Growth of the ice phase in strong cumulonimbus updrafts

A numerical model of ice phase growth in an ascending parcel is used to delineate seeding requirements under the competing embryo and glaciation hypotheses. The strong updraft core is found to remain virtually all liquid until homogeneous freezing occurs, AgI or dry ice seeding having negligible effects with achievable seeding rates. This suggests that the glaciation hypothesis is untenable. Natural hail embryo formation is noted to be limited to updrafts less than 3 to 4 m sec^?1 at cloud base. AgI seeding of such updraft regions at rates currently used is found to produce concentrations of hail embryos sufficient to enhance competition in multi-cell hailstorms, although super-cell storms may require significantly greater seeding rates.     

On the condensed water mass in rising air

Condensed water in a vertical column is related through continuity equations to the updraft speed, column depth, height of the condensation level, and strength of microphysical processes. When the ratio of updraft to characteristic particle fall speed is small, as in stratiform rain, the mass of precipitation in the steady state is proportional to the product of that ratio, column depth, and condensation function. When the ratio is only slightly above unity, two water content regimes are defined by continuity equations. The regime of large water content, an extension of the case described above, occurs when the condensation level is low and when there is rapid conversion of cloud to precipitation. Another regime characterized by a water mass about one-third as large (or less), and absence of precipitation at the ground beneath the updraft column, appears if the condensation level is high enough or cloud conversion slow enough. When the ratio is large, the total water content is usually much smaller than when updrafts and fall speeds are similar, and declines slowly toward a limiting value with increasing ratio. As updrafts intensify, precipitation is more limited near the top of the column, with increasing depth and amount of cloud beneath. In the limit of very fast updraft and weak cloud conversion process, the condensate profile is simply the profile of cloud in air risen from the condensation level without conversion of cloud to precipitation. These findings contrast substantially with those presented by Sulakvelidze,et al., who proposed that the water mass associated with strong updrafts increases with the square of the updraft speed. The difference in results is traced to its origins in different model assumptions.     

Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

The deposition of fog to a wind‐exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind‐induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface‐normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain‐free conditions (0·11 ± 0·05 mm h^?1) and rainy conditions (0·24 ± 0·13 mm h^?1) were about three to six times the eddy‐covariance‐based estimate (0·04 ± 0·002 mm h^?1). Under rain‐free conditions, water‐budget‐based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ～40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ～770 mm year^?1 for the summit cloud forest just below the ridge top (according to the water budget method) and ～785 mm year^?1 for the cloud forest on the lower windward slope (using the eddy‐covariance‐based deposition rate corrected for estimated vertical flux divergence). Copyright © 2006 John Wiley & Sons, Ltd.     

A review of hail growth by stochastic collection in a comulonimbus model

A study of severe convective storms, including tornadoes, indicates that hail is almost always associated with them. Large hail, often having physical characteristics which denote wet growth, is frequently observed to fall in advance of large, damaging tornadoes. Results obtained from a cumulus model, which includes hail growth by stochastic coalescence, support these observations. Hail, rather than rain, is responsible for the large radar reflectivities, and primarily responsible for the mass loading of the updraft, because its size is not dynamically limited as is that of a large raindrop. The applicability of the model, a one-dimensional, time dependent model, is discussed and then the characteristics of hail growth are examined in a variety of conditions, which are determined in nature by aerosols through their capability to initiate condensation, coalescence and drop freezing. Conditions favorable for both dry and wet growth are simulated, corresponding to a cumulonimbus which develops in a shearing environment.     

Numerical simulation of 23 June 2016 Yancheng City EF4 tornadic supercell and analysis of lightning activity

Based on the Weather Research Forecasting (WRF) model that features charging and discharging parameterization, relationships between tornado, hail and lightning were investigated for a tornado-producing (EF4 intensity) supercell thunderstorm over Yancheng City in Jiangsu Province, China, on 23 June 2016. Based on a sounding at 0800, there was a low lifting condensation level, substantial convective available potential energy (CAPE), and strong vertical wind shear near Yancheng City, which promote supercell development. At 1400, observations revealed that hail production and a dramatic increase of positive cloud-to-ground flash rates occurred simultaneously, maximizing five minutes later. The tornado occurred 30 min after the hail production. The time of minimum positive cloud-to-ground flash rates was 15 min later. The simulation indicated that the tornadic supercell moved eastward and that positive cloud-to-ground flash rates increased dramatically at 1400, the same as observed, but their maximum was 5 min later than observed. The simulated updraft volume peaked at 1425 and the simulated downdraft volume maximized 5 min later, when the mesocyclone formed. Simulated reflectivities showed no hook echo and horizontal winds for different height at mid-low levels had a different cyclonic shear at 1430, favorable to mesocyclone formation. Based on the simulated results, the region of positively charged graupel ascended resulting from the region of high liquid water content was lifted by the strong updraft, forming a mid-level strong positive charge region. A lower negative charge region formed by the inductive charging mechanism of collisions between graupel and droplets at the bottom of the cloud, conducive to positive cloud-to-ground flashes.     

Occurrence conditions of positive cloud-to-ground flashes in severe thunderstorms

The purpose of this study was to understand the reasons why frequent positive cloud-to-ground (+CG) flashes occur in severe thunderstorms. A three-dimensional dynamics-electrification coupled model was used to simulate a severe thunderstorm to permit analysis of the conditions that might easily cause +CG flashes. The results showed that strong updrafts play an important role in the occurrence of intracloud flashes. However, frequent +CG flashes require not only strong updrafts but also strong downdrafts in the lower cloud region, conditions that correspond to the later phase of the mature stage and the period of the heaviest solid precipitation of a thunderstorm. During this stage, strong updrafts elevated each charge area in the updraft region to a higher level, which resulted in an inverted tripole charge structure. A wide mid-level region of strong positive charge caused largely by positively charged graupel, presented in the middle of the updraft region because of a non-inductive ice-ice collisional charging mechanism. The charge structure in the downdraft region was consistently more complex and revealed several vertically stacked charge regions, alternating in polarity. Much of the graupel/hail outside the updrafts was lowered to cloud-base by strong downdrafts. In this area, the graupel/hail was charged negatively because of the transportation of negatively charged graupel/hail from higher regions of negative charge in the updrafts, and via the inductive charging mechanism of collisions between graupel/hail and cloud droplets at the bottom of the cloud. Consequently, a large region of negative charge formed near the ground. This meant that +CG flashes were initiated more easily in the lower inverted dipole, i.e., the middle region of positive charge and lower region of negative charge. Frequent +CG flashes began almost synchronously with dramatic increases in the storm updrafts, hail volume, and total flash rate. Therefore, the occurrence of +CG flashes appears a good indicator of storm intensification and it could have some use as a predictor of severe weather in the form of hail.     

Detailed observational study of a weak Echo region

High resolution 3-cm weather radar was used to observe the three dimensional structure of a weak echo region of a severe convective storm in northern Minnesota, U.S.A. Observations show that a seemingly steady-state updraft in the lower portion of the storm broke into an unsteady structure in the upper levels. This is interpreted as indicating that precipitation loading gradually decelerated the upper reaches of the updraft, resulting in periodic breakout of a new updraft core on the inflow side. This periodic behavior of the updraft may be related to hailstreaks and what has been termed the multi-celled storm.     

Numerical simulation on the evolution of cloud particles in 3-D convective cloud

A 3-D convective cloud model with compressible non-hydrostatic dynamics and the spectral bin microphysics of a 2-D slab-symmetric model has been used to simulate an observed supercell storm occurring on 29 June, 2000 near Bird City, Kansas, USA. The main objective of this paper is to study the evolution of particles in this convective storm with bin spectral microphysics scheme. Graupels form and grow through two mechanisms, deposition and riming, with the riming process dominant on top of the inflow and in the upper portion of main updraft. Over the outflow and during the developing and mature stages of the storm, graupel particles mainly grow through deposition with dominant unimodal spectra. Most fall out after growing up. Reducing initial relative humidity disturbance (increasing initial potential temperature disturbance) has negative impact on the formation and growth of graupels over the inflow (outflow). This study shows that large graupel and hail could be suppressed by altering the deposition and coalescence process over the inflow and main updraft. At different locations of the convective cells and with different initial humidity and potential temperature disturbance, the graupel formation and growth mechanisms are different, so as to the feasible hail suppression locations and methods. Supported by National Natural Science Foundation of China (Grant Nos. 40537034, 40805057), and Foundation of Key Laboratory of Meteorological Disaster of Ministry of Education in Nanjing University of Information Science & Technology (Grant No. KLME060202)     

Hail frequency in the Pretoria — Witwatersrand area during 1962 to 1972

Summary Hail observations were made in the Transvaal, South Africa, during the decade 1962 to 1972. The hail-reporting network covered an area of 2800 km² with Pretoria in the northeast and the also densely-populated Witwatersrand in the south. A panel of about 800 voluntary observers contributed 10 560 hail reports during this period. Isopleths of hail frequency within the network were plotted. The pattern was strikingly similar to the isohyetal map. More rain and hail occurred in the urban areas than elsewhere and there was a relationship to topography. Pretoria received less hail than the Witwatersrand, a ridge about 300 m higher than Pretoria.     

Evaluation of the Physical and Chemical Properties of Eyjafjallajökull Volcanic Plume Using a Cloud-Resolving Model

The Eyjafjallajökull volcanic eruption, which occurred on April 14, 2010, caused many environmental, air traffic and health problems. An attempt has been made to demonstrate for the first time that certain improvements could be made in the quantitative prediction of the volcanic ash parameters, and in the accounting of the processes in the immediate vicinity of the volcano, using a cloud-resolving model. This type of explicit modeling by treatment of volcanic ash and sulfate chemistry parameterization, with input of a number parameters describing the volcanic source, is the way forward for understanding the complex processes in plumes and in the future plume dispersion modeling. Results imply that the most significant microphysical processes are those related to accretion of cloud water, cloud ice and rainwater by snow, and accretion of rain and snow by hail. The dominant chemical conversion rates that give a great contribution to the sulfate budget are nucleation and dynamic scavenging and oxidation processes. A three-dimensional numerical experiment has shown a very realistic simulation of volcanic ash and other chemical compounds evolution, with a sloping structure strongly influenced by the meteorological conditions. In-cloud oxidation by H₂O₂ is the dominant pathway for SO₂ oxidation and allows sulfate to be produced within the SO₂ source region. The averaged cloud water pH of about 5.8 and rainwater pH of 4.5 over simulation time show quantitatively how the oxidation may strongly influence the sulfate budget and acidity of volcanic cloud. Compared to observations, model results are close in many aspects. Information on the near field volcanic plume behavior is essential for early preparedness and evacuation. This approach demonstrates a potential improvement in quantitative predictions regarding the volcanic plume distribution at different altitudes. It could be a useful tool for modeling volcanic plumes for better emergency measures planning.     

The cloud-microphysical cause of torrential rainfall amplification associated with Bilis (0604)

After its landfall in China’s mainland in 2006, Typhoon Bilis brought about torrential rainfall amplification at the edge of Guangdong, Jiangxi, and Hunan provinces, causing severe disasters. From a cloud-microphysical perspective, we discuss the differences of cloud-microphysical processes before and during the precipitation amplification and possible causes of the rainfall amplification by using high-resolution simulation data. The results show that the cloud-microphysical characteristics during the above two periods are significantly different. With the distinct increase in the rainfall intensity, the cloud hydrometeor contents increase markedly, especially those of the ice-phase hydrometeors including ice, snow and graupel, contributing more to the surface rainfall. The clouds develop highly and vigorously. Comparisons of conversion rates of the cloud hydrometeors between the above two periods show that the distinct increases in the cloud water content caused by the distinct enhancement of the water vapor condensation rate contribute to the surface rainfall mainly in two ways. First, the rain water content increases significantly by accretion of cloud water by rain water, which thus contributes to the surface rainfall. Second, the accretion of cloud water by snow increases significantly the content of snow, which is then converted to graupel by accretion of snow by graupel. And then the graupel melts into rain water, which subsequently contributes to the surface rainfall amplification. In summary, a flow chart is given to clarify the cloud-microphysical cause of the torrential rainfall amplification associated with Bilis.     

Near-surface water fluxes and their controls in a sloping heterogeneously layered volcanic soil beneath a supra-wet tropical montane cloud forest (NW Costa Rica)

Tropical montane cloud forests (TMCF) receive additional (‘occult’) inputs of water from fog and wind-driven rain. Together with the concomitant reduction in evaporative losses, this typically leads to high soil moisture levels (often approaching saturation) that are likely to promote rapid subsurface flow via macropores. Although TMCF make up an estimated 6.6% of all remaining montane tropical forest and occur mostly in steep headwater areas that are protected in the expectation of reduced downstream flooding, TMCF hillslope hydrological functioning has rarely been studied. To better understand the hydrological response of a supra-wet TMCF (net precipitation up to 6535 mm y⁻¹) on heterogeneously layered volcanic ash soils (Andosols), we examined temporal and spatial soil moisture dynamics and their contribution to shallow subsurface runoff and stormflow for a year (1 July 2003–30 June 2004) in a small headwater catchment on the Atlantic (windward) slope near Monteverde, NW Costa Rica. Particular attention was paid to the partitioning of water fluxes into lateral subsurface flow and vertical percolation. The presence of a gravelly layer (C-horizon) at ~25 cm depth of very high hydraulic conductivity (geometric mean: 502 mm h⁻¹) intercalated between two layers of much lower conductivity (7.5 and 15.7 mm h⁻¹ above and below, respectively), controlled both surface infiltration and delayed vertical water movement deeper into the soil profile. Soil water fluxes during rainfall were dominated by rapid lateral flow in the gravelly layer, particularly at high soil moisture levels. In turn, this lateral subsurface flow controlled the magnitude and timing of stormflow from the catchment. Stormflow amount increased rapidly once topsoil moisture content exceeded a threshold value of ~0.58 cm³ cm⁻³. Responses were not affected appreciably by rainfall intensity because soil hydraulic conductivities across the profile largely exceeded prevailing rainfall intensities.     

Rates of erosion of till in the nearshore zone

Measurements of vertical erosion of till in water depths ranging from 1.1 m to 5 m have been obtained using a modified micro-erosion meter. Measurements in this environment are hindered by poor underwater visibility and losses due to high erosion rates and ice action. There is considerable spatial and temporal variability in the erosion rates measured but average values show a general increase from 11 mm y^?1 in 6 m of water to about 35 mm y^?1 in 2.3 m of water and even higher rates closer to shore. The measured values in shallow water are in good agreement with long-term rates extrapolated from shoreline recession. It is suggested that erosion occurs through abrasion and fluid stressing, and that these mechanisms are aided by softening of the upper surface of the till, possibly through cyclic ‘fatigue failure’.     

The dynamical and liquid water structure of the small cumulus as determined from its environment

This paper examines the evidence for the model of a small cumulus cloud represented as a quasi static but turbulent entity, growing on the upshear side and decaying on the downshear side. While the air just outside the cloudy outline is, on average, stationary relative to the embedding airmass, there is a slight flow, upwards and forward as though the updraft has induced upward motion in the clear air outside the cloud, on the growing side. On the decaying side the motion is downwards and away from the cloud.This is a flow pattern which is not consistent with the air flowing around the cloud as it moves forward but it agrees well with the picture given. Decayed remnants of cloud are found throughout the air previously occupied by the cloud. The cloud outline moves through the embedding air at a velocity which is almost as large as the relative motion of the subcloud feeding airflow (which is almost free from internal wind shear in strong convection).The mixing of dry air from above the inversion yields the observed diluted liquid water content in small cumuli, if such mixing is allowed to proceed until the cloud density equals that of the surrounding air. Quantitative conditions relating the liquid water to inversion temperature and moisture changes, and to the stability of the environment are presented. The strong vertical mixing from the top of the cloud downwards is important to microphysical processes.     

积云并合扩展层化型积层混合云的数值模拟分析

积层混合云是我国的主要降水云系,也是人工影响天气的主要作业对象,从云降水物理的角度来研究云系的形成和发展维系具有重要的意义.基于积层混合云的重要性,本文从个例研究入手,利用中尺度数值模式WRF,模拟2005年5月17~18日发生在我国西南山区（主要以贵州省为主）的积层混合云降水过程.发现这次过程是由对流云并合扩大层化形成的.云系形成以后,云系附近会不断有对流云生成,并在移动过程中并合进入云系,补充云系发展维系所需的含水量和能量,促使云系不断维持.在积层混合云系的内部,对流云和层状云区不断地发生作用.对流云给周围的层状云不断输送含水量和能量,支持着层状云的发展.云系内部两种云相互作用的结果体现在：对流云内的上升气流速度逐渐渐小,层状云的上升气流速度不断维持,总上升气流面积区扩大.对流云的降水量不断减小,而层状云的降水不断维持,带来了大面积持续时间很长的降水.     

Macroscopic impacts of cloud and precipitation processes in shallow convection

This paper presents application of the EULAG model combined with a sophisticated double-moment warm-rain microphysics scheme to the model intercomparison case based on RICO (Rain in Cumulus over Ocean) field observations. As the simulations progress, the cloud field gradually deepens and a relatively sharp temperature and moisture inversions develop in the lower troposphere. Two contrasting aerosol environments are considered, referred to as pristine and polluted, together with two contrasting subgridscale mixing scenarios, the homogeneous and the extremely inhomogeneous mixing. Pristine and polluted environments feature mean cloud droplet concentrations around 40 and 150 mg^?1, respectively, and large differences in the rain characteristics. Various measures are used to contrast evolution of macroscopic cloud field characteristics, such as the mean cloud fraction, the mean cloud width, or the height of the center of mass of the cloud field, among others. Macroscopic characteristics appear similar regardless of the aerosol characteristics or the homogeneity of the subgrid-scale mixing.     

Identifying individual rain events from pluviograph records: a review with analysis of data from an Australian dryland site

Rainfall is routinely reported as falling in ‘events’ or ‘storms’ whose beginning and end are defined by rainless intervals of a nominated duration (minimum inter‐event time, MIT). Rain events commonly exhibit fluctuations in rain rate as well as periods when rain ceases altogether. Event characteristics such as depth, mean rain rate, and the surface runoff volume generated, are defined in relation to the length of the rain event. These derived properties are dependent upon the value of MIT adopted to define the event, and the literature reveals a wide range of MIT criteria. Surprisingly little attention has been paid to this dependency, which limits the inter‐comparison of results in published work. The diversity in criteria also diminishes the usefulness of historical data on event durations, rain rates, etc., in attempts to document changes in the rainfall climate. This paper reviews the range of approaches used in the recognition of rain events, and a 5 year pluviograph record from an arid location is analysed. Changing MIT from 15 min to 24 h (lying within the range of published criteria) alters the number of rain events from 550 to 118. The mean rain rate declines from 2·04 mm h^?1 to 0·94 mm h^?1, and the geometric mean event duration rises from 0·66 h to 3·98 h. This wide variation in the properties of rain events indicates that more attention needs to be paid to the selection and reporting of event criteria in studies that adopt event‐based data analysis. The selection of a MIT criterion is shown to involve a compromise between the independence of widely‐spaced events and their increasingly variable intra‐event characteristics. Copyright © 2008 John Wiley & Sons, Ltd.     

Rain event properties in nature and in rainfall simulation experiments: a comparative review with recommendations for increasingly systematic study and reporting

In hydrology and geomorphology, less attention has been paid to rain event properties such as duration, mean and peak rain rate than to rain properties such as drop size or kinetic energy. A literature review shows a lack of correspondence between natural and simulated rain events. For example, 26 studies that report event statistics from substantial records of natural rain reveal a mean rain rate of just 3·47 mm h^?1 (s.d. 2·38 mm h^?1). In 17 comparable studies dealing with extreme rain rates including events in cyclonic, tropical convective, and typhoon conditions, a mean maximum rain rate (either hourly or mean event rain rate) of 86·3 mm h^?1 (s.d. 57·7 mm h^?1) is demonstrated. However, 49 studies using rainfall simulation involve a mean maximum rain rate of 103·1 mm h^?1 (s.d. 81·3 mm h^?1), often sustained for > 1 h, exceeding even than of extreme rain events, and nearly 30 times the mean rain rate in ordinary, non‐exceptional, rain events. Thus rainfall simulation is often biased toward high rain rates, and many of the rates employed (in several instances exceeding 150 mm h^?1) appear to have limited relevance to ordinary field conditions. Generally, simulations should resemble natural rain events in each study region. Attention is also drawn to the raindrop arrival rate at the surface. In natural rain, this is known to vary from < 100 m^?2 s^?1 to > 5000 m^?2 s^?1. Arrival rate may need to be added to the list of parameters that must be reproduced realistically in rainfall simulation studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Excess 3He in deep water on the East Pacific Rise

Helium isotope measurements show that water on the crest and flanks of the East Pacific Rise has the highest enrichment in ³He so far observed in the oceans; the ³He/⁴He ratio anomaly relative to atmospheric helium is + 32% at the mid-depth maximum in the profiles. The corresponding ³He solubility anomaly relative to saturation with atmospheric helium is +50%. These data indicate that active sea-floor spreading sites on the crests of the mid-ocean rises are the sources of primordial helium injected into the ocean from the earth's interior. The ³He/⁴He ratio in this flux is approximately 1.6 × 10^?5, about 11 times the atmospheric ratio of 1.4 × 10^?6. The total flux of ³He into the atmosphere is 4.6 atoms cm^?2 earth-surface sec^?1, most of which (4.0 atoms cm^?2 sec^?1) is supplied by the oceanic flux. The corresponding atmospheric residence time for ³He is 10⁶ years, which, within the large uncertainties of supply and demand (thermal escape), is consistent with the requirement for a steady state.     

Fog precipitation in the Sierra de las Minas Biosphere Reserve,Guatemala

Fog precipitation occurs when fog droplets are filtered by the forest canopy and coalesce on the vegetative surfaces to form larger water droplets that drip to the forest floor. This study examines the quantity of throughfall compared with incident precipitation produced by the canopy of a lower montane rain forest (2100 m) and an upper montane cloud forest (2550 m) in the Sierra de las Minas Biosphere Reserve, Guatemala. Fog precipitation was measured with throughfall and precipitation gauges from 23 July 1995 to 7 June 1996. Fog precipitation occurred during sampling periods when throughfall exceeded incident precipitation. Fog precipitation contributed <1% of total water inputs in the cloud forest at 2100 m during the 44‐week period, whereas fog precipitation contributed 7·4% at 2550 m during the same period. The depth equivalent of fog precipitation was greater at 2550 m (203·4 mm) than at 2100 m (23·4 mm). The calculation of fog precipitation in this study is underestimated. The degree of underestimation may be evident in the difference in apparent rainfall interception between 2100 m (35%) and 2550 m (4%). Because the apparent interception rate at 2550 m is significantly lower than 2100 m, the canopy probably is saturated for longer periods as a result of cloud water contributions. Data show a seasonal pattern of fog precipitation most evident at the 2550 m site. Fog precipitation represented a larger proportion of total water inputs during the dry season (November to May). Because cloud forests generate greater than 1 mm day^?1 of fog precipitation in higher elevations of the Sierra de las Minas, the conservation of the cloud forest may be important to meet the water demands of a growing population in the surrounding arid lowlands. Copyright © 2003 John Wiley & Sons, Ltd.