Continental Clouds Research Articles

In-situ observations of cloud microphysics over Arabian Sea during dust transport events

The unique in situ measurements of clouds and precipitation within the shallow and deep cumulus over the north-eastern Arabian Sea region during the Indian monsoon are illustrated in this study with a focus on droplet spectral parameters. The observational period showed a significant incursion of Arabian dust and the presence of giant cloud condensation nuclei (GCCN), modifying the cloud and precipitation spectral properties. Warm rain microphysics supported the mixed-phase development in these clouds and exhibited hydrometeors of snow, graupel and large aggregates as part of ice processes. Cloud base droplet number concentration is about 142 ± 79 cm−3 which is one third of the cloud condensation nuclei (CCN) number concentration at 0.2% supersaturation. A rapid broadening of droplet size distribution (DSD) near to the cloud base was noted in contrast to polluted continental clouds. Relationship between the relative dispersion ( ε; the ratio of DSD spectral width ( σ ) to mean radius ( rm )) and liquid water adiabatic fraction (AF) indicates that the entrainment effect has increased relative dispersion significantly (2–3 times larger) in these clouds. Effective radius ( reff ) is found to be proportional to mean volume radius ( rv ) with a proportionality constant ( β ) that varies between 1.0–1.6, depending on the spectral dispersion parameter. Drop size distributions for the small cloud droplets with size range 2–50 μ m and the large drizzle drops (or ice hydrometeors) with size range 100–6400 μ m are parameterized using the gamma function distributions useful for large-scale cloud models.

Cloud Characteristics and Their Effects on Solar Irradiance According to the ICON Model, CLOUDNET and BSRN Observations

We investigated the liquid water path and global solar irradiance (Q) at ground level according to the ICON model; CLOUDNET measurements in Lindenberg, Munich, and Jülich; and BSRN observations in Lindenberg. This research is focused on stratiform non-precipitating clouds. The liquid water path (LWP) is underestimated, while Q is overestimated. The lower LWP is due to liquid water content underestimation practically in all atmosphere layers and a lower frequency of liquid cloud occurrence compared to observations. This is partly associated with the structure of the cloud nucleation scheme of the ICON model and with the default cloud condensation nuclei (CCN) number concentration. An increase in CCNs from 250 cm−3 (typical background value for the region of interest) to 1700 cm−3 (characteristic of polluted continental clouds) leads to an increase in the grid-scale liquid water path by 40% and a decrease in Q by 12% in overcast conditions. However, we also showed that the liquid water path is not a key factor of Q overestimation. The main factor is an inaccurate description of the cloud spatial structure, where the correct prediction of the ratio of direct to global irradiance as a spatial characteristic of clouds plays a more important role than the standard cloud fraction.

A Statistical Framework for Evaluating Rain Microphysics in Model Simulations and Disdrometer Observations

AbstractStatistical analyses of a large disdrometer data set and a diverse set of model simulations for convection using the Regional Atmospheric Modeling System were conducted, with the mutual goal of providing insights into precipitation formation and microphysical processes. We demonstrate that a two‐moment bulk microphysical model successfully captures the dominant observed modes of variability in rainfall related to rainfall intensity and raindrop size distributions. The model reproduced the general distribution of observed precipitation groups (PGs) derived from Principal Component Analysis. The multi‐variable analysis also uncovered some shortcomings in the model as well as limitations of the disdrometer data. The model solutions were constrained in their predicted drop size distributions (DSDs) due to the fixed DSD parameters assumed in a two‐moment microphysics scheme. A case study from the Mid‐latitude Continental Clouds and Convection Experiment field project demonstrated how model results can be used to contextualize the disdrometer observations which are limited in sample size, spatial coherence, and detection of small drops and low drop concentrations. The case study also showed that the spatial patterns of the statistically derived PGs revealed by the model are consistent with the hypothesized microphysical processes that determine surface rain DSDs. This work demonstrates how leveraging the strengths of observations and models together can improve our understanding and representation of rain microphysical processes.

Reduced terrestrial evaporation increases atmospheric water vapor by generating cloud feedbacks

Reduced terrestrial evaporation directly warms the surface by reducing latent cooling, but also indirectly modifies surface climate by altering atmospheric processes. We use a global climate model to explore two end cases of terrestrial evaporation, comparing the climate of SwampLand, a world where land is always fully saturated with water, to that of DesertLand, where land is always completely lacking in soil moisture. When we suppress evaporation to create a desert-like planet, we find that temperatures increase and precipitation decreases in the global mean. We find an increase in atmospheric water vapor over both land and ocean in the DesertLand simulation. Suppressing evaporative cooling over the continents reduces continental cloud cover, allowing more energy input to the surface and increasing surface moist static energy over land. The residence time of atmospheric water vapor increases by about 50 percent. Atmospheric feedbacks such as changes in air temperatures and cloud cover contribute larger changes to the terrestrial surface energy budget than the direct effect of suppressed evaporation alone. Without the cloud feedback, the land surface still warms with suppressed land evaporation, but total atmospheric water vapor decreases, and the anomalous atmospheric circulations over the continents are much shallower than in simulations with cloud changes; that is, the cloud feedback changes the sign of the water vapor response. This highlights the importance of accounting for atmospheric feedbacks when exploring land surface change impacts on the climate system.

Explicitly Modeling the Effects of Cloud Condensation Nuclei on Warm Rain Initiation

Abstract A bin (or spectral) model is developed to investigate the sensitivity of warm rain initiation to cloud condensation nuclei (CCN). It explicitly represents CCN with a formula whose parameters come from the Twomey relationship (or CCN measurements). By seamlessly integrating CCN activation and drop collection with thousands of bins, the model can replicate the effect of CCN on rain initiation, providing a benchmark to test the process parameterizations in rain initiation. The model is used to simulate two extreme cases with CCN parameters of maritime and continental clouds, respectively, where other actual cases usually lie between these two extreme cases. Its simulations show that rain can initiate within half an hour or less as observed in cumulus clouds. The fast rain initiation modeled is attributed mainly to a new process: the condensational conversion of cloud drops to raindrops via collision–coalescence initiators (or drops with radius between 28 and 100 μm). Since the new process is more important in rain initiation than the autoconversion of cloud drops to raindrops when large CCN exist, it is suggested that the process be parameterized into the weather and climate models to better represent CCN and subsequently remove the common bias of “too dense clouds.” Significance Statement The current weather and climate models represent aerosols via implicit parameterizations and have a bias of “too dense clouds.” Their implicit parameterizations of aerosols usually overlook (or misrepresent) some cloud processes. In this paper and its preceding part (Zeng and Li 2020) we proposed a new framework to explicitly parameterize one subset of aerosols: cloud condensation nuclei (CCN). To embody an explicit parameterization of CCN, we still need quantitative information to connect CCN activation and rain initiation, which motivates this study. In the study we developed an accurate microphysical model to simulate the growth of small CCN to large raindrops, providing information on the sensitivity of rain initiation to CCN. We performed many sensitivity simulations and found the condensational conversion of cloud drops to raindrops via collision–coalescence initiators is a vital process in warm rain initiation. Since the process has been overlooked by all the weather and climate models, the study suggests that the process be introduced in the weather and climate models to properly represent the fast warm rain initiation observed and subsequently remove the bias of “too dense clouds.”

Quantification of antibiotic resistance genes (ARGs) in clouds at a mountain site (puy de Dôme, central France)

Antibiotic resistance in bacteria is becoming a major sanitary concern worldwide. The extensive use of large quantities of antibiotics to sustain human activity has led to the rapid acquisition and maintenance of antibiotic resistant genes (ARGs) in bacteria and to their spread into the environment. Eventually, these can be disseminated over long distances by atmospheric transport. Here, we assessed the presence of ARGs in clouds as an indicator of long-distance travel potential of antibiotic resistance in the atmosphere. We hypothesized that a variety of ARGs can reach the altitude of clouds mainly located within the free troposphere. Once incorporated in the atmosphere, they are efficiently transported and their respective concentrations should differ depending on the sources and the geographical origin of the air masses. We deployed high-flow rate impingers and collected twelve clouds between September 2019 and October 2021 at the meteorological station of the puy de Dôme summit (1465 m a.s.l., France). Total airborne bacteria concentration was assessed by flow cytometry, and ARGs subtypes of the main families of antibiotic resistance (quinolone, sulfonamide, tetracycline; glycopeptide, aminoglycoside, β-lactamase, macrolide) including one mobile genetic element (transposase) were quantified by qPCR. Our results indicate the presence of 29 different ARGs' subtypes at concentrations ranging from 1.01 × 103 to 1.61 × 104 copies m−3 of air. Clear distinctions could be observed between clouds in air masses transported over marine areas (Atlantic Ocean) and clouds influenced by continental surfaces. Specifically, quinolones (mostly qepA) resistance genes were prevalent in marine clouds (54 % of the total ARGs on average), whereas higher contributions of sulfonamide, tetracycline; glycopeptide, β-lactamase and macrolide were found in continental clouds. This study constitutes the first evidence for the presence of microbial ARGs in clouds at concentrations comparable to other natural environments. This highlights the atmosphere as routes for the dissemination of ARGs at large scale.

Exploring Satellite-Derived Relationships between Cloud Droplet Number Concentration and Liquid Water Path Using a Large-Domain Large-Eddy Simulation

Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (N_d) and liquid water path (LWP). Specifically, this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between N_d and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the N_d, we have also considered adiabatic N_d (N_Ad) from the LES model for comparison. The joint histogram analysis shows that the N_Ad-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower N_Ad and LWP; the corresponding mean LWP (LWP) shows a weak relation with N_Ad. The CP shows a larger spread at higher N_Ad (>50 cm^–3), and the LWP increases non-monotonically with increasing N_Ad in both cases. Nevertheless, both lack the negative N_Ad-LWP relationship at higher N_Ad, the entrainment effect on cloud droplets. In contrast, the model simulated N_d-LWP clearly illustrates a much more nonlinear (an increase in LWP with increasing N_d and a decrease in LWP at higher N_d) relationship, which clearly depicts the cloud lifetime and the entrainment effect. Additionally, our analysis demonstrates a regime dependency (marine and continental) in the N_Ad-LWP relation from the satellite retrievals. Comparing local vs large-scale statistics from satellite data shows that continental clouds exhibit only a weak nonlinear N_Ad-LWP relationship. Hence a regime-based N_d-LWP analysis is even more relevant when it comes to warm continental clouds and their comparison to satellite retrievals.

Methodology to determine the coupling of continental clouds with surface and boundary layer height under cloudy conditions from lidar and meteorological data

Abstract. The states of coupling between clouds and surface or boundary layer have been investigated much more extensively for marine stratocumulus clouds than for continental low clouds, partly due to more complex thermodynamic structures over land. A manifestation is a lack of robust remote sensing methods to identify coupled and decoupled clouds over land. Following the idea for determining cloud coupling over the ocean, we have generalized the concept of coupling and decoupling to low clouds over land, based on potential temperature profiles. Furthermore, by using ample measurements from lidar and a suite of surface meteorological instruments at the U.S. Department of Energy's Atmospheric Radiation Measurement Program's Southern Great Plains site from 1998 to 2019, we have developed a method to simultaneously retrieve the planetary boundary layer (PBL) height (PBLH) and coupled states under cloudy conditions during the daytime. The new lidar-based method relies on the PBLH, the lifted condensation level, and the cloud base to diagnose the cloud coupling. The coupled states derived from this method are highly consistent with those derived from radiosondes. Retrieving the PBLH under cloudy conditions, which has been a persistent problem in lidar remote sensing, is resolved in this study. Our method can lead to high-quality retrievals of the PBLH under cloudy conditions and the determination of cloud coupling states. With the new method, we find that coupled clouds are sensitive to changes in the PBL with a strong diurnal cycle, whereas decoupled clouds and the PBL are weakly related. Since coupled and decoupled clouds have distinct features, our new method offers an advanced tool to separately investigate them in climate systems.

Environmental sensitivities of shallow-cumulus dilution – Part 2: Vertical wind profile

Abstract. This second part of a numerical study on shallow-cumulus dilution focuses on the sensitivity of cloud dilution to changes in the vertical wind profile. Insights are obtained through large-eddy simulations of maritime and continental cloud fields. In these simulations, the speed of the initially uniform geostrophic wind and the strength of geostrophic vertical wind shear in the cloud and subcloud layer are varied. Increases in the cloud-layer vertical wind shear (up to 9 ms-1km-1) lead to 40 %–50 % larger cloud-core dilution rates compared to their respective unsheared counterparts. When the background wind speed, on the other hand, is enhanced by up to 10 m s−1 and subcloud-layer vertical wind shear develops or is initially prescribed, the dilution rate decreases by up to 25 %. The sensitivities of the dilution rate are linked to the updraft strength and the properties of the entrained air. Increases in the wind speed or vertical wind shear result in lower vertical velocities across all sets of experiments with stronger reductions in the cloud-layer wind shear simulation (27 %–47 %). Weaker updrafts are exposed to mixing with the drier surrounding air for a longer time period, allowing more entrainment to occur (i.e., the “core-exposure effect”). However, reduced vertical velocities, in concert with increased cloud-layer turbulence, also assist in widening the humid shell surrounding the cloud cores, leading to entrainment of more humid air (i.e., the “core–shell dilution effect”). In the experiments with cloud-layer vertical wind shear, the core-exposure effect dominates and the cloud-core dilution increases with increasing shear. Conversely, when the wind speed is increased and subcloud-layer vertical wind shear develops or is imposed, the core–shell dilution effect dominates to induce a buffering effect. The sensitivities are generally stronger in the maritime simulations, where weaker sensible heat fluxes lead to narrower, more tilted, and, therefore, more suppressed cumuli when cloud-layer shear is imposed. Moreover, in the experiments with subcloud wind shear, the weaker baseline turbulence in the maritime case allows for a larger turbulence enhancement, resulting in a widening of the transition zones between the cores and their environment, leading to the entrainment of more humid air.

The interseasonal features of precipitation microphysics over Thiruvananthapuram and Kolkata - the two tropical stations of Indian sub-continent

A comparative study of precipitation microphysics along with the possible causative mechanisms for the inter-seasonal variation of raindrop size distribution over Thiruvananthapuram and Kolkata – the two tropical stations of Indian sub-continent has been highlighted in this paper. In addition to the contrasting geographical and climatic variations, the importance of the two locations lies in the fact that the rainfall over Thiruvananthapuram represents the Arabian Sea branch of south-west monsoon while that over Kolkata represents the Bay of Bengal branch. The study reveals a distinct domination of raindrops of diameter 2 mm and above for the pre-monsoon months (March–May) over Kolkata with respect to Thiruvananthapuram, while for the monsoon months (June–September), no such appreciable differences are noticed. Correspondingly, the continental and maritime clouds also show their contrasting impacts in the rainfall microphysics during these inter-seasonal phases for the two locations. It has been observed that rainfall evolving from the continental clouds during the pre-monsoon period over Kolkata produces distinct diurnal variation of rainfall occurrences, which are also associated with the presence of raindrop of diameter 3 mm and above. Correspondingly, such variations are absent for the maritime clouds which dominate the monsoon rainfall for both the locations. Thus it can be said that the nature of clouds along with the surface temperature plays an important role in the varying microphysics of precipitation during the inter-seasonal phases of monsoon over these two cities of India.

Unraveling the characteristics of precipitation microphysics in summer and winter monsoon over Mumbai and Chennai – the two urban-coastal cities of Indian sub-continent

The characteristic features of inter-seasonal variability of precipitation microphysics for the year 2019 over Mumbai and Chennai – the two urban-coastal cities of Indian sub-continent have been studied in this paper. The importance of choosing the regions lies in the fact that these two cities are well-known for the occurrences of severe rainfall events due to the effect of south-west and north-east monsoon during the summer and winter season of Indian sub-continent respectively. It has been observed that raindrops of diameter 2‐–4 mm dominate the summer rainfall over Chennai with respect to winter rainfall. Correspondingly, while comparing the two cities together, the impact of the south-west and north-east monsoon in conjunction with the interplay of the continental and maritime effect on the microphysics of precipitation over these cities is strongly visible in the analysis. The study reveals that the rainfall evolving from the continental clouds along with contrasting surface temperature produces distinct diurnal variation for the summer (winter) rainfall over Chennai (Mumbai) while such variations are not visible for the maritime clouds. It has been observed that the drops of larger diameter along with higher radar reflectivity dominated the summer rainfall of Chennai with respect to Mumbai. While for the winter rainfall, such distinct variations are visible only for higher rainfall regimes from 16 mm/h and above where the domination of larger drops is visible for the Mumbai rainfall. Thus the difference in surface temperature which in turn escalates the convective environment in the atmosphere can be termed as one of the important factors responsible for this inter-seasonal variability of precipitation microphysics.

A new classification of satellite-derived liquid water cloud regimes at cloud scale

Abstract. Clouds are highly variable in time and space, affecting climate sensitivity and climate change. To study and distinguish the different influences of clouds on the climate system, it is useful to separate clouds into individual cloud regimes. In this work we present a new cloud classification for liquid water clouds at cloud scale defined using cloud parameters retrieved from combined satellite measurements from CloudSat and CALIPSO. The idea is that cloud heterogeneity is a measure that allows us to distinguish cumuliform and stratiform clouds, and cloud-base height is a measure to distinguish cloud altitude. The approach makes use of a newly developed cloud-base height retrieval. Using three cloud-base height intervals and two intervals of cloud-top variability as an inhomogeneity parameter provides six new liquid cloud classes. The results show a smooth transition between marine and continental clouds as well as between stratiform and cumuliform clouds in different latitudes at the high spatial resolution of about 20 km. Analysing the micro- and macrophysical cloud parameters from collocated combined MODIS, CloudSat and CALIPSO retrievals shows distinct characteristics for each cloud regime that are in agreement with expectation and literature. This demonstrates the usefulness of the classification.

Keys to Differentiating between Small- and Large-Drop Icing Conditions in Continental Clouds

AbstractUsing observations from research aircraft flights over the Great Lakes region, synoptic and mesoscale environments that appear to drive a relationship between liquid water content, drop concentration, and drop size are investigated. In particular, conditions that fell within “small drop” and “large drop” regimes are related to cloud and stability profiles, providing insight regarding whether the clouds are tied to the local boundary layer. These findings are supported by analysis of flight data from other parts of North America and used to provide context for several icing incidents and accidents where large-drop icing was noted as a contributing factor. The relationships described for drop size discrimination in continental environments provide clues that can be applied for both human- and model-generated icing forecasts, as well as automated icing algorithms.

Effects of Sea Spray on Microphysics and Intensity of Deep Convective Clouds Under Strong Winds

AbstractDeep convective clouds similar to those arising in the tropical cyclone eyewall are simulated using a parcel model and 2‐D slab symmetric cloud model with spectral bin microphysics (the Hebrew University Cloud Model). The size distribution of sea spray particles (SSPs) at cloud base is calculated using the Lagrangian‐Eulerian bin microphysics model. The model describes the SSP production, advection, and formation of the size distribution of SSP in the hurricane atmospheric boundary layer at different strong wind speeds. The SSP distributions calculated by the Lagrangian‐Eulerian bin microphysics model are used in the parcel model and the Hebrew University Cloud Model to investigate the microphysical and dynamical effects of SSP on clouds. The SSPs ascending in cloud updrafts dramatically increase the number concentration of cloud drops within a wide range of drop sizes. As a result, sea spray creates clouds with unique property combinations of both maritime and continental types. These clouds have droplet size distributions characterized by a high drop concentration and a low effective radius, as in continental clouds. At the same time, the presence of SSP of a few hundred microns in radii triggers intense rain just above the cloud base, which is typical of extreme maritime clouds. In the presence of large sea spray drops, the smallest cloud condensational nuclei, including the smallest SSP, are activated, giving rise to the permanent in‐cloud nucleation of small droplets, which produce a high concentration of small ice crystals above the level of homogeneous freezing. We showed that the SSP substantially increased the maximum vertical velocity, cloud water content, and mass contents of ice particles. The results are compared with available observed data.

Effects of Sea Spray on the Dynamics and Microphysics of an Idealized Tropical Cyclone

AbstractThe effect of sea spray particles (SSP) on the intensity and microphysical structure of an idealized tropical cyclone (TC) is investigated using the Weather Research and Forecasting Model with a new spectral bin microphysics package. The SSP size distribution in the hurricane boundary layer is calculated using a Lagrangian–Eulerian bin-microphysics model (LEM) with high spatial resolution and extremely high drop size resolution. Sea spray ascending in updrafts within the eyewall of a TC dramatically increases the concentration of cloud drops within a wide range of sizes and decreases the effective drop radius to the values typical of polluted continental clouds. At the same time, the presence of spray drops of a few hundred microns in radii trigger intense rain just above cloud base. As a result, sea spray creates clouds that have a unique combination of maritime and continental properties. Outside the eyewall, clouds remain extremely maritime. SSP are shown to increase substantially the maximum vertical velocity, the cloud water content, and the mass contents of ice particles within the eyewall. As a result, SSP lead to TC intensification, axis symmetrization, and a decrease in eyewall radius. We found a new mechanism where wind-generated sea spray particles invigorate the inner cloud bands by nucleating more cloud droplets, which leads to more water vapor condensation and greater latent heating. The dependence of this process on surface wind speed constitutes a positive feedback loop that can lead to higher hurricane intensity. The results of simulations of cloud microstructure in the hurricane eyewall are supported by observations.

A Continuing Investigation of Diurnal and Location Trends in an Ice Crystal Icing Engine Event Database

<div class="section abstract"><div class="htmlview paragraph">Due to ongoing efforts by the aviation industry, much has been learned over the last several years regarding jet engine power loss and compressor damage events caused by the ingestion of high concentrations of ice crystal particles into the core flow path. Boeing has created and maintained a database of such ice crystal icing (ICI) events to aid in analysis and further study of this phenomenon. This article provides a general update on statistics derived from the Boeing event database, and provides more details on specific event clusters of interest. A series of three flight campaigns have, over the past five years, collected in-situ data in deep convective clouds that will be used for the assessment of the new FAA CFR Part 33 ice crystal environmental envelope Appendix D, and the equivalent EASA CS-25 Appendix P. The most recent Boeing engine event study in 2015 focused on oceanic cloud systems that caused events in Southeast Asia, a region expected to have similar cloud properties as the first flight campaign in Darwin, Australia. The current study will examine ICI engine events over South America and Africa, which are caused by large deep convective continental clouds of the type that were not the main focus of the three Appendix D/P flight campaigns. Continental event clouds are compared to the previously studied oceanic event clouds, in order to ascertain whether any systematic differences exist that should be considered when assessing the representativeness of the flight campaign datasets. The results may also be useful to those developing nowcasting algorithms identifying high ice concentration regions using satellite measurements.</div></div>

Sensitivity of liquid cloud optical thickness and effective radius retrievals to cloud bow and glory conditions using two SEVIRI imagers

Abstract. Retrievals of cloud properties from geostationary satellite sensors offer extensive spatial and temporal coverage and resolution. The high temporal resolution allows the observation of diurnally resolved cloud properties. However, retrievals are sensitive to varying illumination and viewing geometries, including cloud glory and cloud bow conditions, which can lead to irregularities in the diurnal data record. In this study, these conditions and their effects on liquid cloud optical thickness and effective radius retrievals are analyzed using the Cloud Physical Properties (CPP) algorithm. This analysis is based on the use of Spinning Enhanced Visible and Infrared Imager (SEVIRI) reflectances and products from Meteosat-8 and Meteosat-10, which are located over the Indian and Atlantic Ocean, respectively, and cover an extensive common area under different viewing angles. Comparisons of the retrievals from two full days, over ocean and land, and using different spectral combinations of visible and shortwave-infrared channels, are performed, to assess the importance of these factors in the retrieval process. The sensitivity of the cloud-bow- and cloud-glory-related irregularities to the width of the assumed droplet size distribution is analyzed by using different values of the effective variance of the size distribution. The results suggest for marine stratocumulus clouds an effective variance of around 0.05, which implies a narrower size distribution than typically assumed in satellite-based retrievals. For the case with continental clouds a broader size distribution (effective variance around 0.15) is obtained. This highlights the importance of appropriate size distribution assumptions and provides a way to improve the quality of cloud products in future climate data record releases.

Effects of Continental Clouds on Surface Aitken and Accumulation Modes

AbstractRelationships between clouds and surface aerosol are investigated at the Oklahoma Atmospheric Radiation Measurements Southern Great Plains site. Cloud chemical transformations, coalescence, and Brownian capture increase material within cloud droplets. Then when cloud droplets evaporate, as they often do, their residuals are larger than unnucleated particles. This creates a bimodal particle size distribution by moving some Aitken mode (25‐ to 100‐nm diameter) particles to the accumulation mode (100‐ to 825‐nm diameter). Clouds also block solar radiation that causes photochemical reactions that produce small particles that grow by coagulation and condensation into the Aitken mode. Highly significant positive correlations of remotely sensed cloud fraction (cf) with time‐lagged Aitken and accumulation mode mean particle diameter (mpd) isolated the effects of cloud processing on both modes. Positive correlations of cf with accumulation concentrations and negative correlations of cf with Aitken concentrations also provided evidence of cloud processing. Photochemical production of very small particles under clear daylight skies worked together with cloud processing to enhance Aitken mpd and concentration correlations with cf. Cloud‐processed aerosol was evident only during daylight and when the boundary layer mixing height exceeded the remotely sensed cloud base altitude. Greater cf, especially consecutive high cf hours increased accumulation and Aitken mpd and accumulation concentrations while it decreased Aitken concentrations. Lower cf, especially consecutive hours of no clouds, decreased overall mpd and did not enhance the accumulation mode. All of these results implicated clouds as a source of the accumulation mode, and thus, clouds seemed to explain bimodal aerosol over the mid‐North American continent.

The Role of Secondary Ice Processes in Midlatitude Continental Clouds

AbstractClouds contribute very large uncertainties to our understanding of Earth's climate system. This is partly attributed to the insufficient predictive abilities of ice formation processes in clouds and the ramifications for the hydrological cycle and climate. To improve predictions of ice particle concentrations in clouds, a better understanding of the relative contributions of ice nucleating particles and secondary ice processes (SIPs) is needed. To address this challenging question, we combine ice nucleation measurements via immersion freezing of particles filtered from rainwater, with satellite‐retrieved cloud top glaciation temperatures (Tg) of the same clouds, while considering the chemical composition of the rainwater, the particles, and the particles' mass loads. In addition, laboratory‐derived ice nucleation parameterization of K‐feldspar was implemented in an ice nucleation model in order to reconstruct Tg considering primary ice nucleation only. We show that the observed Tg does not correlate with the median freezing temperature of the drops from the laboratory measurements froze (T50), and are significantly warmer than the model prediction. This suggests that SIP play a major role in glaciating the investigated clouds system. Furthermore, we show that the difference between Tg and T50 best correlates with the size of the cloud droplets at −5 °C, indicating that SIP is controlled by cloud droplet sizes. Hence, our results suggest that the effect of SIP on Tg, and therefore on Earth's radiation budget, may be significant.

Technical note: Updated parameterization of the reactive uptake of glyoxal and methylglyoxal by atmospheric aerosols and cloud droplets

Abstract. We present updated recommendations for the reactive uptake coefficients for glyoxal and methylglyoxal uptake to aqueous aerosol particles and cloud droplets. The particle and droplet types considered were based on definitions in GEOS-Chem v11, but the approach is general. Liquid maritime and continental cloud droplets were considered. Aerosol types include sea salt (fine and coarse), with varying relative humidity and particle size, and sulfate/nitrate/ammonium as a function of relative humidity and particle composition. We take into account salting effects, aerosol thermodynamics, mass transfer, and irreversible reaction of the organic species with OH in the aqueous phase. The new recommended values for the reactive uptake coefficients in most cases are lower than those currently used in large-scale models, such as GEOS-Chem. We expect application of these parameterizations will result in improved representation of aqueous secondary organic aerosol formation in atmospheric chemistry models.