Raindrop Growth Research Articles

Microphysical characteristics of torrential predecessor rain events over the Yangtze River Delta Area and the related tropical cyclones

The precipitation structures and microphysical characteristics of predecessor rain events (PREs) over the Yangtze River Delta area and related tropical cyclones (TCs) from 2014 to 2019 were investigated using Dual-frequency Precipitation radar data from Global Precipitation Measurement (GPM) for drop size distributions (DSDs). Results showed that the total mean rain rate of PREs was larger than that of TCs, primarily due to higher convective and stratiform rain rates, with enhanced fractional coverage of convective rain in PREs. Examination of microphysical characteristics revealed that a greater quantity of small-sized droplets and a smaller quantity of medium- as well as large-sized droplets contributed towards PREs in comparison with TCs. The conclusion still holds when partitioning DSDs based on different precipitation rate categories. Further investigation of DSDs using gamma functions illustrated that precipitation in PREs had lower average mass-weighted diameters (Dm) and enhanced normalized number concentration (Nw) compared with TCs; partitioning precipitation into convective and stratiform components illustrated a larger Dm and lower Nw in TCs than PREs. The analysis of microphysical and thermodynamical processes using the reanalysis data indicates that relatively intense convective activity with drier conditions may be favorable to enhancing raindrop growth through collision-coalescence processes, as a result of larger Dm in TCs than PREs. The empirical relations (Z–R algorithms) applied in different rain regimes (stratiform, convective, and total PREs) revealed significant diversities, relying on weather conditions and geographical locations. Plain language summaryThe Yangtze River Delta area is an important economic belt in China. Under climate change, observed frequent occurrences of weather extremes of heavy rainfall and tropical cyclones (TCs) exert adverse effects on economic development in this region. Thus, a deep understanding of the mechanism of TCs torrential rainfall in the Yangtze River Delta area is urgently necessary. In this study, we investigated the precipitation patterns and microphysical characteristics of predecessor rain events (PREs) in the Yangtze River Delta region, and their association with TCs in the South China Sea-Western North Pacific Ocean (SCS-WNPO) area from 2014 to 2019. We found that PREs had a higher total mean precipitation rate than TCs. Further examination using gamma functions demonstrated that PREs exhibited lower average mass-weighted diameters (Dm) and higher normalized intercept parameters (Nw) than TCs. This pattern persisted when distinguishing between convective and stratiform precipitation components. We believe that our study makes a significant contribution to the literature because these results provide valuable insights into the distinct precipitation characteristics of PREs and TCs in the study region and contribute to a better understanding of tropical cyclone-related rainfall patterns, and act as a scientific basis for disaster mitigation.

In situ observations of rain rate and precipitation microphysics over the coastal area of South China: Perspectives for satellite validation

Abstract This study investigates precipitation observed by a set of collocated ground-based instruments in Zhuhai, a coastal city located at the southern tip of the Pearl River Delta of Guangdong Province in South China. Seven months of ground-based observations from a tipping-bucket rain gauge (RG), two laser disdrometers (PARSIVEL and PWS), and a vertically-pointing Doppler Micro Rain Radar-2 (MRR), spanning from December 2021 to July 2022, are statistically evaluated to provide a reliable reference for China’s spaceborne precipitation measurement mission. Rainfall measurement discrepancies are found between the instruments, though the collocated deployment mitigates uncertainties originating from spatial/temporal variabilities of precipitation. The RG underestimates hourly rain amounts at the observation site, opposite to previous studies, leading to 18.2% percent bias (Pbias) of hourly rain amounts when compared to the PARSIVEL. With the same measurement principle, the hourly-accumulated rain between the two laser disdrometers has a Pbias of 15.3%. Discrepancies between MRR and disdrometers are assumed to be due to different temporal/spatial resolution, instrument sensitivities and observation geometry, with a Pbias of mass-weighted mean diameter and normalized intercept parameter of gamma size distribution less than 9%. The vertical profiles of drop size distribution (DSD) derived from the MRR are further examined during extreme rainfalls in the East Asia monsoon season (May, June, and July). Attributed to the abundant moisture which favors the growth of raindrops, coalescence is identified as the predominant effective process and the raindrop mass-weighted mean diameter increases by 33.7% when falling from 2000 m to 600 m during the extreme precipitation event in May.

Raindrop Size Distributions Simulated Using a Bin Microphysics Scheme: Different Biases in Stratiform and Convective Rain From an Extratropical Cyclone

AbstractBin microphysics schemes prognose the raindrop size distribution (RSD), which can be directly evaluated through comparison with disdrometer observations. This evaluation will provide implications on the reliability of simulated cloud microphysics by bin microphysics schemes. In this study, the RSDs of a precipitation event associated with an extratropical cyclone passing South Korea are simulated using a bin microphysics scheme and compared with those observed by a ground‐based disdrometer. The simulated mean RSD overall agrees with the observation. However, notable overestimations appear in the large‐ (3.3–4.3 mm) and small‐ (0.56–1.88 mm) diameter ranges, which respectively stem from the biases in two different time periods, one dominated by stratiform rain and the other largely involved with convective rain. In the stratiform‐rain‐dominated period, the melting of snow is the largest contributor to RSDs. The overestimation in the large‐diameter range in this period can be associated with overly active ice–ice collection at upper levels, which generates a local maximum in RSD at the diameter of 3.3 mm that is not seen in the observed RSDs. In the convective‐rain‐involved period, the warm‐rain collision–coalescence is the largest contributor to RSDs. The overestimation in the small‐diameter range and underestimation in the large‐diameter range imply that the collisional growth of raindrops is represented to be weaker than that in reality. The findings in this study suggest that the RSDs simulated using a bin microphysics scheme can have some systematic biases associated with misrepresentation of some microphysical processes.

Seasonal variations in precipitation microphysics over East China based on GPM DPR observations

The disdrometer-observed raindrop size distribution (DSD) in East China has been shown to vary across seasons and rainfall types. In this study, as an extensive research, the seasonal variations of vertical structures and associated microphysical processes of precipitation were investigated using nine-year measurements of dual-frequency precipitation radar on board the Global Precipitation Measurement satellite. The spatial distribution and vertical profiles of the reflectivity factor (Ze), mass-weighted mean diameter (Dm), and generalized intercept parameter (Nw) showed different magnitudes of variability among seasons (and rain types). Generally, the collisional-coalescence process was dominant for sufficient raindrop growth in convective precipitation, followed by the breakup of melted large ice particles. Summer convective precipitation exhibited typical monsoonal features with the largest Ze and Dm values, whereas extremely weak winter convective precipitation exhibited stratiform-like characteristics. For stratiform precipitation, the major microphysical processes were competitive between the breakup and coalescence processes during different seasons. In contrast, collisional coalescence was predominant in the shallow convective precipitation throughout the year. The distribution of rainfall and DSD parameters with respect to storm top height also varied. Specifically, the storm (convective core) top converged around the upper (middle) troposphere during extreme rainfall, indicating moderate convection where Dm reached its maximum with moderate log10Nw. Warm-rain processes also play an important role in raindrop growth. Moreover, the discrepancies between the extreme rainfall and the highest echo top suggest a weak linkage between the heaviest rainfall and tallest storms. This study provides a comprehensive picture of the seasonal variability in precipitation microphysics in East China.

The Characteristics of Raindrop Size Distribution at Windward and Leeward Side over Mountain Area

To analyze the difference in the microphysical development characteristics of orographic rainfall, several Parsivel disdrometers were installed along the windward and leeward slope of a mountain. There were differences in the raindrop size distribution according to the difference in height and distance from the center of the mountain. In low-altitude coastal areas and adjacent areas, the number concentration of raindrops smaller than 1 mm was relatively lower than in mountainous areas, and the rain rate increased with the growth in the size of the raindrops. On the other hand, a higher rain rate was observed as the number concentration of raindrops smaller than 1 mm increased in the hillside area. The increase in the number concentration of small raindrops was evident at the LCL (lifting condensation level) altitude. The main factors affecting the increase in the rain rate on the windward and leeward slopes were the concentration of raindrops and the growth of raindrops, which showed regional differences. As a result of a PCA (principal component analysis), it was found that raindrop development by vapor deposition and weak convection were the main rainfall development characteristics on the windward and leeward slopes, respectively. The difference in regional precipitation development characteristics in mountainous areas affects the parameters of the rainfall estimation relational expression. This means that the rainfall relation calculated through the disdrometer observation data observed in a specific mountainous area can cause spatial and quantitative errors.

Extreme Precipitation Produced by Relatively Weak Convective Systems in the Tropics and Subtropics

AbstractThis study investigates warm‐season extreme precipitation events (EPEs) with the focus on the less‐attended weak convection (WeEPEs) using spaceborne radar observations. WeEPEs are compared to EPEs with intense convection (InEPEs), including their frequency/location, convective structures, and storm environments. WeEPEs and InEPEs both account for about 30% of the EPEs, highlighting the importance of WeEPEs. Overland WeEPEs mainly occur over key monsoon regions and maximize over windward sides of coastal mountains likely due to orographic enhancement. WeEPEs develop under conditions with deep moist profiles and strengthened low‐level onshore flows. Radar, microwave, and lightning proxies all suggest a weak updraft and limited ice contents in WeEPEs. However, vertical radar profiles of WeEPEs exceptionally increase (∼15 dBZ) downward below the melting level, indicating substantial raindrop growth and/or concentration increase through very active warm‐rain processes. This study provides useful guidelines for evaluating high‐resolution model simulations and satellite rainfall retrievals on extreme precipitation.

Microphysical process of precipitating hydrometeors from warm-front mid-level stratiform clouds revealed by ground-based lidar observations

Abstract. Mid-level stratiform precipitations during the passage of warm fronts were detailedly observed on two occasions (light and moderate rain) by a 355 nm polarization lidar and water vapor Raman lidar, both equipped with waterproof transparent roof windows. The hours-long precipitation streaks shown in the lidar signal (X) and volume depolarization ratio (δv) reveal some ubiquitous features of the microphysical process of precipitating hydrometeors. We find that for the light-rain case precipitation that reaches the surface begins as ice-phase-dominant hydrometeors that fall out of a shallow liquid cloud layer at altitudes above the 0 ∘C isotherm level, and the depolarization ratio magnitude of falling hydrometeors increases from the liquid-water values (δv<0.09) to the ice/snow values (δv>0.20) during the first 100–200 m of their descent. Subsequently, the falling hydrometeors yield a dense layer with an ice/snow bright band occurring above and a liquid-water bright band occurring below (separated by a lidar dark band) as a result of crossing the 0 ∘C level. The ice/snow bright band might be a manifestation of local hydrometeor accumulation. Most falling raindrops shrink or vanish in the liquid-water bright band due to evaporation, whereas a few large raindrops fall out of the layer. We also find that a prominent δv peak (0.10–0.40) always occurs at an altitude of approximately 0.6 km when precipitation reaches the surface, reflecting the collision–coalescence growth of falling large raindrops and their subsequent spontaneous breakup. The microphysical process (at ice-bright-band altitudes and below) of moderate rain resembles that of the light-rain case, but more large-sized hydrometeors are involved.

Microphysical Origin of Raindrop Size Distributions During the Indian Monsoon

AbstractSurface raindrop size distributions over a rain shadow region during the Indian summer monsoon are clustered using the k‐means algorithm. The rainfall for five dominant clusters has distinct vertical features in polarimetric radar and micro rain radar. The deep convection with low cloud bases and high liquid water is associated with sharply increasing radar reflectivity at low levels and the largest drops at the surface. The large drops, originated by ice processes, break while falling below the melting layer, causing a peak in raindrops smaller than 0.5 mm diameter near the cloud base, where the falling raindrops rapidly grow by collision‐coalescence without breaking. The number concentration in the stratiform rain is relatively moderate; hence a gradual raindrop growth, and uniform vertical reflectivity. However, few raindrops in stratiform rain grow to a larger diameter than in convective rain, for the same rain rate due to the absence of collisional breakup.

Vertical Characteristics of Raindrops Size Distribution over Sumatra Region from Global Precipitation Measurement Observation

The climatology of the vertical profile of raindrops size distribution (DSD) over Sumatra Region (10° S – 10° N, 90° E – 110° E) has been investigated using Global Precipitation Measurement (GPM) level 2 data from January 2015 to June 2018. DSD's vertical profile was observed through a vertical profile of corrected radar reflectivity (Ze) and two parameters of normalized gamma DSD, i.e., mass-weight mean diameter (Dm) and total drops concentration (Nw). Land-ocean contrast and rain type dependence of DSD over Sumatra were clearly observed. The values of Dm and Nw were larger in the land than in the ocean. Negative and positive gradients of Dm toward the surface were dominant during stratiform and convective rains, respectively, consistent with the Z gradient. Moreover, the negative gradient of stratiform rain in the ocean is larger than in land. Thus, the depletion of large drops is dominant over the ocean, which is due to the break-up process that can be observed from the increase of Nw. Raindrop growth of convective rains is more robust over the ocean than land that can be seen from a larger value of Dmgradient. The BB strength is slightly larger over land and coastal region than over the ocean, indicating that the riming process is more dominant over land and coastal regions than the ocean. Doi: 10.28991/esj-2021-01274 Full Text: PDF

Lightning and precipitation: The possible electrical modification of observed raindrop size distributions

Many studies of cloud electrification have suggested that the presence of precipitation in the mixed phase region of the cloud is essential for charge separation and lightning initiation in clouds. However, observations of the rain gush phenomenon, a transient amplification in near-surface intensity after an overhead lightning also suggest that the lightning discharge can substantially enhance precipitation intensity at the ground. But the microphysical link between lightning and enhanced precipitation intensity after lightning is not well understood. With the observation of a transient amplification in the rain intensity and broadening of the corresponding Raindrop Size distribution (RDSD) after the lightning, it is inferred here that the lightning-induced atmospheric ions and prevailing electrical forces may potentially modulate the RDSD as well as the rain intensity by influencing the collision-coalescence process and the growth rate of raindrops after lightning. The time delay between the lightning and subsequent increase in rain intensity at the Earth's surface was observed to be between 2‐4 min. Also, a good correlation was observed between the variations in lightning frequency and the rain intensity during thunderstorms with an average time lag of 4 min. Piepgrass et al. (1982) have reported a good correlation between lightning frequency and rainfall when the precipitation lagged the lightning by times of 4 and 9 min. These observations indicate that the association between lightning frequency and rainfall with 4-min time lag (the shorter one) may be a result of the lightning-induced growth of raindrops below the melting layer rather than the enhancement in precipitation in the mixed phase region of cloud. The new knowledge, coupled with our related work (MUDIAR et al., 2018) on stratiform clouds provides a compelling basis for the parameterization of the electrical processes in weather/climate models.

Microphysics of Stratiform and Convective Precipitation During Meiyu Season in Eastern China

AbstractThe microphysical structure of Meiyu precipitation in Eastern China is investigated using two‐dimensional video disdrometer (2DVD) and S‐band polarimetric radar observations. The constrained‐gamma raindrop size distribution (DSD) model derived from 2DVD observations performs well in representing Meiyu DSDs. The vertical variability of polarimetric variables and retrieved DSD parameters are then investigated. The results show different patterns of vertical behavior for convective and stratiform rain due to different ice‐phase and precipitation microphysical processes. The radar reflectivity of stratiform rain presents a distinct bright band (with an average echo top between 6 and 7 km). In contrast, the convective rain shows a larger reflectivity with a relatively higher echo top at 8 km. Polarimetric signatures of convective rain above the 0°C isotherm imply the coexistence of rimed particles and aggregates, which is indispensable for the intense precipitation on the ground. However, with a bulk precipitation formed below the melting layer, warm rain processes are still critical pathways for the growth of raindrops and the subsequent generation of heavy rainfall. Furthermore, both convective and stratiform rain is dominated by raindrops <4 mm, and the increase in their rain intensity can mostly be attributed to the increase in raindrop concentration. The identified maritime nature of convective rain has a much higher (roughly more than two times) number concentration of raindrops than that of convection in a similar climate region. This study provides a more comprehensive picture of Meiyu precipitation microphysics in Eastern China.

A Study of the Effects of Anthropogenic Gaseous Emissions on the Microphysical Properties of Landfalling Typhoon Nida (2016) over China

Using the Weather Research and Forecasting model with chemistry module (WRF-Chem), Typhoon Nida (2016) was simulated to investigate the effects of anthropogenic gaseous emissions on the vortex system. Based on the Multi-resolution Emission Inventory for China (MEIC), three certain experiments were conducted: one with base-level emission intensity (CTRL), one with one-tenth the emission of SO2 (SO2_C), and one with one-tenth the emission of NH3 (NH3_C). Results show that the simulations reasonably reproduced the typhoon’s track and intensity, which were slightly sensitive to the anthropogenic gaseous emissions. When the typhoon was located over the ocean, a prolonged duration of raindrop growth and more precipitation occurred in CTRL run. The strongest updraft in CTRL is attributed to the maximum latent heating through water vapor condensation. During the landfalling period, larger (smaller) differential reflectivities in the main-core of the vortex were produced in NH3_C (SO2_C) run. Such opposite changes of raindrop size distributions may lead to stronger (weaker) rainfall intensity, and the ice-related microphysical processes and the relative humidity in low troposphere were two possible influential factors. Moreover, additional ten-member ensemble results in which white noise perturbations were added to the potential temperature field, indicated that the uncertainty of thermodynamic field in the current numerical model should not be ignored when exploring the impacts of aerosol on the microphysics and TC precipitation.

Accurate solution method for the Maxey–Riley equation, and the effects of Basset history

The Maxey–Riley equation has been extensively used by the fluid dynamics community to study the dynamics of small inertial particles in fluid flow. However, most often, the Basset history force in this equation is neglected. Analytical solutions have almost never been attempted because of the difficulty in handling an integro-differential equation of this type. Including the Basset force in numerical solutions of particulate flows involves storage requirements which rapidly increase in time. Thus the significance of the Basset history force in the dynamics has not been understood. In this paper, we show that the Maxey–Riley equation in its entirety can be exactly mapped as a forced, time-dependent Robin boundary condition of the one-dimensional diffusion equation, and solved using the unified transform method. We obtain the exact solution for a general homogeneous time-dependent flow field, and apply it to a range of physically relevant situations. In a particle coming to a halt in a quiescent environment, the Basset history force speeds up the decay as a stretched exponential at short time while slowing it down to a power-law relaxation, ${\sim}t^{-3/2}$, at long time. A particle settling under gravity is shown to relax even more slowly to its terminal velocity (${\sim}t^{-1/2}$), whereas this relaxation would be expected to take place exponentially fast if the history term were to be neglected. An important mechanism for the growth of raindrops is by the gravitational settling of larger drops through an environment of smaller droplets, and repeatedly colliding and coalescing with them. Using our solution we estimate that the rate of growth rate of a raindrop can be grossly overestimated when history effects are not accounted for. We solve exactly for particle motion in a plane Couette flow and show that the location (and final velocity) to which a particle relaxes is different from that due to Stokes drag alone. For a general flow, our approach makes possible a numerical scheme for arbitrary but smooth flows without increasing memory demands and with spectral accuracy. We use our numerical scheme to solve an example spatially varying flow of inertial particles in the vicinity of a point vortex. We show that the critical radius for caustics formation shrinks slightly due to history effects. Our scheme opens up a method for future studies to include the Basset history term in their calculations to spectral accuracy, without astronomical storage costs. Moreover, our results indicate that the Basset history can affect dynamics significantly.

Vertical gradient of stratiform radar reflectivity below the bright band from the Tropics to the extratropical latitudes seen by GPM

This study examined the vertical gradient of radar reflectivity below the detected bright band in stratiform regions from the Tropics to the extratropical latitudes using data from the Ku‐band (13.6 GHz) precipitation radar on board the Global Precipitation Measurement (GPM) Core Observatory. Stratiform precipitation profiles with reflectivity decreasing (increasing) from the melting level toward the surface occur frequently in the tropical oceans (mid‐ and high‐latitude oceans). High fractions of downward increasing stratiform pixels are found over the North Pacific Ocean throughout the year and over East Asia except for winter. In contrast, the North American continent and the adjacent North Atlantic Ocean are characterized by low fractions of downward‐increasing pixels during summer. The difference is consistent with the dominant type of convection over East Asia (warm‐type clouds) and over the North American continent (cold‐type clouds). Even in the tropical oceans such as the Atlantic and eastern Pacific intertropical convergence zones, there are some areas with moderate fractions of downward‐increasing stratiform pixels where the warm rain process dominates. The downward reduction of reflectivity in the stratiform region of MCSs is obviously due to evaporation, which is a function of lower‐tropospheric relative humidity. The downward increase of reflectivity in stratiform regions over the midlatitude oceans appears the result of raindrop growth. This is achieved via the collection of cloud droplets while falling through low‐level clouds produced by large‐scale vertical motion in the lower troposphere due to large‐scale convergence associated with synoptic‐scale systems.

Enhancement of orographic precipitation in Jeju Island during the passage of Typhoon Khanun (2012)

Typhoon Khanun caused over 226mm of accumulated rainfall for 6h (0700 to 1300 UTC), localized around the summit of Mt. Halla (height 1950m), with a slanted rainfall pattern to the northeast. In this study, we investigated the enhancement mechanism for precipitation near the mountains as the typhoon passed over Jeju Island via dual-Doppler radar analysis and simple trajectory of passive tracers using a retrieved wind field. The analysis of vertical profiles of the mountain region show marked features matching the geophysical conditions. In the central mountain region, a strong wind (≥7ms−1) helps to lift low-level air up the mountain. The time taken for lifting is longer than the theoretical time required for raindrop growth via condensation. The falling particles (seeder) from the upper cloud were also one of the reasons for an increase in rainfall via the accretion process from uplifted cloud water (feeder). The lifted air and falling particles both contributed to the heavy rainfall in the central region. In contrast, on the leeward side, the seeder-feeder mechanism was important in the formation of strong radar reflectivity. The snow particles (above 5km) were accelerated by strong downward winds (≤−6ms−1). Meanwhile, the nonlinear jumping flow (hydraulic jump) raised feeders (shifted from the windward side) to the upper level where particles fall. To support these development processes, a numerical simulation using cloud-resolving model theoretically carried out. The accreting of hydrometeors may be one of the key reasons why the lee side has strong radar reflectivity, and a lee side weighted rainfall pattern even though lee side includes no strong upward air motion.

THE MIDLATITUDE CONTINENTAL CONVECTIVE CLOUDS EXPERIMENT (MC3E).

Abstract The Midlatitude Continental Convective Clouds Experiment (MC3E), a field program jointly led by the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Program and the National Aeronautics and Space Administration’s (NASA) Global Precipitation Measurement (GPM) mission, was conducted in south-central Oklahoma during April–May 2011. MC3E science objectives were motivated by the need to improve our understanding of midlatitude continental convective cloud system life cycles, microphysics, and GPM precipitation retrieval algorithms. To achieve these objectives, a multiscale surface- and aircraft-based in situ and remote sensing observing strategy was employed. A variety of cloud and precipitation events were sampled during MC3E, of which results from three deep convective events are highlighted. Vertical structure, air motions, precipitation drop size distributions, and ice properties were retrieved from multiwavelength radar, profiler, and aircraft observations for a mesoscale convective system (MCS) on 11 May. Aircraft observations for another MCS observed on 20 May were used to test agreement between observed radar reflectivities and those calculated with forward-modeled reflectivity and microwave brightness temperatures using in situ particle size distributions and ice water content. Multiplatform observations of a supercell that occurred on 23 May allowed for an integrated analysis of kinematic and microphysical interactions. A core updraft of 25 m s−1 supported growth of hail and large raindrops. Data collected during the MC3E campaign are being used in a number of current and ongoing research projects and are available through the ARM and NASA data archives.

English

Two models for raindrop growth in clouds are developed and compared. A continuous accretion model is solved numerically for drop growth from 20-50 microns, using a polynomial approximation to the collection kernel, and is shown to underestimate growth rates. A Monte Carlo simulation for stochastic growth is also implemented to demonstrate discrete drop growth. The approach models the effect of decreased average time between captures as the drop size increases. It is found that the stochastic model yields a more realistic growth rate, especially for larger drop sizes. It is concluded that the stochastic model showed faster droplet accumulation and hence shorter times for drop growth.

Recirculation and growth of raindrops in simulated shallow cumulus

AbstractThis study investigates growth processes of raindrops and the role of recirculation of raindrops for the formation of precipitation in shallow cumulus. Two related cases of fields of lightly precipitating shallow cumulus are simulated using Large‐Eddy Simulation combined with a Lagrangian drop model for raindrop growth and a cloud tracking algorithm. Statistics from the Lagrangian drop model yield that most raindrops leave the cloud laterally and then evaporate in the subsaturated cloud environmental air. Only 1%–3% of the raindrops contribute to surface precipitation. Among this subsample of raindrops that contribute to surface precipitation, two growth regimes are identified: those raindrops that are dominated by accretional growth from cloud water, and those raindrops that are dominated by selfcollection among raindrops. The mean cloud properties alone are not decisive for the growth of an individual raindrop but the in‐cloud variability is crucial. Recirculation of raindrops is found to be common in shallow cumulus, especially for those raindrops that contribute to surface precipitation. The fraction of surface precipitation that is attributed to recirculating raindrops differs from cloud to cloud but can be larger than 50%. This implies that simple conceptual models of raindrop growth that neglect the effect of recirculation disregard a substantial portion of raindrop growth in shallow cumulus.

Impact of aerosol and freezing level on orographic clouds: A sensitivity study

The response of clouds and precipitation to changes in aerosol properties is variable with the ambient meteorological conditions, which is important for the distribution of water resources, especially in mountain regions. In this study, a detailed bin microphysics scheme is coupled into the Weather Research and Forecasting (WRF) model to investigate how orographic clouds and precipitation respond to changes in aerosols under different thermodynamic profiles. The model results suggest that when the initial aerosol number concentration changes from a clean continental background (4679cm−3) to a polluted urban environment (23,600cm−3), the accumulated surface precipitation amount can be increased up to 14% mainly due to the enhanced riming process which results from more droplets of 10–30μm in diameter. When the freezing level is lowered from 2.85km to 0.9km (above 1000hPa level), the growth of ice-phase particles via riming process is enhanced, leading to more precipitation. However, the response of surface precipitation amount to increase in aerosol particle concentration is not linear with lowering freezing level, and there is a maximum precipitation enhancement caused by aerosols (about 14%) as the freezing level is at 1.4km. Further sensitivity tests show that, the response of riming growth to increase in aerosol particle concentration becomes more significant with lowering the freezing level, but this effect becomes less significant as the freezing level is further lowered due to the limited liquid water. Moreover, the growth of raindrops through collision and coalescence is suppressed with lowering freezing level, due to the shorter distance between the melting level and the ground.

A Lagrangian drop model to study warm rain microphysical processes in shallow cumulus

AbstractIn this study, we introduce a Lagrangian drop (LD) model to study warm rain microphysical processes in shallow cumulus. The approach combines Large‐Eddy Simulations (LES) including a bulk microphysics parameterization with an LD model for raindrop growth. The LD model is one‐way coupled with the Eulerian LES and represents all relevant rain microphysical processes such as evaporation, accretion, and selfcollection among LDs as well as dynamical effects such as sedimentation and inertia. To test whether the LD model is fit for purpose, a sensitivity study for isolated shallow cumulus clouds is conducted. We show that the surface precipitation rate and the development of the raindrop size distribution are sensitive to the treatment of selfcollection in the LD model. Some uncertainty remains for the contribution of the subgrid‐scale turbulence to the relative velocity difference of a pair of LDs, which appears as a factor in the collision kernel. Sensitivities to other model parameters such as the initial multiplicity or the initial mass distribution are small. Overall, sensitivities of the LD model are small compared to the uncertainties in the assumptions of the bulk rain microphysics scheme, and the LD model is well suited for particle‐based studies of raindrop growth and dynamics. This opens up the opportunity to study effects like recirculation, deviations from terminal fall velocity and other microphysical phenomena that so far were not accessible for bin, bulk, or parcel models.