Cloud Aerosol Interaction And Precipitation Enhancement Experiment Research Articles

Variability of Index of Coalescence Activity (ICA) over a rain-shadow region during monsoon and its role in cloud seeding programs in India

The Index of Coalescence Activity (ICA) is an indicator of the microphysical state of the cloud. ICA is related to cloud base temperature and buoyancy of cloud at 500 hPa level. It is a useful parameter for making a decision on the type of seeding to be carried out. During Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) over the Indian region, aircraft operations for in situ measurements of aerosols and clouds were conducted from the base station Hyderabad to cover parts of north peninsular India falling under the rain-shadow region. Special Radiosonde flights were conducted in the afternoon hours prior to aircraft operations from the locations Hyderabad and Mahabubnagar in addition to regular India Meteorological Department (IMD) Radiosonde flights at 00 UTC from Hyderabad. The parameters like cloud base height and buoyancy at 500 hPa level obtained from Radiosonde flights have been used to estimate ICA values during the period of the experiment (June to October 2009–2011). The variability of ICA has been studied using morning and afternoon sounding profiles over the rain-shadow region. Morning hours showed highly negative ICA values, as compared to noon hours. During noon time, positive ICA values are observed before monsoon onset (in 2009, monsoon onset was 21st June over the study area) and on few occasions during post-monsoon (October). Positive ICA values represent that collision-coalescence (CC) process is not fully developed and hence, hygroscopic seeding is the proper decision. While, negative ICA values indicate active CC process, suggesting glaciogenic seeding. The soundings conducted just before the cloud seeding operations provide information about the actual thermodynamic and microphysical state of the cloud. Hence, ICA information is useful for providing guidelines to the operational cloud seeding programs.

Impact of monsoon on below cloud base aerosol hygroscopicity over a rain shadow region of India

Vertically constrained observations of aerosol size distribution and hygroscopicity using the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) measurements during the monsoon seasons of 2018 and 2019 over a typical global climate model grid area are presented. Two regimes of aerosol loading - low and high, were identified within the season. Low aerosol loading is associated with active monsoon conditions and strong westerlies, while high aerosol loading occurs when the westerly airmass weakens or becomes continental. Aerosol hygroscopicity was the lowest (∼0.08) during low aerosol loading days in 2019 and the highest (∼0.3) during high aerosol loading days in 2018. Aitken mode aerosols control the bulk hygroscopicity on high aerosol loading days and at high supersaturation. The refractory Black Carbon (rBC) aerosols accounted for nearly 10% of the total aerosol number concentration during the monsoon. The internally mixed rBC aerosols had thicker coatings for smaller rBC cores and vice-versa. The cloud condensation nuclei (CCN) closure at the cloud base is established from the in situ observations. These observations are first-of-its-kind from the Indian region, covering two contrasting monsoon seasons, and are useful for studying aerosol-cloud interactions and constraining models.

Moisture transport enhanced by the nocturnal low-level jet in association with the passage of a monsoon depression over the Indian subcontinent

Characteristics of moisture transport associated with a monsoon depression (MD) formed over the Bay of Bengal and later moved westward over central India are presented. The investigation is carried out mainly using the Weather Research and Forecast (WRF) cloud-resolving model simulations and the vertically integrated water vapor transport (IVT) to assess the path and features of the moisture transport. The model performance evaluation is carried out using micrometeorological and microwave radiometer observations from the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX). The results indicate that the strengthened LLJ during the nocturnal periods helps to enhance the moisture convergence in the lower troposphere and thus the rainfall, both before the formation of MD as well as during its passage over land. The moisture budget over a selected region along the path of the MD gave further insights on the moisture convergence, associated precipitation and related features during the study period. The analyses using IVT and LLJ strength are illustrated to be useful in identifying areas of intense convection.

Evaluation of high-resolution WRF model forecasts and their use for cloud seeding decisions

The high-resolution (1 km) forecasts are evaluated during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX). The rainfall observations from a network of rain gauge stations and at high spatial resolution are used for verification purposes. Observed variations in the daily rainfall were adequately reproduced by the model. Several verification indices are used to evaluate the skill of the model forecast. The high rainfall events are overestimated by the model during the first half of the monsoon season and vice versa for the second half. The validated model was used to develop cloud seeding scores for a decision support system during the experiment. The systematic comparison of the model-derived seed score and actual seeding locations from the experiment satisfying the seeding criteria are verified. It is demonstrated that the high-resolution model forecast (<1 km) could be very valuable in identifying possible target regions for cloud seeding. A few real time applications of seed score maps generated from these high resolution forecasts are illustrated in the study.

Hygroscopic growth and CCN activation of aerosols during Indian Summer Monsoon over a rain-shadow region

Hygroscopic growth factor and cloud condensation nuclei (CCN) activation properties of aerosols along with their physical and absorption properties were measured during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over a rain-shadow region, Solapur, India. The hygroscopicity of aerosols at both sub- and supersaturated regimes and the responsible factors are investigated during the Indian Summer Monsoon (ISM). During the intermittent rainy, wet, and cloudy days the total aerosol (≈ 966 cm−3), CCN (≈ 587 cm−3 at 0.4% supersaturation, S) number concentrations, and black carbon (≈ 482 ng m−3) mass concentrations were less, which increased to the mean values of ≈ 3754 cm−3, ≈ 974 cm−3, and ≈ 892 ng m−3, respectively during the dry and clear-sky period. Multiple mode aerosol number size distribution prevailed irrespective of the meteorological conditions, while a prominent nucleation mode was observed during the dry conditions. The nucleation mode was consistent from 09:00 hours local time (IST) during the dry conditions, apart from those associated with the local emissions during the sunset. The nucleation mode associated with the sharp enhancement in absorption Angstrom exponent after the BC peak during the early morning hours of dry days indicated the secondary organic aerosol formation. The hygroscopic growth factor measurements at 50 and 150 nm and the hygroscopicity parameter-ΚHTDMA values were spread over a wide range. But the mean and the median values were similar for both wet and dry periods. However, the hygroscopicity derived from CCN measurements-ΚCCN at 0.2 and 1.0% Ss were higher than the ΚHTDMA values. The high ΚCCN (≈ 0.44) during wet conditions reduced to ≈ 0.28 during the dry conditions for 1.0% S (corresponding to ~40 nm). The hygroscopicity and the CCN efficiency of aerosols were high during daytime and from midnight to sunrise for the dry and the wet periods, respectively.

Precipitation processes over Indian region under different environmental conditions from in situ measurements

Large-scale transport of air mass modulates the weather by altering the cloud and precipitation microphysics of convective clouds. This study examines how oceanic and continental air mass advected into the convective cloud system determines hydrometeors' vertical distribution using in situ airborne observations from the Cloud-Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) during 2010. Maritime air mass from the Bay of Bengal favours a clean atmosphere (CA) that supports clouds with fewer droplets, and the effective radius crosses above 12 μm below the 10 °C temperature level. Thus CA is mainly characterised by moderate precipitation efficiency and broader hydrometeors size distribution spectra. The mixing of air mass from the Arabian sea, Bay of Bengal, and central Indian region results in a moderately polluted atmosphere (MPA), enhancing convective cloud depth and favours mixed-phase processes and equally strong updrafts and downdrafts. As the concentration of sea salt, dust particles, moisture content, and convective available potential energy (CAPE) increases, MPA profoundly supports the primary and secondary ice nucleation inside the convective system. Despite CA and MPA, wide hydrometeor size distribution is absent in a highly polluted atmosphere (HPA), primarily evolved from continental air mass. We noticed an absence of larger raindrops inside clouds developed in HPA, indicating reduced collisions, making them less efficient with the precipitation process. The raindrop size distribution under the three environmental conditions showed distinct characteristics, implying contrasting cloud microphysical processes and precipitation efficiency.

Impact of middle atmospheric humidity on boundary layer turbulence and clouds

An attempt is made to connect the moisture present above the boundary layer and its association with the cloud development and boundary layer turbulence using Large Eddy Simulation (LES) and aircraft observations. The dry boundary layer (BL) with shallow clouds observed during Cloud-Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the arid Indian peninsula is investigated in this study. The constant altitude aircraft observations at different elevations are used to verify LES derived fluxes and variances. LES simulations with the drier conditions above BL resulted in deeper, warmer and drier BL with an enhanced boundary layer turbulent kinetic energy. A dynamic feedback from dry air intrusion into the BL resulted in energetic BL eddies in the dry conditions and a doubling of moisture exchange coefficients. The liquid water content in cloud updrafts rather than in downdrafts were depleted significantly in dry conditions. LES sensitivity indicates that the middle atmospheric water vapor alone could influence the shallow to deep cumulus cloud transitions in the monsoon regime in a dramatic way, by changing the population of cumulus, congestus or deep cumulus clouds. A 30% drying above the BL could drastically reduce the liquid water path and cloud albedo by 10–15%.

Evaluation of PBL parameterization schemes against direct observations during a land depression over Central India

Detailed observations from the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Phase III integrated observational campaign 2014 are used to validate six different planetary boundary layer (PBL) schemes in Weather Research and Forecasting (WRF) model during a land depression over central India in summer monsoon season. This work is a continuation of our previous study on a dry event associated with a heatwave over the same region (Sathyanadh et al. 2017). Four local and two nonlocal PBL schemes are evaluated with hourly observations of air temperature, eddy covariance fluxes, friction velocity, wind, and radiation components for the model performance of the complete energy budget and weather parameters. The models simulated a slightly warmer and drier surface layer with strong winds and overestimated sensible heat flux and ground heat flux during the wet event. The errors in the surface layer temperature are closely associated with errors in the sensible heat flux and incoming radiation, while the errors associated with moisture are linked to the excessive surface layer mixing. The interrelationships between various surface layer parameters and PBL heights are investigated to identify the key controlling factors of PBL evolution in the context of land depression.

Role of liquid phase in the development of ice phase in monsoon clouds: Aircraft observations and numerical simulations

The effect of warm microphysical properties on ice processes is investigated in developing monsoon (MON) and premonsoon (PRE) cumulus clouds growing under different thermodynamic and aerosol conditions. We used airborne observations of cloud microphysical properties during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) and idealized large eddy simulation (LES) with the Weather Research & Forecasting (WRF) model coupled with the spectral-bin microphysics scheme. Airborne observations showed that ice microphysical properties including ice number concentration and ice water content (IWC) strongly depend on cloud base height, boundary layer water vapor mixing ratio, and cloud drop effective radius at the freezing level. The MON clouds in both high and low aerosol concentrations, characterized by a moister boundary layer and warmer cloud bases compared with PRE clouds, have broader drop size distributions (DSD) above the freezing level and produce more ice particles. Idealized WRF-LES simulations of MON and PRE clouds support the observed results with larger IWC in both polluted and clean MON clouds when compared to polluted PRE clouds, and the moisture differences play a more important role in altering the ice microphysical characteristics than aerosol number concentration. Cloud droplet and rain drop properties in both PRE and MON clouds greatly affect ice processes and change ice particle number concentration and types, through impacting drop freezing and riming. The larger IWC in the simulated MON clouds compared with PRE clouds is mainly due to enhanced deposition and riming as a result of the increased boundary layer moisture.

Observed diurnal and intraseasonal variations in boundary layer winds over Ganges valley

The diurnal and intraseasonal variability in the surface winds are examined using high resolution measurements during the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) Phase III Integrated Ground based Experiment, conducted in the Ganges valley during 2014–15. The yearlong observations of lower boundary layer wind from the Ganges valley are discussed in this study. Wind speed and directions at the surface levels obtained from SOund Detection And Ranging (SODAR) and tower are compared and found, mostly SODAR measurements are consistent with tower during high wind conditions (greater than 4 ms−1) except under weak circumstances. Our analysis revealed that, the mean wind speed found to be a maximum (7.86 ms−1) during the monsoon season in the air layer between 10 m and 200 m level and shows a sharp increase near to the surface (at 20 m level). Analysis also depicts seasonal variations in wind speed are predominant in the lower boundary layer and it is closely linked to the large-scale flow patterns/disturbances. The turbulent kinetic energy (TKE) was found to be maximum at 165 m (0.62 m2s-2) during the pre-monsoon due to enhanced surface heating. The 93% and 75% of the wind speed profiles exhibited unstable conditions in the monsoon and post monsoon conditions. Moreover, the unstable layers are characterized by high wind speed and temperature structure function (CT2)in day time. On large scale, the wind speed gets strengthened (reduced) in the wet (dry) spell in the lower levels with distinct diurnal cycle in the Indo Gangetic Plain. The advection of moisture in the nocturnal periods in connection with the wet spells in the near surface level is observed and the wind speeds in the daytime (above 150 m) are also found to be stronger in the range 3.5–5 ms−1 in the dry spells due to the enhanced mixing. The diurnal cycle is more evident in the dry spells in the lower levels. Rainfall peak found in July 2014 is connected with the quasi-biweekly mode periodicity of 2–16 days and further, power spectrum analysis reveals that the presence of high frequency oscillations of periodicity less than 10 days and between 10 and 20 days in the surface layer parameters.

Simultaneous Observations of Nitrogen Dioxide, Formaldehyde and Ozone in the Indo-Gangetic Plain

ABSTRACT This study reports the concentrations of nitrogen dioxide (NO2) and formaldehyde (HCHO), retrieved using the Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique and collocated observations of surface ozone (O3) conducted over the Indo-Gangetic Plain (IGP) during the 2014 monsoon period as part of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX). The average daytime NO2 mixing ratio was 0.81 ± 0.20 ppbv (parts per billion by volume) (range: 0.08–6.06 ppbv). NO2 was observed to decrease during the morning between 06:00 and 09:00 local time and then stabilise for the rest of the day. The average daytime HCHO mixing ratio was 1.93 ± 0.60 ppbv (range: 0.32–8.81 ppbv). Unlike NO2, HCHO, driven by daytime photochemical formation from hydrocarbon precursors, increased during the early morning. The average O3 mixing ratio was 30.0 ± 13.0 ppbv (range: 2.7–81.9 ppbv) during the daytime and 22.5 ± 10.2 ppbv (range: 1–63 ppbv) during the nighttime. Analyses of the back trajectories indicatedfound that the NO2 mixing ratios during CAIPEEX-2014 were affected by long-range transport from thermal power plants situated about 110 km to the south but the HCHO mixing ratios and O3 production were influenced by local emissions. These observations suggest that in rural IGP, ozone concentrations are affected by local emission rather than by long-range transport.

Thermodynamics and Microphysics Relation During CAIPEEX-I

Influence of the environmental thermodynamics on the microphysics of deep cumulus clouds over different parts of India is studied using in situ airborne observations from the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) during 2009. This study provides an understanding of the thermodynamics–microphysics relation over the Indian summer-monsoon region. Relatively stronger updraft and turbulence are noted in the pre-monsoon cloud base layers compared to that of the monsoon clouds. It is illustrated from the in situ observations as well as from a microphysical parcel model that the vertical variation of cloud droplet number concentration (CDNC) has a well-defined peak at a certain height above the cloud base. This elevated CDNC peak is found to be connected with the cloud parcel buoyancy and cumulative convective available potential energy (cCAPE). Higher parcel buoyancy above the cloud base of dry pre-monsoon clouds is associated with stronger in-cloud updraft velocity, higher supersaturation and higher droplet number concentration (in addition to aerosol effect). Higher adiabatic fraction and lower entrainment rate are observed in polluted clouds where boundary layer moisture is low, compared to clean clouds. Relative dispersion of droplet size distribution is found to vary concurrently with air mass characteristics and aerosol number concentration observed over different locations during the experiment. Aerosol–precipitation relationships are also investigated from the observation. Maximum reflectivity and rain rates showed a direct link with boundary layer water vapor content rather than with subcloud aerosol number concentration.

WRF model sensitivity to choice of PBL and microphysics parameterization for an advection fog event at Barkachha, rural site in the Indo-Gangetic basin, India

The present study evaluates the performance of four planetary boundary layer (PBL) parameterization schemes combined with five cloud microphysics schemes in Weather Research Forecasting (WRF) model, specifically for an advection fog event occurred during 4–6 December 2014 at Barkachha, rural site in the Indo-Gangetic plain (IGP). For this purpose, the model was configured over the IGP with 2-km horizontal resolution, and results are compared with detailed micrometeorological data (surface meteorological parameters and fluxes, radiative fluxes, and surface layer wind profiles) gathered during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Integrated Ground Observational Campaign (IGOC) site located in the IGP. The meteorological conditions conducive for the fog formation have been evaluated. All of the tested PBL-microphysics combination showed substantial bias for surface temperature, radiation fluxes, and wind speed. None of the combination found to be superior in predicting the fog event; however, the local MYNN2.5 combination with the WSM3, WSM6, and Lin microphysics obtained slightly better result at the study location. In general, judging from all simulations of liquid water content (as an indicator for the fog), the above combinations were able to simulate the current fog event but the fog onset, duration, and dissipation were particularly offset.

Characterization of hydrometeors and precipitation over the Indian monsoon region using aircraft measurements

In-situ observations of cloud microphysical properties, carried out over different parts of Indian sub-continent using an instrumented research aircraft during Phase-I of Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) from June to September 2009, were studied. Different cloud probes were used to characterize the hydrometeor and precipitation types in the monsoon clouds. The results revealed that all liquid phase hydrometeors were present at temperatures −12 °C to 15 °C. Most of the presence of rain drops were found in the liquid water content (LWC) range from 0.5 to 2 g/m3. In general, rain drops are initiated when the droplet effective radius (Re) exceeded 12 μm. Rain dominated at the tops of young growing convective clouds even at temperatures colder than −10 °C. Mixed phase hydrometeors were present at temperatures from −2 °C to −18 °C. The cases where mixed phase precipitation occurred at temperatures warmer than about −7 °C were associated with influx of transported dust aerosol at the upper (supercooled) region of these cloud systems. Ice only hydrometeors were found at temperatures extending from −10 °C to −22 °C. Most of the monsoon rain is produced by warm and cold cloud/mixed-phase processes in the cloud. The combined Re from two different cloud probes is useful for validation of satellite derived cloud microphysical parameter.

Rain-shadow: An area harboring “Gray Ocean” clouds

The characteristics of monsoon convective clouds over the rain-shadow region of north peninsular India have been investigated using in situ aircraft cloud microphysical observations collected during Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX). The parameters considered for characterization are: liquid water content (LWC), cloud vertical motion (updraft, downdraft: w), cloud droplet number concentration (CDNC) and effective radius (Re). The results are based on 15 research flights which were conducted from the base station Hyderabad during summer monsoon season. The clouds studied were developing congestus. The clouds have low CDNC and low updraft values resembling the oceanic convective clouds. The super-saturation in clouds is found to be low (≤0.2%) due to low updrafts. The land surface behaves like ocean surface during monsoon as deduced from Bowen ratio. Microphysically the clouds showed oceanic characteristics. However, these clouds yield low rainfall due to their low efficiency (mean 14%). The cloud parameters showed a large variability; hence their characteristic values are reported in terms of median values. These values will serve the numerical models for rainfall simulations over the region and also will be useful as a scientific basis for cloud seeding operations to increase the rainfall efficiency. The study revealed that monsoon convective clouds over the rain-shadow region are of oceanic type over the gray land, and therefore we christen them as “Gray Ocean” clouds.

Thermodynamic structure of the convective boundary layer (CBL) over the Indian monsoon region during CAIPEEX campaigns

Abstract. Spatial and temporal variability in the convective boundary layer (CBL) height for the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) study period are examined using the data collected from high-resolution radiosondes during May–September 2009 over the Indian monsoon region. In total, 57 radiosonde launchings were carried out at ∼ 11:00–17:00 IST over six different stations covering a large geographical region, ranging from latitude ∼ 13 to 32° N and longitude 73 to 92° E. Of the total 57 launchings, 17 were made during cloudy conditions during which relative humidity (RH) was found to be greater than 83 % for an ∼ 1.0 km layer at various altitudes below 6 km. Within the layer the difference between saturated equivalent potential temperature and equivalent potential temperature is small, and it satisfies the condition that RH &gt; 83 % for about 1 km is considered as the cloudy layer. There are eight cases when the cloud-topped boundary layer (CTBL) and 19 cases when fair-weather boundary layer (FWBL) is observed. The CBL heights are obtained using thermodynamic profiles, which vary from ∼ 0.4 to 2.5 km a. g. l. The formation of the cloud layers above the boundary layer generally lowers the CBL height and is responsible for its day-to-day variability. The development of the cloud beneath the boundary layer generally elevates the CBL, which is also responsible for the large day-to-day variability in the CBL. The FWBL identified using relative invariance of the thermodynamic profiles varies from ∼ 2.0 to 5.5 km, which is clearly marked by a local minimum in the refractivity gradient. During cloudy days, the CBL is found to be shallow and the surface temperature lower when compared to clear-sky days. The CBL and the lifting condensation level (LCL) heights are randomly related and are found to be at a lower height during cloudy days when compared to clear-sky days. Finally, the typical comparison between the CBL height obtained using thermodynamic profiles and backscattering profiles using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) is examined.

Aerosol–Cloud Interaction in Deep Convective Clouds over the Indian Peninsula Using Spectral (Bin) Microphysics

Abstract The Weather Research and Forecasting (WRF) Model coupled with a spectral bin microphysics (SBM) scheme is used to investigate aerosol effects on cloud microphysics and precipitation over the Indian peninsular region. The main emphasis of the study is in comparing simulated cloud microphysical structure with in situ aircraft observations from the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX). Aerosol–cloud interaction over the rain-shadow region is investigated with observed and simulated size distribution spectra of cloud droplets and ice particles in monsoon clouds. It is shown that size distributions as well as other microphysical characteristics obtained from simulations such as liquid water content, cloud droplet effective radius, cloud droplet number concentration, and thermodynamic parameters are in good agreement with the observations. It is seen that in clouds with high cloud condensation nuclei (CCN) concentrations, snow and graupel size distribution spectra were broader compared to clouds with low concentrations of CCN, mainly because of enhanced riming in the presence of a large number of droplets with a diameter of 10–30 μm. The Hallett–Mossop ice multiplication process is illustrated to have an impact on snow and graupel mass. The changes in CCN concentrations have a strong effect on cloud properties over the domain, amounts of cloud water, and the glaciation of the clouds, but the effects on surface precipitation are small when averaged over a large area. Overall enhancement of cold-phase cloud processes in the high-CCN case contributed to slight enhancement (5%) in domain-averaged surface precipitation.

Microphysical characteristics of convective clouds over ocean and land from aircraft observations

The Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) is a field campaign conducted in India with an instrumented research aircraft. On 29 October 2010, a cyclonic circulation over the Bay of Bengal persisted throughout the day. A special mission was conducted over the Bay of Bengal on this day with the objective of characterizing marine and continental clouds on the same day and finding contrasting/similar signatures of their microphysical properties. The research aircraft sampled growing convective clouds over the ocean and over land. High concentrations of aerosol and cloud condensation nuclei (CCN) were observed over land compared to ocean. Over ocean, higher liquid water content (LWC) and lower cloud droplet number concentrations (Nc) were observed, and droplets reached the threshold of precipitation initiation at lower cloud depths. Over land, clouds contained lower LWC and higher Nc, hence droplets did not reach the threshold of precipitation initiation at a warm temperature as in ocean clouds. Over the ocean larger droplets or drizzle were observed at lower cloud depth than over land. The maximum LWC was found to be very similar at higher altitudes. The convective clouds over land were modified by pollution aerosol with contrasting microphysical properties to those over the ocean.

Cloud‐edge mixing: Direct numerical simulation and observations in Indian Monsoon clouds

AbstractA direct numerical simulation (DNS) with the decaying turbulence setup has been carried out to study cloud‐edge mixing and its impact on the droplet size distribution (DSD) applying thermodynamic conditions observed in monsoon convective clouds over Indian subcontinent during the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX). Evaporation at the cloud‐edges initiates mixing at small scale and gradually introduces larger‐scale fluctuations of the temperature, moisture, and vertical velocity due to droplet evaporation. Our focus is on early evolution of simulated fields that show intriguing similarities to the CAIPEEX cloud observations. A strong dilution at the cloud edge, accompanied by significant spatial variations of the droplet concentration, mean radius, and spectral width, are found in both the DNS and in observations. In DNS, fluctuations of the mean radius and spectral width come from the impact of small‐scale turbulence on the motion and evaporation of inertial droplets. These fluctuations decrease with the increase of the volume over which DNS data are averaged, as one might expect. In cloud observations, these fluctuations also come from other processes, such as entrainment/mixing below the observation level, secondary CCN activation, or variations of CCN activation at the cloud base. Despite large differences in the spatial and temporal scales, the mixing diagram often used in entrainment/mixing studies with aircraft data is remarkably similar for both DNS and cloud observations. We argue that the similarity questions applicability of heuristic ideas based on mixing between two air parcels (that the mixing diagram is designed to properly represent) to the evolution of microphysical properties during turbulent mixing between a cloud and its environment.

The influence of wind speed on surface layer stability and turbulent fluxes over southern Indian peninsula station

Surface to atmosphere exchange has received much attention in numerical weather prediction models. This exchange is defined by turbulent parameters such as frictional velocity, drag coefficient and heat fluxes, which have to be derived experimentally from high-frequency observations. High-frequency measurements of wind speed, air temperature and water vapour mixing ratio (eddy covariance measurements), were made during the Integrated Ground Observation Campaign (IGOC) of Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) at Mahabubnagar, India (16∘44′N, 77∘59′E) in the south-west monsoon season. Using these observations, an attempt was made to investigate the behaviour of the turbulent parameters, mentioned above, with respect to wind speed. We found that the surface layer stability derived from the Monin–Obukhov length scale, is well depicted by the magnitude of wind speed, i.e., the atmospheric boundary layer was under unstable regime for wind speeds >4 m s−1; under stable regime for wind speeds <2 m s−1 and under neutral regime for wind speeds in the range of 2–3 m s−1. All the three stability regimes were mixed for wind speeds 3–4 m s−1. The drag coefficient shows scatter variation with wind speed in stable as well as unstable conditions.