Building on the hypothesis of wall and jet structural modes proposed in Part 1 (Choudhary et al. 2024), this study reports coherent patterns and vortical structures associated with the jet mode by further analysing our experimental particle image velocimetry datasets. Instantaneous velocity fields are binned based on dominant streamwise Fourier modes, focusing on submodes with wavelengths $\lambda _x\approx 5{z_{T}}$ (submode 1) and $\lambda _x\approx 2.5{z_{T}}$ (submode 2); $z_{T}$ is outer length scale of the flow. Two-point correlations of streamwise velocity fluctuations for the total and modal fields reveal near-periodic coherent patterns inclined backwards (similar to $14<^>{\circ }$ ) in the outer region and forwards (similar to $9<^>{\circ }$ ) in the inner region. Vortical structures in conditionally averaged velocity fluctuation vector fields are examined using linear stochastic estimation (LSE) with anticlockwise vorticity (prograde) at the outer energy site as the condition. The vortical structure of submode 1 is a three-vortex system with (i) a robust clockwise vortex in the inner region and (ii) a saddle-point topology in the outer region. The vortical structure of submode 2 is a backward-leaning vortex packet. The LSE fields indicate Q1-Q3 events in the inner region contributed by 'non-local' eddies through the interaction of outer and inner submode 1 vortices. Quadrant analysis reveals that Q1-Q3 events due to 'non-local' eddies outweigh Q2-Q4 contributions of 'local' eddies, producing counter-gradient momentum diffusion below mean velocity maximum. These findings further substantiate the hypothesis of wall and jet structural modes and indicate that the region below mean velocity maximum in wall jets significantly differs from a turbulent boundary layer.

Statistical and fractal properties of the scalar interface in a plane turbulent wall jet of air are investigated experimentally. Seeding is introduced for only the jet flow. These seed particles do not diffuse or evaporate quickly, in contrast to smoke that is commonly used in air flows. Three cameras are placed one above the other and operated in a single-frame mode to image the instantaneous scalar interface between the seeded jet and unseeded ambient fluid with sub-Kolmogorov spatial resolution and an unprecedented dynamic range (large-to-small scale ratio) of 2450. The interface in each image is marked by a simple thresholding technique. A well-converged probability density function (p.d.f.) of the interface height from the wall is obtained from an ensemble of 2945 usable images. The p.d.f. is found to be Gaussian to an excellent approximation, with the most probable location at 1.57 times the outer length scale of the wall jet (marking half the maximum velocity). Fractal dimension of the interface in each usable image is computed using the box counting algorithm and compensated plots for an objective determination. The mean fractal dimension over all usable images is 1.361 ± 0.002, which agrees very well with the near-universal value reported by Sreenivasan and coworkers for a variety of turbulent flows.

In this study, a spatial spectral analysis of turbulent plane wall jets is conducted using two-dimensional particle image velocimetry (2D-PIV) at three different nozzle Reynolds numbers: 10,244, 15,742, and 21,228. To accomplish this, four cameras are positioned side-by-side, capturing the longest field of view in the study of wall jets. The obtained PIV fields are utilized to construct spatial spectra, aiming to understand the model spectral contribution to the variance of velocity fluctuations and Reynolds shear stress. The study reveals that the jet mode exhibits wavelengths that scale with the jet length scale, denoted as z_T . This mode contains two dominant submodes with wavelengths of 5 z_T and 2.5 z_T respectively. In the region above, the velocity maximum, the presence of the jet mode is observed, while the region below it exhibits a robust bimodal behavior attributed to both the wall and jet modes.

Amplitude modulation coefficient R_AM for the plane, turbulent wall jet (henceforth wall jet) data published by Gupta et al. (J Fluid Mech 891:A11 [1]) is computed using Hilbert transform method (a combination of Fourier and Hilbert transforms) and compared with the R_AM data from Bhatt and Gnanamanickam (Phys Rev Fluids 5:74604 [2]) at matching Reynolds numbers (Re _τ as well as Re _δ ). Discrepancies between the two curves are discussed with focus on lateral confinement of the wall jet. Wavelet transform method (a combination of Fourier and wavelet transforms) is also used to compute R_AM and validated against the Hilbert transform method calculations. Furthermore, wavelet transform method is used to compute the frequency modulation coefficient R_FM . Results for R_AM and R_FM computed for the three Re _j experiments reported by Gupta et al. (J Fluid Mech 891:A11 [1]) display Reynolds number similarity of the amplitude and frequency modulation in wall jets over the range of Reynolds numbers covered.

Friction drag at the surface in atmospheric boundary layers (ABLs) is the most challenging flux quantity to infer from observations and simulations of mean wind speed profiles and requires an appeal to similarity theories. Recently, we have reported a new scaling framework and predictive model for drag in non-convective, smooth-walled laboratory turbulent boundary layers (lab TBLs) (Dixit et al. in Phys Fluids 32(4) [1]). Here, we evaluate this predictive model for a specific class of ABLs called truly neutral ABLs (TNABLs) through large-eddy simulations (LES). TNABL is free from convection effects at its upper as well as lower boundaries and therefore bears the closest correspondence to lab TBLs in that respect. From the viewpoint of testing scaling laws, TNABLs present attractive friction Reynolds numbers two to three orders of magnitude higher than the largest Reynolds numbers achieved in lab TBLs to date. The LES study reported here uses the weather and research forecasting (WRF) model in ideal simulation mode. Specifically, we attempt to understand the role of surface roughness in the LES of TNABLs toward the applicability of our predictive drag model. Our results show that the drag scaling and predictive model show promising potential to handle TNABLs provided that the roughness length typical of laboratory experiments is set for WRF simulations.

Cloud seeding experiments for modifying clouds and precipitation have been underway for nearly a century; yet practically all the attempts to link precipitation enhancement or suppression to the presence of seeding materials within clouds remain elusive. In 2019, the Cloud–Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) investigated residuals of cloud hydrometeors in seeded and non-seeded clouds with an airborne mini aerosol mass spectrometer (mAMS). The mAMS was utilized in conjunction with a counterflow virtual impactor (CVI) inlet with a cutoff diameter size of approximately 7 µm. The evaporated cloud droplets from the CVI inlet as cloud residuals were evaluated through the mAMS. The chlorine (Cl) associated with hygroscopic materials, i.e. calcium chloride (CaCl2) and potassium (K), which serve as the oxidizing agents in the flares, is found in relatively higher concentrations in the seeded clouds compared to the non-seeded clouds. In convective clouds, Cl and K as cloud residuals were found even at a vertical distance of 2.25 km from the cloud base. Major findings from the seeding impact are an increase in the number concentration of small (< 20 µm) droplets and an indication of raindrop formation at 2.25 km above the cloud base. It is demonstrated that the seed particle signature can be traced inside clouds along with the microphysical impacts.

Plane turbulent wall jets are traditionally considered to be composed of a turbulent boundary layer (TBL) topped by a half-free jet. However, certain peculiar features, such as counter-gradient momentum flux occurring below velocity maximum in experiments and numerical simulations, suggest a different structure of turbulence therein. Here, we hypothesize that turbulence in wall jets has two distinct structural modes, wall mode scaling on wall variables and free-jet mode scaling on jet variables. To investigate this hypothesis, experimental data from our wall jet facility are acquired using single hot-wire anemometry and two-dimensional particle image velocimetry at three nozzle Reynolds numbers 10 244, 15 742 and 21 228. Particle image velocimetry measurements with four side-by-side cameras capture the longest field of view studied so far in wall jets. Direct spatial spectra of these fields reveal modal spectral contributions to variances of velocity fluctuations, Reynolds shear stress, shear force, turbulence production, velocity fluctuation triple products and turbulent transport. The free-jet mode has wavelengths scaling on the jet length scale ${z_{T}}$ , and contains two dominant submodes with wavelengths $5{z_{T}}$ and $2.5{z_{T}}$ . The region of flow above the velocity maximum shows the presence of the outer jet mode whereas the region below it shows robust bimodal behaviour attributed to both wall and inner jet modes. Counter-gradient momentum flux is effected by the outer jet mode intruding into the region below velocity maximum. These findings support the hypothesis of wall and free-jet structural modes, and indicate that the region below velocity maximum could be much complex than a conventional TBL.

The lack of reliable and continuous measurements of the atmospheric boundary layer (ABL) height poses a significant problem for accurate weather forecasting and examining the intricate dynamics between the ABL and the free troposphere (FT). To address this crucial scientific problem, the present study investigates the ABL height from various simultaneous observations of multi-remote sensing instruments Micropulse lidar (MPL), Ceilometer (CL), Wind profiler radar (WPR) and Radiosonde (RS) and four different PBL schemes (Asymmetrical Convective Model version 2 (ACM2), Yonsei University (YSU), Mellor-Yamada Nakanishi and Nino level 2.5 (MYNN2), and Bougeault -Lacarrere (BouLac) during different sky and surface conditions. The diurnal variation of ABL heights between observation and model differs by-230 m. The ACM2 and MYNN2 show better performance ( R > 0.85) of ABL height than other numerical schemes. The ABL height from different observations such as CL, WPR, MPL, RS and simulated ACM2 and MYNN2 is about 1098 +/- 196 m, 1303 +/- 217 m, 1461 +/- 391 m, and 1716 +/- 639 m, 1808 +/- 407 m, and 1585 +/- 393 m respectively, during the radiosonde launching time (-11:00 -12:00 IST). The difference in mean ABL height is observed due to their different measurement techniques and tracer identity between observation and model simulations. The study underscores a consistent and abrupt reduction in ABL height between observations ( <480 m), model simulations (-700 m), and re-analysis data sets (-550 m) during wet-surface conditions, highlighting the model 's ability to replicate consistent ABL behaviour in different surface conditions. While in cloudy conditions, the WRF-model underestimates-50 -200 m than the observed ABL. This discrepancy is mainly observed due to the underestimation of sensible heat flux and downward shortwave radiation in model simulations. Due to the distinct ABL growth rate difference between the model (-100 -200 m/h) and observations (-105 -130 m/h), the time of attaining peak ABL height differs by one hour among them during different sky conditions. This is attributed to the large deviation in surface temperature, the incoming solar radiation, and the sensible heat fluxes.

We present a novel generalized scaling framework and predictive model for wall friction in turbulent flows. The scaling is derived from the dynamical equations, and total mean-flow kinetic energy and the velocity profile shape factor are used as surrogates for dynamical and boundary condition effects. Veracity of the present approach is assessed using data from the literature spanning unprecedented ranges of flow types, Reynolds numbers, accelerations, and history effects. Unlike previous models that solely apply to standard flows, the present framework reconciles nonstandard flows with standard flows and enables accurate estimates of wall friction in numerical simulations and experiments without resolving the viscous sublayer or using the law of the wall.

The demand for effective methods to augment precipitation over arid regions of India has been increasing over the past several decades as the changing climate brings warmer average temperatures. In the fourth phase of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX IV), a scientific investigation was conducted over a rain-shadow region of the Western Ghats mountains in India. The primary objective was to investigate the efficacy of hygroscopic seeding in convective clouds and to develop a cloud seeding protocol. CAIPEEX IV followed the World Meteorological Organization (WMO) recommendations in a peer-reviewed report with physical, statistical, and numerical investigations. The initial results of the campaign in the monsoon period of 2018 and 2019 with two instrumented aircraft, a ground-based dual-polarization C-band radar, a network of rain gauges, radiosondes, and surface aerosol measurements are reported here. The hygroscopic seeding material was detected in cloud droplets and key cloud microphysical processes in the seeding hypothesis were tracked. The formidable challenges of assessing seeding impacts in convective clouds and the results from 150 seed and 122 no-seed samples of randomized experiments are illustrated. Over 5,000 cloud passes from the airborne campaign provided details about the convective cloud properties as the key indicators for a seeding strategy and the evaluation protocol. The experimental results suggest that cloud seeding can be approached scientifically to reduce uncertainty. The results from this study should interest the scientific community and policymakers concerned with climate change’s impact on precipitation and how to mitigate rainfall deficiencies.

This data set is obtained from an aircraft campaign Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) conducted over the Indian subcontinent to measure cloud and aerosol properties. Data presented in the paper Black Carbon as residuals of monsoon clouds can be found. The data consist of in-cloud and ambient atmosphere Black Carbon measurements and cloud properties. The data set consists of the following: 1. Mean Aerosol Size Distribution (#/cm**3 ) below cloud base 2. Temperature (°C ), Total Droplet Concentration (#/cm*3 ), Refrectrory Black Carbon (rBC) concentration (#/cm*3 ) 3. rBC mixing state data 4. Data for Figure1, rBC inside the cloud and ambient atmosphere (#/cm**3 ). 5. Mean Relative Humidity (%) and coating thickness (nm) with standard deviations. 6. Scattering Inacasdence Ratio- Scattering Incasdance Time_ Coating thickness 7. Statistics of small drop, mid and large drop concentrations (#/cm**3 )

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.

An asymptotic $-1/2$ power-law scaling and a semi-empirical finite-$Re$ model were recently presented by Dixit et al. (2020) for skin friction in zero-pressure-gradient (ZPG) turbulent boundary layers (TBLs). In this work, a new derivation is presented which shows that these relations (i) fundamentally represent a dynamically-consistent scaling of skin friction for nominally two-dimensional ZPG TBLs and fully-developed pipes and channels, and (ii) apply individually to each of these flows. The new theoretical arguments are based on transfer of kinetic energy from mean flow to large eddies of turbulence and depend neither on flow geometry nor outer boundary condition, both of which distinguish one type of flow from the other. Using skin friction data from the literature, it is demonstrated that the finite-$Re$ model describes, as predicted by the theory, data from individual flows remarkably well; these data cover the complete range of laboratory/simulation Reynolds numbers to date. It is, however, observed that performance of the model degrades while attempting to describe data from all flows in a universal fashion. Differences in outer boundary condition and large-scale structures amongst different types of flows appear to be responsible for this degradation. An empirical correction based on Clauser's shape factor, is proposed to absorb the outer boundary condition effects into the scaling of skin friction. This correction leads to a new universal scaling and a robust, semi-empirical, universal finite-$Re$ model for skin friction in ZPG TBLs, pipes and channels. Remarkable collapse of data from all flows in the new scaling underscores the importance of a dynamically-consistent approach towards revealing universality of skin friction in wall turbulence.

10th International Cloud Modeling Workshop What: More than 120 cloud modeling researchers participated in a virtual workshop to discuss recent progress in representing dynamics-microphysics interactions in numerical models and pathways to improve our understanding across a variety of scales. When: 26-30 July 2021 Where: Online

Data from experiments on two-dimensional turbulent wall jets suggest existence of two distinct layers, the wall (inner) layer, and the jet (outer) layer, each having its own universal scaling independent of the local Reynolds number \({\text{Re}}_{\tau }\). This view is distinct from most earlier approaches that are either not clear about what the outer flow is, or consider the wall jet to be comprising of two regions—the region below the velocity maximum is a turbulent boundary layer having its own two-layer structure with a logarithmic overlap, and the region above the velocity maximum is a half free jet. These regions are smoothly patched at the velocity maximum. The present view considers the outer flow to be a universal full jet (rather than a half jet), centered at the velocity maximum, which overlaps with the universal inner wall flow. The hypothesis of a scale-aware overlap of these universal scaling regions leads to the prediction of an \({\text{Re}}_{\tau }\)-dependent power-law velocity profile in the overlap layer. Further, an intermediate variable approach is shown to effectively absorb this \({\text{Re}}_{\tau }\) dependence leading to a universal power-law profile for mean velocity in the overlap layer. Experiments show strong support for this description.

Friction factor models for turbulent flow in smooth pipes express friction factor λ as a function of the bulk Reynolds number ReD and may be broadly grouped into two categories: power-law models and log-law models. While the former stem from the spectral scaling arguments applied to eddy momentum transfer close to the wall, the latter are derived from the mean velocity log law and are known to be consistent with the attached eddy model of wall turbulence structure. Interestingly, none of these models individually describes the entire range of Reynolds numbers (Re) accessed to date, without requiring adjustment of coefficients and/or exponents, i.e., these models are not universal. In this work, we present a new semi-empirical universal model that combines, without introducing any additional empirical coefficients, the essence of both power-law and log-law models. Due to this, our model successfully describes the variation of friction factor over the entire range of Reynolds numbers (more than four decades in ReD) at once. The physical basis for our model is the observation that at finite Reynolds numbers, the flow appears to be a small perturbation of the so-called ultimate regime of smooth-pipe turbulence, as far as friction is concerned; in the ultimate regime, λ→0 asymptotically as ReD→∞. The new model has significant potential toward accurate estimation of friction factor or flow rate in smooth pipe flows.

Continuous aerosol and cloud condensation nuclei (CCN) measurements carried out at the ground observational facility situated in the rain-shadow region of the Indian subcontinent are illustrated. These observations were part of the Cloud Aerosol Interaction Precipitation Enhancement Experiment (CAIPEEX) during the Indian summer monsoon season (June to September) of 2018. Observations are classified as dry–continental (monsoon break) and wet–marine (monsoon active) according to the air mass history. CCN concentrations measured for a range of supersaturations (0.2 %–1.2 %) are parameterized using Twomey's empirical relationship. CCN concentrations at low (0.2 %) supersaturation (SS) were high (>1000cm-3) during continental conditions and observed together with high black carbon (BC∼2000ngm-3) and columnar aerosol loading. During the marine air mass conditions, CCN concentrations diminished to ∼350cm-3 at 0.3 % SS and low aerosol loading persisted (BC∼800ngm-3). High CCN activation fraction (AF) of ≅0.55 (at 0.3 % SS) was observed before the monsoon rainfall, which reduced to ≅0.15 during the marine air mass and enhanced to ≅0.32 after that. There was mostly monomodal aerosol number size distribution (NSD) with a mean geometric mean diameter (GMD) of ≅85 nm, with least (≅9 %) contribution from nucleation mode (<30 nm) particles persisted before the monsoon, while multimode NSD with ≅19 % of nucleation mode particles was found during the marine air mass. Critical activation diameters (dcri) for 0.3 % SS were found to be about 72, 169, and 121 nm prior to, during, and after the marine conditions, respectively. The better association of CCN with aerosol absorption, and the concurrent accumulation mode particles during continental conditions, points to the possibility of aged (oxygenated) carbonaceous aerosols enhancing the CCN activity prior to the marine conditions. An enhancement in CCN concentrations and k values during the daytime along with absorption Ångström exponent was observed during the marine conditions. Best closure obtained using measured critical diameter and ammonium sulfate composition during continental conditions emphasizes the role of aged aerosols contributing to the accumulation mode, enhancing the CCN efficiency. The overestimation of CCN and less hygroscopicity of accumulation mode aerosols during the marine air mass indicate the role of size-dependent aerosol composition in CCN activity during the period.

Studies in the literature on two-dimensional, fully developed, turbulent wall jets on flat surfaces, have invariably reckoned on either the nozzle initial conditions or the asymptotic conditions far downstream, as scaling parameters for the streamwise variations of length and velocity scales. These choices, however, do not square with the notion of self-similarity, which is essentially a 'local' concept. We first demonstrate that the streamwise variations of velocity and length scales in wall jets show remarkable scaling with local parameters, i.e. there appear to be no imposed length and velocity scales. Next, it is shown that the mean velocity profile data suggest the existence of two distinct layers - the wall (inner) layer and the full-free jet (outer) layer. Each of these layers scales on the appropriate length and velocity scales and this scaling is observed to be universal, i.e. independent of the local friction Reynolds number. Analysis shows that the overlap of these universal scalings leads to a Reynolds-number-dependent power-law velocity variation in the overlap layer. It is observed that the mean-velocity overlap layer corresponds well to the momentum-balance mesolayer and there appears to be no evidence for an inertial overlap; only the meso-overlap is observed. Introduction of an intermediate variable absorbs the Reynolds-number dependence of the length scale in the overlap layer and this leads to a universal power-law overlap profile for mean velocity in terms of the intermediate variable.

A new asymptotic −1/2 power-law scaling is derived from the momentum integral equation for the drag in flat-plate turbulent boundary layers. In the limit of infinite Reynolds number, the appropriate velocity scale for drag is found to be M/ν, where M is the boundary layer kinematic momentum rate and ν is the fluid kinematic viscosity. Data covering a wide range of Reynolds numbers remarkably collapse to a universal drag curve in the new variables. Two models, discrete and continuous, are proposed for this universal drag curve, and a robust drag estimation method, based on these models, is also presented.

This study reports on new particle formation (NPF) and characteristic features observed from a rural site falling in the rainshadow of the Western Ghats in peninsular India. A total of 35 NPF events observed during August 2018 January 2019 are classified and analyzed here. The apparent formation rates ranged from 0.2 to 10.0 cm(-3) s(-1), while the growth rates of nucleation mode particles ranged from 1.2 to 13.8 nm h(-1). The frequency of occurrence was least during August (core monsoon) and highest during post-monsoon. The local winds were calm and southeasterly to easterly (from the urban centre) supplying the essential precursor gases during October and November, leading to a frequent occurrence of nucleation events. Observations suggest that an increased condensation sink could limit the NPF while promoting Aitken mode growth. The newly formed particles accounted for about 10-80% of the total aerosol concentration. These newly formed particles were able to act as cloud condensation nuclei after growing to approximately 50 nm with an average activation fraction of 0.4.