gain for the surface but the spatial discrepancy between fog water input recorded by fog collectors and the liquid water content indicates that drizzle, i.e. droplets outside the resolved range, may contribute to the larger total water deposition at Vogelfederberg.</p>

A depolarisation lidar-based method for the determination of liquid-cloud microphysical properties

Atmospheric Measurement Techniques ◽

10.5194/amt-8-237-2015 ◽

2015 ◽

Vol 8 (1) ◽

pp. 237-266 ◽

Cited By ~ 23

Author(s):

D. P. Donovan ◽

H. Klein Baltink ◽

J. S. Henzing ◽

S. R. de Roode ◽

A. P. Siebesma

Keyword(s):

Water Content ◽

Multiple Scattering ◽

Liquid Water ◽

Cloud Droplet ◽

Optimal Estimation ◽

Liquid Water Content ◽

Cloud Base ◽

Base Region ◽

Microphysical Properties ◽

Inversion Procedure

Abstract. The fact that polarisation lidars measure a depolarisation signal in liquid clouds due to the occurrence of multiple scattering is well known. The degree of measured depolarisation depends on the lidar characteristics (e.g. wavelength and receiver field of view) as well as the cloud macrophysical (e.g. cloud-base altitude) and microphysical (e.g. effective radius, liquid water content) properties. Efforts seeking to use depolarisation information in a quantitative manner to retrieve cloud properties have been undertaken with, arguably, limited practical success. In this work we present a retrieval procedure applicable to clouds with (quasi-)linear liquid water content (LWC) profiles and (quasi-)constant cloud-droplet number density in the cloud-base region. Thus limiting the applicability of the procedure allows us to reduce the cloud variables to two parameters (namely the derivative of the liquid water content with height and the extinction at a fixed distance above cloud base). This simplification, in turn, allows us to employ a fast and robust optimal-estimation inversion using pre-computed look-up tables produced using extensive lidar Monte Carlo (MC) multiple-scattering simulations. In this paper, we describe the theory behind the inversion procedure and successfully apply it to simulated observations based on large-eddy simulation (LES) model output. The inversion procedure is then applied to actual depolarisation lidar data corresponding to a range of cases taken from the Cabauw measurement site in the central Netherlands. The lidar results were then used to predict the corresponding cloud-base region radar reflectivities. In non-drizzling condition, it was found that the lidar inversion results can be used to predict the observed radar reflectivities with an accuracy within the radar calibration uncertainty (2–3 dBZ). This result strongly supports the accuracy of the lidar inversion results. Results of a comparison between ground-based aerosol number concentration and lidar-derived cloud-droplet number densities are also presented and discussed. The observed relationship between the two quantities is seen to be consistent with the results of previous studies based on aircraft-based in situ measurements.

Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-4835-2010 ◽

2010 ◽

Vol 10 (10) ◽

pp. 4835-4848 ◽

Cited By ~ 30

Author(s):

O. Geoffroy ◽

J.-L. Brenguier ◽

F. Burnet

Keyword(s):

Water Content ◽

Liquid Water ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Liquid Water Content ◽

Tuning Parameter ◽

Parametric Function ◽

Generalized Gamma ◽

Parametric Functions ◽

Generalized Gamma Function

Abstract. Parametric functions are currently used to represent droplet spectra in clouds and to develop bulk parameterizations of the microphysical processes and of their interactions with radiation. The most frequently used parametric functions are the Lognormal and the Generalized Gamma which have three and four independent parameters, respectively. In a bulk parameterization, two parameters are constrained by the total droplet number concentration and the liquid water content. In the Generalized Gamma function, one parameter is specified a priori, and the fourth one, like the third parameter of the Lognormal function, shall be tuned, for the parametric function to statistically best fit observed droplet spectra. These parametric functions are evaluated here using droplet spectra collected in non-or slightly precipitating stratocumulus and shallow cumulus. Optimum values of the tuning parameters are derived by minimizing either the absolute or the relative error for successively the first, second, fifth, and sixth moments of the droplet size distribution. A trade-off value is also proposed that minimizes both absolute and relative errors for the four moments concomitantly. Finally, a parameterization is proposed in which the tuning parameter depends on the liquid water content. This approach significantly improves the fit for the smallest and largest values of the moments.

The Potential of a Spaceborne Cloud Radar for the Detection of Stratocumulus Clouds

Journal of Applied Meteorology ◽

10.1175/1520-0450-36.6.676 ◽

1997 ◽

Vol 36 (6) ◽

pp. 676-687 ◽

Cited By ~ 24

Author(s):

Neil I. Fox ◽

Anthony J. Illingworth

Keyword(s):

Water Content ◽

Liquid Water ◽

Cloud Droplet ◽

Liquid Water Content ◽

Effective Radius ◽

Cloud Base ◽

Sensitivity Threshold ◽

Marine Stratocumulus ◽

Cloud Radar ◽

Stratocumulus Clouds

Abstract The radar reflectivity and liquid water content of stratocumulus clouds have been computed from cloud droplet spectra recorded during more than 4000 km of cloud penetrations by an aircraft, and the probability of detecting various values of liquid water content as a function of the radar sensitivity threshold has been derived. The goal of the study is to specify the sensitivity required for any future spaceborne cloud radar. In extensive marine stratocumulus deeper than about 200 m, occasional but ubiquitous drizzle-sized droplets of up to 200 μm dominate the radar return and increase it by between 10 and 20 dB above the cloud droplet contribution to the return, making radar detection easier, although the concentration of the drizzle drops is so low that they have no effect on the liquid water content or effective radius. These occasional drizzle-sized droplets are present throughout the vertical and horizontal extent of such clouds but should evaporate within 200 m of cloud base. On occasion, the drizzle can fall farther and may yield a false measurement of cloud-base altitude, but such cases can be recognized by examining the vertical profile of reflectivity. A radar sensitivity threshold of −30 dBZ would detect 80%, 85%, and 90% of the marine stratocumulus, with a liquid water content above 0.025, 0.05, and 0.075 g m−3, respectively. Because nonprecipitating drizzle droplets are rare in continental stratocumulus, the equivalent figures are reduced to 38%, 33%, and 25%. Improving the sensitivity to −40 dBZ increases detection probability to nearly 100% for both types of cloud. These figures are based on the assumption that the cloud is deep enough to fill the radar pulse volume.

Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-17633-2009 ◽

2009 ◽

Vol 9 (4) ◽

pp. 17633-17663 ◽

Cited By ~ 1

Author(s):

O. Geoffroy ◽

J.-L. Brenguier ◽

F. Burnet

Keyword(s):

Water Content ◽

Liquid Water ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Liquid Water Content ◽

Tuning Parameter ◽

Parametric Function ◽

Generalized Gamma ◽

Parametric Functions ◽

Generalized Gamma Function

Abstract. Parametric functions are currently used to represent droplet spectra in clouds and to develop bulk parameterizations of the microphysical processes and of their interactions with radiation. The most frequently used parametric functions are the Lognormal and the Generalized Gamma which have three and four independent parameters, respectively. In a bulk parameterization, two parameters are constrained by the total droplet number concentration and the liquid water content. In the Generalized Gamma function, one parameter is specified a priori, and the fourth one, like the third parameter of the Lognormal function, shall be tuned, for the parametric function to statistically best fit observed droplet spectra. These parametric functions are evaluated here using droplet spectra collected in non-or slightly precipitating stratocumulus and shallow cumulus. Optimum values of the tuning parameters are derived by minimizing either the absolute or the relative error for successively the first, second, fifth, and sixth moments of the droplet size distribution. A trade-off value is also proposed that minimizes both absolute and relative errors for the four moments concomitantly. Finally, a parameterization is proposed in which the tuning parameter depends on the liquid water content. This approach significantly improves the fit for the smallest and largest values of the moments.