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In the afternoon of 28 July 2005, a damaging F2 tornado in Birmingham, United Kingdom, was one of three tornadoes developing on the immediate cool side of a surface warm front. An analysis is performed to find out why the narrow zone on this side of the front was apparently so favourable for tornadoes in spite of lower surface temperatures. It is investigated how three ingredients for tornadogenesis are distributed and how these distributions evolve. These ingredients — the presence of a convective storm, low-level wind shear, and a low LCL height — and the presence of SBCAPE, which is a prerequisite for surface-based convective storms, are studied using surface, upper-air and satellite data, as well as a convection-permitting model. At some distance on the cool side of the front, strong wind shear existed across the lowest 1 km (around 15 m s −1 bulk shear), but no surface-based instability to support surface-based convection. In contrast, the air mass on the front's warm side exhibited weak SBCAPE (50–200 J kg −1 ) but only fairly weak low-level shear (0–1 km bulk shear ~ 6 m s −1 ). The analysis suggests that between these regions, within a narrow zone on the cold side of the front, surface-based instability was considerably higher (~ 450 J kg −1 of SBCAPE) and the 0- to 1-km bulk wind shear was estimated to be 10.5 m s −1 . Moreover, the higher relative humidity in this zone resulted in a lower lifted condensation level (~ 200 m in this zone, compared to ~ 700 m on the warm side of the front). It is concluded that an overlap of all these four ingredients only existed within this zone that was only about 30 km wide.

Proximity soundings from reanalysis data for tornado events in Europe for the years 1958 to 1999 and in the US for the years 1991 to 1999 have been used for generating distributions of parameter combinations important for severe convection. They include parcel updraft velocity (WMAX) and deep-layer shear (DLS), lifting condensation level (LCL) and deep-layer shear (DLS), and LCL and shallow-layer shear (LLS) for weak and significant tornadoes. We investigate how well they discriminate between weak and significant tornadoes. For Europe, these distributions have been generated for unrated, F0 and F1 tornadoes as well to discover if the unrated tornadoes can be associated with the weak tornadoes. The pattern of parameter combination distributions for unrated tornadoes in Europe strongly resembles the pattern of F0 tornadoes. Thus, the unrated tornadoes are likely to consist of mostly F0 tornadoes. Consequently, the unrated tornadoes have been included into the weak tornadoes and distributions of parameter combinations have been generated for these. In Europe, none of the three combinations can discriminate well between weak and significant tornadoes, but all can discriminate if the unrated tornadoes are included with the weak tornadoes (unrated/weak). In the US, the combinations of LCL and either of the shear parameters discriminate well between weak and significant tornadoes, with significant tornadoes occurring at lower LCL and higher shear values than the weak ones. In Europe, the shear shows the same behavior, but the LCL behaves differently, with significant tornadoes occurring at higher LCL than the unrated/weak ones. The combination of WMAX and DLS is a good discriminator between unrated/weak and significant tornadoes in Europe, but not in the US, with significant tornadoes occurring at a higher WMAX and DLS than the unrated/weak tornadoes.

The impact of soil moisture on convective precipitation, convective indices, surface energy balance components, and near-surface meteorological variables is analysed for seven intensive observation periods of the Convective and Orographically induced Precipitation Study (COPS) conducted in summer 2007 using a non-hydrostatic limited-area atmospheric prediction model. The control runs are compared to sensitivity experiments under dry (-25 %) and wet (+25 %) initial soil moisture conditions. In the wet experiment, surface fluxes produce moister and cooler boundary layers with increased equivalent potential temperatures. Furthermore, the lifting condensation level and the level of free convection are lowered for all analysed regions, even under different synoptic controls. The comparison of boundary-layer and mid-tropospheric forcing regimes reveal that the impact of soil moisture on the atmosphere is not systematically higher for boundary-layer forcing. Whereas the Bowen ratio exhibits a clear dependence on soil moisture conditions, the impact on precipitation is complex and strongly depends on convective inhibition. A considerable, but non-systematic dependence of convective precipitation on soil moisture exists in the analysed complex orography. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields.

A simple mass-flux cumulus parameterization scheme suitable for large-scale atmospheric models is presented. The scheme is based on a bulk-cloud approach and has the following properties: (1) Deep convection is launched at the level of maximum moist static energy above the top of the boundary layer. It is triggered if there is positive convective available potential energy (CAPE) and relative humidity of the air at the lifting level of convection cloud is greater than 75%; (2) Convective updrafts for mass, dry static energy, moisture, cloud liquid water and momentum are parameterized by a one-dimensional entrainment/detrainment bulk-cloud model. The lateral entrainment of the environmental air into the unstable ascending parcel before it rises to the lifting condensation level is considered. The entrainment/detrainment amount for the updraft cloud parcel is separately determined according to the increase/decrease of updraft parcel mass with altitude, and the mass change for the adiabatic ascent cloud parcel with altitude is derived from a total energy conservation equation of the whole adiabatic system in which involves the updraft cloud parcel and the environment; (3) The convective downdraft is assumed saturated and originated from the level of minimum environmental saturated equivalent potential temperature within the updraft cloud; (4) The mass flux at the base of convective cloud is determined by a closure scheme suggested by Zhang (J Geophys Res 107(D14), doi: 10.1029/2001JD001005 , 2002) in which the increase/decrease of CAPE due to changes of the thermodynamic states in the free troposphere resulting from convection approximately balances the decrease/increase resulting from large-scale processes. Evaluation of the proposed convection scheme is performed by using a single column model (SCM) forced by the Atmospheric Radiation Measurement Program’s (ARM) summer 1995 and 1997 Intensive Observing Period (IOP) observations, and field observations from the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE) and the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The SCM can generally capture the convective events and produce a realistic timing of most events of intense precipitation although there are some biases in the strength of simulated precipitation.

The aim of the present study is to forecast thunderstorms over Kolkata (22°32′N, 88°20′E), India, during the pre-monsoon season (April–May) with graph spectral distance and entropy analysis. Graph vertices represent points connected by lines or edges, and lifting condensation level, convective condensation level, level of free convection, freezing level, level of neutral buoyancy and the surface level are taken as the input of the graph vertices. The variation in the most probable distance between the vertices is investigated. The result reveals a particular orientation of the vertex distances for thunderstorm days which is significantly different from the non-thunderstorm days. The reference graphs for thunderstorm and non-thunderstorm days are formed using the most probable vertex distances. The spectral distance between the reference graph and the graphs corresponding to thunderstorms are computed with the data collected during the period 1997–2009. The entropies, or the measure of disorderliness or uncertainty, are estimated for the graph distance matrices. The result shows that the thunderstorm days possess lower distance entropy than the non-thunderstorm days. This indicates that the reference graph that has been constructed for thunderstorms is more consistent. The result further depicts that the forecast accuracy through the present method is 98% with 1 h lead time, whereas the accuracy is 93% with 6 h lead time. The forecast is validated with the India Meteorological Department observations for the years 2007, 2008 and 2009. Copyright © 2011 Royal Meteorological Society

This paper presents the observations of the thermodynamic structure of the Marine Atmospheric Boundary Layer (MABL) over Bay of Bengal (BoB), Northern Indian Ocean (NIO) and Arabian Sea (AS) regions, using radiosonde observations carried out as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during March–May 2006. Most of the profiles show the general structure of tropical MABL which consists of surface layer, sub-cloud layer (mixed layer), transition layer, cloud layer and trade wind inversion (TWI). However, in few soundings over BoB and AS, it was observed that the cloud layer is remarkably well-mixed, forming a double mixed layer (DML) structure. This feature is seen to be closely associated with the low level anticyclonic system over these sectors. The average profiles of vertical wind (corresponding to the region of observation) obtained from NCEP showed that there is an increase in the vertical velocity on the days when such a double mixed layer was observed and the altitude region of this high ω coincides with that of the DML. The study also reveals some important differences in the MABL structure during convectively active and weak periods. The spatial variation of the parameters like mixed layer (ML) depth, height of lifting condensation level (LCL), level of free convection (LFC), and trade wind inversion (TWI) are presented and discussed in detail. The TWI observations are first of their kind from the BoB region. The TWI top is found to be at a higher height compared to the observations over other oceanic regions, and the relationship between the TWI altitude and lapse rate of specific humidity is used to explain why the TWI was formed at a higher height. The observations of the MABL parameters are also compared with those reported from the previous campaigns over these regions.

Building on recent observational evidence showing disproportionate increases in temperature and aridity over land in a warming climate, this study examines simulated land–ocean contrasts in fully coupled projections from the Third Coupled Model Intercomparison Project (CMIP3) archive. In addition to the projection of disproportionate changes in temperature and moisture over land, the analysis reveals contrasts in clouds and radiative fluxes that play a key role in the eventual equilibration of the planetary energy budget in response to forcing. Despite differences in magnitude, the nature of the feedbacks governing the land–ocean contrast are largely robust across models, notwithstanding the large intermodel differences in cloud parameterizations, and suggest the involvement of fundamental constraints. The model responses are consistent with previously proposed ideas maintaining that relative humidity (RH) over land decreases with warming because precipitation and the hydrological cycle are governed primarily by transports of moisture from the oceans, where increases in lower-tropospheric temperature and saturated humidity fail to keep pace with those over land. Here, it is argued additionally that constraints on RH imply systematic changes in the cloud distribution and radiative feedbacks over land, as decreased RH raises the lifting condensation level, even as moist instability increases, and suppresses convective clouds. This effect is shown to be particularly strong at low latitudes where the dynamical influence of competing sources of maritime deep convection may further suppress convection. It is found that as a result of the coincidence between strong warming and a muted net greenhouse feedback associated with decreases in RH and clouds, the mean increase in outgoing longwave radiation (OLR) over land (1.0 W m−2 K−1) in transient simulations at 2200 is almost double that over the ocean (0.6 W m−2 K−1), and a strong negative net top-of-atmosphere (TOA) radiative perturbation emerges as the simulations approach and attain equilibrium. However, over the oceans a positive radiative imbalance persists and the increase in water vapor and other greenhouse gases does not allow a local TOA equilibration to occur. The contrast results in an increase in the transport of energy from ocean to land relative to the twentieth century that is accompanied by lasting increases in both OLR and absorbed shortwave radiation globally. A conceptual model to describe the simulated variability is proposed that involves the following: 1) the differing albedos and lower-tropospheric lapse rates over land and ocean, 2) the nonlinearity of the saturated lapse rate in a warming environment, and 3) the disproportionate response in temperature, moisture, clouds, and radiation over land versus ocean. It is noted that while the land–ocean contrast plays a key role in achieving global radiative equilibrium, it entails disproportionate increases in temperature and aridity over land and therefore is likely to be associated with substantial environmental impacts.

The correlation between aerosol particulate matter with aerodynamic diameter ≤2.5 μ m (PM2.5) and cloud base height (CBH) of low clouds (CBH lower than 1.5 km a.g.l.) at Baltimore and New York, United States, for an 8 year period (2007–2014) was investigated using information from the Automated Surface Observing System (ASOS) observations and collocated U.S. Environmental Protection Agency (EPA) observations. The lifting condensation level (LCL) heights were calculated and compared with the CBH. The monthly average observations show that PM2.5 decreases from 2007 to 2014 while there is no significant trend found for CBH and LCL. The variability of the LCL height agrees well with CBH but LCL height is systematically lower than CBH (~180 m lower). There was a significant negative correlation found between CBH–LCL and PM2.5. All of the cloud cases were separated into polluted and clean conditions based on the distribution of PM2.5 values. The distributions of CBH–LCL in the two groups show more cloud cases with smaller CBH–LCL in polluted conditions than in clean conditions.

Land–atmosphere interaction at climatological time scales in a large area that includes the West African Sahel has been explicitly explored in a regional climate model (RegCM) simulation using a range of diagnostics. First, areas and seasons of strong land–atmosphere interaction were diagnosed from the requirement of a combined significant correlation between soil moisture, evaporation, and the recycling ratio. The northern edge of the West African monsoon area during June–August (JJA) and an area just north of the equator (Central African Republic) during March–May (MAM) were identified. Further analysis in these regions focused on the seasonal cycle of the lifting condensation level (LCL) and the convective triggering potential (CTP), and the sensitivity of CTP and near-surface dewpoint depressions HIlow to anomalous soil moisture. From these analyses, it is apparent that atmospheric mechanisms impose a strong constraint on the effect of soil moisture on the regional hydrological cycle.

This study examines 6 yr of cloud properties observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra satellite in five prominent marine boundary layer (MBL) cloud regions (California, Peru, Canary, Angola, and Australia) and investigates their relationships with near-surface meteorological parameters obtained from NCEP reanalyses. About 62 000 independent scenes are used to examine the instantaneous relationships between cloud properties and meteorological parameters that may be used for global climate model (GCM) diagnostics and parameterization. Cloud liquid water path (LWP) generally increases with lower-tropospheric stability (LTS) and lifting condensation level (LCL), whereas cloud drizzle frequency is favored by weak LTS and negligible cold air advection. Cloud fraction (CF) depends strongly on variations in LTS, and to a lesser extent on surface air temperature advection and LCL, although the relationships vary from region to region. The authors propose capturing the effects of these three parameters on CF via their linear combination in terms of a single parameter, the effective lower-tropospheric stability (eLTS). Results indicate that eLTS offers a marked improvement over LTS alone in explaining the median CF variations within the different study regions. A parameterization of CF in terms of eLTS is provided, which produces results that are improved over those of Klein and Hartmann’s LTS-only parameterization. However, the new parameterization may not predict the observed variability correctly, and the authors propose a method that might address this shortcoming via a statistical approach.

This paper examines the performances of various cumulus convective parameterization schemes in the tropical atmosphere using an aqua-planet atmospheric General Circulation Model forced by zonally symmetric but latitudinally varying sea surface temperature (SST) and solar angle. The intertropical convergence zone (ITCZ) is represented by intense precipitation. The assigned Control experiment with a specific SST distribution, as designated by the Aqua Planet Experiment, yields a single ITCZ when Zhang’s scheme or Manabe’s scheme is employed, whereas a double ITCZ occurs when Tiedtke’s scheme is used. The key to the occurrence of a double ITCZ is latitudinal variation in evaporation within the boundary layer. Such variation is induced mainly by latitudinal variation in the zonal wind speed, with the existence of a calm belt at the equator and a maximum wind speed located off the equator, arising from the evaporation–wind feedback (EWF) mechanism. The latitudinal distribution of evaporation results in a decrease in the height of the lifting condensation level in areas off the equator and an increase at the equator. The occurrence of a single ITCZ in Zhang’s scheme is attributed to the use of a Convective Available Potential Energy criterion by which convection occurs more readily at the equator. As a result, a precipitation maximum is maintained at the equator via a prevailing Conditional Instability of the Second Kind mechanism.

Edwards and Thompson have made comprehensive and thorough comments concerning the methods, ac¬curacy, and results in a case study by Kellenbenz et al. These comments questioned the representativeness of model-derived soundings and graphics used, as well as the modification methods employed with soundings presented in the 18 July 2004 case study. Other issues included the application of previous database studies to lifting condensation level (LCL) values found in the 18 July 2004 examination, as well as a focus that em¬phasized a single tornadic storm. In this response, the authors address problems and oversights with the 18 July 2004 case study, using new data to conduct a reanalysis of the environment on that evening. Additionally, a large independent database of Rapid Update Cycle (RUC) analysis soundings associated with supercell tornadoes is used to provide the context from which to carefully evaluate LCL results from the 18 July 2004 case relative to significant and violent tornadoes. Results indicate that although the LCL height associated with the F4 tornadic supercell in the original case study was probably overestimated, the background LCL environment was still unusually high for a violent tornado. New material presented in this response reinforces the conclusion that, when LCL heights are at the far upper end of empirical study distributions associated with significant tornadoes, undue weight should not be given to LCL as a tornado probability reduction factor when CAPE-storm relative helicity (SRH) combinations and deep-layer shear are also strong.

To begin exploring the underlying mechanisms that couple vegetation to cloud formation processes, we derive the lifting condensation level (LCL) to estimate cumulus cloud base height. Using a fully coupled land–ocean–atmosphere general circulation model (HadCM3LC), we investigate Amazonian forest feedbacks on cloud formation over three geological periods; modern-day (a.d. 1970–1990), the last glacial maximum (LGM; 21 kya), and under a future climate scenario (IS92a; a.d. 2070–2090). Results indicate that for both past and future climate scenarios, LCL is higher relative to modern-day. Statistical analyses indicate that the 800 m increase in LCL during the LGM is related primarily to the drier atmosphere promoted by lower tropical sea surface temperatures. In contrast, the predicted 1,000 m increase in LCL in the future scenario is the result of a large increase in surface temperature and reduced vegetation cover.

The linkages between soil moisture dynamics and convection triggers, defined here as the first crossing between the boundary layer height (hBL) and lifting condensation level (hLCL), are complicated by a large number of interacting processes occurring over a wide range of space and time scales. To progress on this problem, a soil–plant hydrodynamics model was coupled to a simplified ABL budget to explore the feedback of soil moisture on convection triggers. The soil–plant hydraulics formulation accounted mechanistically for features such as root water uptake, root water redistribution, and midday stomatal closure, all known to affect diurnal cycles of surface fluxes and, consequently, ABL growth. The ABL model considered the convective boundary layer as a slab with a discontinuity at the inversion layer. The model was parameterized using the wealth of data already collected for a maturing Loblolly pine plantation situated in the southeastern United States. A 30-day dry-down simulation was used to investigate the possible feedback mechanisms between soil moisture and convective rainfall triggers. Previous studies, which made use of surface flux measurements to drive an ABL model, have postulated that a negative feedback was possible, which could award the ecosystem with some degree of self-regulation of its water status. According to model simulation results here, this negative feedback is unlikely. However, drastic changes in external water sources to the ABL are needed for triggering convection when soil moisture is depleted. The apparent negative feedback originated from a decoupling between the water vapor sources needed to produce convection triggers and surface water vapor fluxes.

The effects of upstream relative humidity (RH) on low-level wind and precipitation patterns for low-speed, statically stable flows over a mountain are investigated using idealized two- and three-dimensional numerical-simulation experiments in which RH is increased from 0% to 100%. For RH less than some critical threshold, the flow upstream becomes less decelerated as RH is increased; for RH greater than this threshold, the flow upstream becomes more decelerated as RH is increased. This increasing deceleration with RH is due to locally enhanced static stability resulting from enhanced condensation near the freezing level. Analyses from the simulations indicate that the lifted condensation level and the height of the freezing level are significant control parameters for the upstream-flow deceleration in the steady-state solutions. Dimensional analysis using these control parameters (as well as others) brings forth new nondimensional parameters that are shown to enter into analytic formulas for the orographic upstream-flow deceleration in a moist atmosphere.

Continental fair-weather cumuli exhibit significant diurnal, day-to-day, and year-to-year variability. This study describes the climatology of cloud macroscale properties, over the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site. The diurnal cycle of cloud fraction, cloud-base height, cloud-top height, and cloud thickness were well defined. The cloud fraction reached its maximum value near 1400 central standard time. The average cloud-base height increased throughout the day, while the average cloud thickness decreased with time. In contrast to the other cloud properties, the average cloud-chord length remained nearly constant throughout the day. The sensitivity of the cloud properties to the year-to-year variability of precipitation and day-to-day changes in the height of the lifting condensation level (zLCL) and surface fluxes were compared. The cloud-base height was found to be sensitive to both the year, zLCL, and the surface fluxes of heat and moisture; the cloud thickness was found to be more sensitive to the year than to zLCL; the cloud fraction was sensitive to both the low-level moisture and the surface sensible heat flux; and cloud-chord length was sensitive to zLCL. Distributions of the cloud-chord length over the ACRF SGP site were computed and were well fit by an exponential distribution. The contribution to the total cloud fraction by clouds of each cloud-chord length was computed, and it was found that the clouds with a chord length of about 1 km contributed most to the observed cloud fraction. This result is similar to observations made with other remote sensing instruments or in modeling studies, but it is different from aircraft observations of the contribution to the total cloud fraction by clouds of different sizes.

The present paper presents an investigation of water vapor's condensation to liquid substance in highly convective flow conditions. An airfoil geometry was chosen to demonstrate the applicability of the model developed. The flow is considered subsonic and compressible, at high Reynolds number. The contribution of turbulence effects is accounted for by the Spalart–Allmaras model, which is suitable for such type applications. The study of condensation is based on a mixture two-phase model, which allows for interpenetrating substances, while following the economical single-fluid approach. Furthermore, the phases may move at different velocities (slip velocity) in a manner that is described by conservation equations. Results are presented for several flight conditions, concerning the angle of attack and the ambient-air humidity levels. Since the condensation effects are investigated on an airfoil configuration, emphasis has been given to the liquid mass-fraction distribution in several critical areas, where considerable formation of liquid droplets is observed. Results show that for humidity levels higher than 0.7, high liquid concentration areas are indicated onto the rear part of the foil. Attention is focused on the dispersion of the condensation cloud and on the areas of mass-transfer interaction between liquid droplets and humid air.

A two-dimensional cloud model with bin microphysics was used to investigate the effects of cloud condensation nuclei (CCN) concentrations and thermodynamic conditions on convective cloud and precipitation developments. Two different initial cloud droplet spectra were prescribed based on the total CCN concentrations of maritime (300 cm − 3 ) and continental (1000 cm − 3 ) air masses, and the model was run on eight thermodynamic conditions obtained from observational soundings. Six-hourly sounding data and 1-hourly precipitation data from two nearby weather stations in Korea were analyzed for the year 2002 to provide some observational support for the model results. For one small Convective Available Potential Energy (CAPE) (∼ 300 J kg − 1 ) sounding, the maritime and continental differences were incomparably large. The crucial difference was the production of ice phase hydrometeors in the maritime cloud and only water drops in the continental cloud. Ice phase hydrometeors and intrinsically large cloud drops of the maritime cloud eventually lead to significant precipitation. Meanwhile negligible precipitation developed from the continental cloud. For the three other small CAPE soundings, generally weak convective clouds developed but the maritime and continental clouds were of the same phases (both warm or both cold) and their differences were relatively small. Model runs with the four large CAPE (∼ 3000 J kg − 1 ) soundings demonstrated that the depth between the freezing level (FL) and the lifting condensation level (LCL) was crucial to determine whether a cloud becomes a cold cloud or not, which in turn was found to be a crucial factor to enhance cloud invigoration with the additional supply of freezing latent heat. For two large CAPE soundings, FL–LCL was so deep that penetration of FL was prohibitive, and precipitation was only mild in the maritime clouds and negligible in the continental clouds. Two other soundings of similarly large CAPE had small FL–LCL, and both the maritime and continental clouds became cold clouds. Precipitation was strong for both but much more so in the maritime clouds, while the maximum updraft velocity and the cloud top were slightly higher in continental clouds. Although limited to small CAPE cases, more precipitation for smaller FL–LCL for a selected group of precipitation and thermodynamic sounding data from Korea was in support of these model results in its tendency. These results clearly demonstrated that the CCN effects on cloud and precipitation developments critically depended on the given thermodynamic conditions and not just the CAPE but the entire structure of the thermodynamic profiles had to be taken into account.

On the evening of 18 July 2004, several tornadoes occurred with two supercell thunderstorms over eastern North Dakota. The second and smaller in diameter of these storms produced an F4 tornado in an environment with lifting condensation level (LCL) heights that were atypically high according to recent statistical studies about supercell tornado environments. Surface dewpoints were also underforecast by computer models. These two issues are examined in this paper, which provides an overview of this event. The synoptic setting and environment characteristics suggest that evapotranspiration (ET) was responsible in part for enhancing surface moisture. It is likely that ET affected instability and convection initiation. This study also found that the presence of steep low-level lapse rates juxtaposed with low-level convective available potential energy along a surface trough may have contributed to tornado development in a high LCL environment where wind and instability characteristics were otherwise favorable for supporting supercell tornadoes.

Using satellite cloud data from the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) and collocated precipitation rates from the Advanced Microwave Scanning Radiometer (AMSR), it is shown that rain rate is closely related to the amount of very thick high cloud, which is a better proxy for precipitation than outgoing longwave radiation (OLR). It is also shown that thin high cloud, which has a positive net radiative effect on the top-of-atmosphere (TOA) energy balance, is nearly twice as abundant in the west Pacific compared to the east Pacific. For a given rain rate, anvil cloud is also more abundant in the west Pacific. The ensemble of all high clouds in the east Pacific induces considerably more TOA radiative cooling compared to the west Pacific, primarily because of more high, thin cloud in the west Pacific. High clouds are also systematically colder in the west Pacific by about 5 K. The authors examine whether the anvil cloud temperature is better predicted by low-level equivalent potential temperature (ΘE), or by the peak in upper-level convergence associated with radiative cooling in clear skies. The temperature in the upper troposphere where ΘE is the same as that at the lifting condensation level (LCL) seems to influence the temperatures of the coldest, thickest clouds, but has no simple relation to anvil cloud. It is shown instead that a linear relationship exists between the median anvil cloud-top temperature and the temperature at the peak in clear-sky convergence. The radiatively driven clear-sky convergence profiles are thus consistent with the warmer anvil clouds in the EP versus the WP.

A one-dimensional numerical model of a warm convective cloud is presented. This model is used to study the effectiveness of the action on the cloud by hygroscopic particles with the aim of intensifying precipitation. The numerical simulation takes into account the processes of condensation, coagulation, and sedimentation of cloud droplets and makes it possible to obtain spatiotemporal characteristics of cloud development. A system of differential equations describing time variations in the temperature, pressure, and watervapor supersaturation during the adiabatic rise of a continuous air flow is solved. The evolution of the size distribution of cloud droplets is described by a kinetic equation. A continuous acting source of droplets with the size distribution calculated with consideration for condensation properties and dispersion characteristics of condensation nuclei (natural and additionally introduced during the action) is specified at the cloud-base level. The cloud top is formed owing to the evaporation of droplets in the barrier atmospheric layer over the cloud. The influence of changes in the barrier-layer height on the structure of cloud parameters and precipitation-formation processes is analyzed. The introduction of additional hygroscopic particles into a cloud is shown to act as a trigger mechanism initiating the processes of coagulation and sedimentation in the cloud medium. In this case, a positive effect of action by fine particles can be achieved if a certain reserve of sufficiently large droplets is present in the cloud. The results of calculating the dependence of the action effect on the height of the barrier layer, restricting cloud development, are presented.

Based on results from a three-dimensional cloud-resolving model, it is shown that simulated convective storm motions are affected by thermodynamic as well as kinematic properties of the environment. In addition to the mean wind and its vertical shear, the effect on isolated storm motion of parameters such as bulk convective available potential energy (CAPE), the vertical distribution of buoyancy in the profile, the heights of the lifting condensation level (LCL) and level of free convection (LFC), and cloud-base temperature is considered. Storm motions show at least some sensitivity to all input parameters. Consistent with previous studies, hodograph radius has the most pronounced effect, but the vertical distribution of shear (which also influences the mean wind) affects storm evolution and propagation, even when the effective hodograph radius is unchanged. Among the thermodynamic parameters, the most significant variations occur when the LCL–LFC configuration is modified or when cloud-base temperature is changed. The effects of increases in bulk CAPE act mainly to increase the temporal variability of storm motions. This temporal variability is found to consist both of oscillations about a mean state and trends (accelerations) and is related to increases in the complexity of storm evolution with increasing CAPE. The results point to the importance of environmental factors that enhance storm intensity and rotation, which play a key role in determining storm deviate motion.

Cumulus formation and convection initiation are examined near a cold front–dryline “triple point” intersection on 24 May 2002 during the International H2O Project (IHOP). A new Lagrangian objective analysis technique assimilates in situ measurements using time-dependent Doppler-derived 3D wind fields, providing output 3D fields of water vapor mixing ratio, virtual potential temperature, and lifted condensation level (LCL) and water-saturated (i.e., cloud) volumes on a subdomain of the radar analysis grid. The radar and Lagrangian analyses reveal the presence of along-wind (i.e., longitudinal) and cross-wind (i.e., transverse) roll circulations in the boundary layer (BL). A remarkable finding of the evolving radar analyses is the apparent persistence of both transverse rolls and individual updraft, vertical vorticity, and reflectivity cores for periods of up to 30 min or more while moving approximately with the local BL wind. Satellite cloud images and single-camera ground photogrammetry imply that clouds tend to develop either over or on the downwind edge of BL updrafts, with a tendency for clouds to elongate and dissipate in the downwind direction relative to cloud layer winds due to weakening updrafts and mixing with drier overlying air. The Lagrangian and radar wind analyses support a parcel continuity principle for cumulus formation, which requires that rising moist air parcels achieve their LCL before moving laterally out of the updraft. Cumuli form within penetrative updrafts in the elevated residual layer (ERL) overlying the moist BL east of the triple point, but remain capped by a convection inhibition (CIN)-bearing layer above the ERL. Dropsonde data suggest the existence of a convergence line about 80 km east of the triple point where deep lifting of BL moisture and locally reduced CIN together support convection initiation.

The pressure distribution in a squeeze film air bearing depends on the water vapor pressure and the saturation vapor pressure of water at ambient temperature. When the water vapor pressure is compressed beyond the saturation vapor pressure, a portion of the water vapor condenses, thus reducing the total gas pressure in the air bearing. This condensation mechanism thus reduces the flying height of the slider. Using a magnetic recording system for experiments, we quantified the effect of condensation on flying height by measuring the dependence of magnetic spacing on relative humidity. We developed a numerical model based on the proposed mechanism that matches the experimental data and provides a detailed view of the air-bearing response to humidity.

A new Lagrangian analysis technique is developed to assimilate in situ boundary layer measurements using multi-Doppler-derived wind fields, providing output fields of water vapor mixing ratio, potential temperature, and virtual potential temperature from which the lifting condensation level (LCL) and relative humidity (RH) fields are derived. The Lagrangian analysis employs a continuity principle to bidirectionally distribute observed values of conservative variables with the 3D, evolving boundary layer airflow, followed by temporal and spatial interpolation to an analysis grid. Cloud is inferred at any grid point whose height z > zLCL or equivalently where RH ≥ 100%. Lagrangian analysis of the cumulus field is placed in the context of gridded analyses of visible satellite imagery and photogrammetric cloud-base area analyses. Brief illustrative examples of boundary layer morphology derived with the Lagrangian analysis are presented based on data collected during the International H2O Project (IHOP): 1) a dryline on 22 May 2002; 2) a cold-frontal–dryline “triple point” intersection on 24 May 2002. The Lagrangian analysis preserves the sharp thermal gradients across the cold front and drylines and reveals the presence of undulations and plumes of water vapor mixing ratio and virtual potential temperature associated with deep penetrative updraft cells and convective roll circulations. Derived cloud fields are consistent with satellite-inferred cloud cover and cloud-base locations.

Several east–west-oriented bands of clouds and light rain formed on 20 July 2005 over eastern Montana and the Dakotas. The cloud bands were spaced about 150 km apart, and the most intense band was about 20 km wide and 300 km long, featuring areas of maximum radar reflectivity factor of about 50 dBZ. The cloud bands formed poleward of an area of lower-tropospheric frontogenesis, where air of modest convective available potential energy was being lifted. During initiation and maintenance of the bands, mesoscale regions of dry symmetric and inertial instability were present in the region of the bands, suggesting a possible mechanism for the banding. Interpretation of the extant instabilities in the region of the bands was sensitive to the methodology to assess the instability. The release of these instabilities produced circulations with enough vertical motion to lift parcels to their lifting condensation level, resulting in the observed cloud bands. A high-resolution, numerical weather prediction model demonstrated that forecasting these types of events in such real-time models is possible, although the timing, evolution, and spacing of the bands were not faithfully reproduced. This case is compared to two previous cases in the literature where banded convection was associated with a combination of conditional, symmetric, and inertial instability.

The problem of condensation of water vapor from humid air flowing in a duct in turbulent flow is formulated theoretically. Vapor condensing at the dew-point temperature of the vapor-air mixture diffuses to the wall of the duct through an air film. The flow of the condensate is laminar. The condensing vapor releases both convection and latent heats to the wall of the duct. Thus, it is treated as a combined heat and mass transfer problem. The mass, momentum, and energy balance equations for the vapor-air mixture flowing in the duct and the diffusion equation for the vapor species are considered. Ti, the temperature at gas-to-liquid interface, at which condensation takes place, is estimated with the help of the heat balance and mass balance equations at interface. The local and average values of the condensation Nusselt number, condensate Reynolds number, gas-liquid interface temperature, and pressure drop are estimated from the numerical results for different values of the system parameters, such as relative humidity and temperature of air at inlet, gas phase Reynolds number, and total pressure at inlet. The gas phase convection Nusselt and Sherwood numbers are also computed. A comparison of the present work with experimental data, for the case of in-tube condensation of vapor from humid air, shows satisfactory agreement.

On clear, calm summer days, cloud lines are often observed above the expanding urban area along the railroads from Tokyo. The cloud lines were simulated using a numerical model with a simplified urban surface parameterization. The horizontal distributions of the simulated clouds agree well with the observed clouds in satellite images. Some sensitivity tests indicate that the urban thermal effect enhances cloud formation but suppresses clouds in the adjacent rural area. The simulated clouds are consistent with the observations when the urban maximum sensible heat flux is substantially larger than the rural one. With a reduction of the thermal contrast, cloud suppression in the rural area is gradually weakened. The mechanisms of the cloud contrast are as follows: 1) Thermals form in the mixed layer above the land surface. 2) Because of the larger sensible heat flux, the mixed layer tends to be higher in the urban area; thus, thermals reach the lifting condensation level more easily than they do in the rural area. 3) The wide compensating downdrafts of the strong urban thermals cover the entire rural area and suppress thermals and clouds there.

This analysis uses long time-series of daily mean data from the three BERMS flux sites in central Saskatchewan to explore biases in ERA-40 at a close-by grid-point, and to study the relationships between surface variables and fluxes and cloud cover in the observed and model data sets. On the seasonal timescale the biases in ERA-40 of temperature and humidity are small, but the model has a high bias in evaporation, and except in mid-summer a low bias of reflective cloud, which gives a high bias in the surface downward net shortwave flux. In summer, on days with high observed cloud cover, the model has too little cloud, and a corresponding high sensible heat flux and a warm, dry bias; while on summer days when observed cloud is lower, the model biases are generally reversed. The internal relationships between near-surface relative humidity, linked to the mean lifting condensation level, cloud cover and the surface radiation fluxes are however very similar in model and data. Although cause and effect cannot be determined from these biases alone, given the complexity of the interactions, they suggest possible errors in both the land-surface model and the atmospheric cloud parameterizations.

Water is always present in the atmospheric air in the form of vapour. As air of low temperature expands through the turbine condensation may occur. Condensation gives rise to several problems in materialization of the air cycle as it results in temperature rise thus to different conditions than the expected at the turbine exit. Also in most air cycle applications, liquid water must be removed from the air stream before it gets to the cooled space. In the case that turbine exit temperature is very low; there is the possibility of ice particles formation. A method allowing the evaluation of air properties when condensation occurs is required in order to examine its effect on the cycle and optimize the operating envelope. The present paper describes a thermodynamic equilibrium method for predicting the occurrence of condensation and calculating the mixture properties once condensation has occurred. The method has been validated against the experimental results from a turbocharger turbine. The experimental results show that condensation can cause significant alteration to the turbine exit conditions. It is demonstrated that condensation can be predicted and the mixture properties after condensation can also be accurately evaluated.

Sounding parameters are examined to determine whether they can help distinguish between Alberta, Canada, severe thunderstorms that spawn significant tornadoes (F2–F4), weak tornadoes (F0–F1), or nontornadic severe storms producing large hail. Parameters investigated included storm-relative helicity (SRH), precipitable water (PW), and storm convergence. The motivation for analyzing these parameters is that, in theory, they might affect the rate of change of vertical vorticity generation through vortex stretching, vortex tilting, and baroclinic effects. Precipitable water showed statistically significant differences between significant tornadic storms and those severe storms that produced weak tornadoes or no tornadoes. All significant tornadic cases in the dataset used had PW values exceeding 22 mm, with a median value of 24 mm. Values of PW between 19 and 23 mm were generally associated with weak tornadic storms. Computed values of storm convergence, height of the lifted condensation level, and normalized most unstable CAPE did not discriminate between any of the three storm categories. The SRH showed discrimination of significant tornadoes from both weak tornadic and nontornadic severe storm groups. The Alberta data suggest that significant tornadoes tended to occur with SRH > 150 m2 s−2 computed for the 0–3-km layer whereas weak tornadoes were typically formed for values between 30 and 150 m2 s−2. Threshold values of SRH were lower than those suggested in studies based on storm observations throughout much of the United States.

We predicted the spatial pattern of hourly probability of cloud cover in the Luquillo Experimental Forest (LEF) in North-Eastern Puerto Rico using four different models. The probability of cloud cover (defined as “the percentage of the area covered by clouds in each pixel on the map” in this paper) at any hour and any place is a function of three topographic variables: aspect, slope and the difference between elevation and lifting condensation level. We chose the best models based on multiple statistics including the Akaike Information Criterion (AIC), scaled deviance and extra-dispersion scale. As a result, the generalized linear model (GLM) and one generalized linear mixed model (GLMM) with exponential spatial structure were the best candidate models. The probabilities of cloud cover in both our simulations and the observations increased with elevation, and were higher at night. They decreased in the morning after the sun rose until early afternoon, and then increased again for the rest of the day until night, apparently in response to the movement of the lifting condensation level. Two types of satellite images were available to calibrate our models: the higher spatial resolution, but expensive and infrequent Landsat-7 Enhanced Thermal Mapper plus (ETM+) images and the frequent, free, but low spatial resolution Moderate Resolution Imaging Spectroradiometer (MODIS) images. The derived probabilities of cloud cover when calibrated to the two types of remote sensing images were very similar, which justifies our using the free MODIS images instead of the Landsat images to calibrate the models. We applied the model to all months and the results indicated in agreement with the data that the probability of cloud cover is less during the dry season, higher in wet season and moderate for the rest of the year. We found our models could usually predict the probability of cloud cover for each 100 m in elevation level at a certain time with an index of agreement (IoA) of 0.560–0.919 and at a certain location over a day with an IoA of 0.940–0.994, indicating a medium to good model simulation at that particular time or location.

The present study reports on an investigation into the dependence of the surface temperature of metal plates on environmental parameters. Monitored plates were exposed at three locations (one coastal and two inland) in southern Australia. The following parameters were monitored: surface temperature, surface wetness, wind speed, ambient relative humidity, air temperature and lifting condensation level. (The lifting condensation level is the height at which a parcel of air rising from the ground will reach saturation.) An analysis was undertaken of the temperature difference between the exposed plate surfaces and ambient air. It was found that the maximum temperature difference during the day depended on season, wind speed and, to a lesser extent, the lifting condensation level. The maximum undercooling during the night showed a relatively limited range and a weak dependence on wind speed. The rate of rise in temperature difference after sunrise was found to be only slightly dependent on season, but...

During 9–11 November 1998 and 9–10 March 2002, two similar convective lines moved across the central and eastern United States. Both convective lines initiated over the southern plains along strong surface-based cold fronts in moderately unstable environments. Both lines were initially associated with cloud-to-ground (CG) lightning, as detected by the National Lightning Detection Network, and both events met the criteria to be classified as derechos, producing swaths of widespread damaging wind. After moving into areas of marginal, if any, instability over the upper Midwest, CG lightning production ceased or nearly ceased, although the damaging winds continued. The 9 March 2002 line experienced a second phase of frequent CG lightning farther east over the mid-Atlantic states. Analysis of these two events shows that the production of CG lightning was sensitive to the occurrence and vertical distribution of instability. Periods with frequent CG lightning were associated with sufficient instability within the lower mixed-phase region of the cloud (i.e., the temperature range approximately between −10° and −20°C), a lifting condensation level warmer than −10°C, and an equilibrium level colder than −20°C. Periods with little or no CG lightning possessed limited, if any, instability in the lower mixed-phase region. The current Storm Prediction Center guidelines for forecasting these convective lines are presented.

A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature (θυ) and water-vapor mixing ratio (r), as well as the mean vertical profiles of θυ, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as noncloud updrafts. This model has several unique features: 1) cloud cover is determined from the boundary layer JPDF of θυ versus r, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed. Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on Barbados Oceanographic and Meteorological Experiment (BOMEX), it is shown that the CuP model does a good job predicting cloud-base height and cloud-top height. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model.

The system must get across an energy peak of unstable equilibrium during the condensation of pure vapor; as the supersaturated extent of vapor increases and the temperature decreases, the energy peak shortens and vapor condensation becomes easier. The system must get across an energy peak of unstable equilibrium first, and then get into an energy valley of stable equilibrium during the condensation of impure vapor; as the partial pressure of vapor decreases, the energy peak becomes taller, the energy valley more shallow, vapor condensation becomes more difficult and liquid evaporation becomes easier; when the partial pressure of vapor decreases to a certain extent, the energy peak and the energy valley combine into one, and vapor condensation becomes impossible.

The relative humidity (RH) and the dewpoint temperature (td) are two widely used indicators of the amount of moisture in air. The exact conversion from RH to td, as well as highly accurate approximations, are too complex to be done easily without the help of a calculator or computer. However, there is a very simple rule of thumb that can be very useful for approximating the conversion for moist air (RH > 50%), which does not appear to be widely known by the meteorological community: td decreases by about 1°C for every 5% decrease in RH (starting at td= t, the dry bulb temperature, when RH = 100%). This article examines the mathematical basis and accuracy of this and other relationships between the dewpoint and relative humidity. Several useful applications of the simple conversion are presented, in particular the computation of the cumulus cloud-base level (or lifting condensation level) as zLCL >> (20 + t/5)(100 – RH), where zLCL is in meters when t is in degrees Celcius and RH in percent. Finally, a his...

A special observation campaign (X-BAIU), using various instruments (wind profilers, C-band weather radars, X-band Doppler radars, rawinsondes, etc.), was carried out in Kyushu (western Japan) during the Baiu season, from 1998 to 2002. In the X-BAIU-99 and -02 observations, a line-shaped orographic rainband extending northeastward from the Koshikijima Islands appeared in the low-level strong wind with warm-moist airs. The weather radar observation indicated that the rainband was maintained for 11h. The maximum length and width of the rainband observed in 1999 was ~200km and ~20km, respectively. The rainband observed in 2002 was not so developed compared with the case in 1999. The Froude number averaged from sea level to the top of the Koshikijima Islands (~600m) was large (>1), and the lifting condensation level was below the tops of the Koshikijima Islands. Thus, it is suggested that the clouds organizing the rainband are formed by the triggering of the mountains on the airflow passing over them. The vertical profile of horizontal wind in/around the rainband was investigated in the wind profiler observations. In the downdraft region 60km from the Koshikijima Islands, strong wind and its clockwise rotation with increasing height was observed below 3km altitude. In addition, a strong wind component perpendicular to the rainband was observed when the rainband was well developed. These wind behaviors were related to the evolution of the rainband.

A cold-air outbreak east of the Rocky Mountains on 23 January 2003 produced banded clouds and snow across the central and southeastern United States. The bands occurred through two processes: 1) thermal instability in the planetary boundary layer produced horizontal convective rolls (HCRs) over widespread areas, and 2) lake-effect processes downstream of small lakes (fetch < 100 km) produced localized bands. Characteristics of the observed bands associated with the HCRs, such as horizontal scale, depth of circulation, orientation, duration, and dynamics, are explored through observations, previous literature, and theoretical models. Snow from clouds produced by HCRs over land during the cold season has not been extensively studied previously. In this event, cold-air advection over the warm ground led to an upward sensible heat flux, promoting the occurrence of the HCR circulations. As the surface temperature decreased, the height of the lifting condensation level decreased, eventually forming clo...

A line-shaped convective system (LCS) was observed on 19 t h June 2001 in southern Kyushu, Japan in a field experiment called X-BAIU-01. Operational meteorological radar observed a mesoscale line of precipitation in this experiment. A detailed analysis of the radar observation indicated that this line of precipitation had a structure of the LCS, which is a type of mesoscale convective system (MCS). The convective cells in the LCS propagated east-northeastward at the speed of 25 m s - 1 . The LCS formed between two wind profilers. "Dual wind profiler" analysis illustrates mesoscale-flow patterns upstream of the LCS, such as horizontal convergence with upward airflow above 500 m in altitude, and divergence with downward airflow underneath. Moreover, an integrated analysis of the dual wind profiler, operational weather radar and radiosonde measurements revealed the formation of the LCS as follows. The surface atmosphere from the ocean climbs a mountain slope in southern Kyushu up to the lifting condensation level (LCL) by environmental wind, and forms a stratus cloud. The atmosphere continues climbing the mountain slope up to the height of a convergence layer aloft. The strong updraft in the convergence layer changes the type of cloud from stratus to cumulus at this height, and increases precipitation intensity. The atmosphere continues ascending up to the level of free convection (LFC) in the updraft region. After reaching the LFC, the atmosphere increases the ascending speed by buoyancy inside, and develops convective cells. A number of cells are advected by the environmental wind and form the LCS. This process indicates that this type of the LCS does not have a back-building structure, which is one type of the LCS, but has a terrain-induced structure. A refined conceptual model of this type of the LCS is presented based on previous studies and observations reported in this paper. The analysis also indicates that the isolated mountains or islands upstream are not essential for the formation of this type of the LCS, because the profile of the atmosphere does not satisfy the conditions necessary to trigger convections or to excite lee waves by the mountains or islands.

Since 1997, rainfall has been measured by using tipping-bucket rain gauges in a mountainous area as a part of GAME-Tropics (the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME)). Orographic precipitation tends to occur when easterly or southeasterly wind is predominated, wind speed is relatively strong (Average: 4.6m/s, S. D.: 2.1m/s), and the lifted condensation level is low. By investigating orographic precipitation using a regional atmospheric model under various atmospheric conditions, 1) comparatively strong monsoon westerly in the lower troposphere and complex terrain and 2) moist condition in the lower atmosphere including evapotranspiration from the wet ground are significant factors to the spatial pattern of precipitation in the mountainous region.

The precipitation efficiencies for mesoscale convective systems (MCS) over the central United States are calculated. During July–September 2000 and June–September 2001, 24 MCS for which sufficient data were available occurred over or near Missouri. Precipitable water fields from the hourly Rapid Update Cycle (RUC) and radar-derived precipitation grids are used to calculate the precipitation efficiency. Geostationary Operational Environmental Satellite soundings and RUC winds are used to assess the pre-MCS environment. Statistical analysis reveals that precipitation efficiency has a relatively strong positive correlation with the relative humidity in the layer between the surface and the lifting condensation level; significant negative correlations are found between the precipitation efficiency and both the convective inhibition and the environmental wind shear. The latter, inverse relationship between shear and precipitation efficiency supports the findings of previous investigators.

Monthly dispersion characteristics for air quality modeling over the South China Sea offshore the west coast of Borneo are studied using long-term ship measurements. It is found from monthly averages that the stability condition is nearly neutral throughout the year with the exception of April, May, and November which are slightly unstable. The lifting condensation level ranged from 338 to 450 m. The lowest value of the ventilation factor occurred in April and the highest in January. The friction velocity for each month is also provided to determine the vertical eddy diffusivity and horizontal and vertical dispersion coefficients.

Taking account of the variation of the water vapor content and the temperature in the air parcel, a new model for the estimation of the cloud droplet size, the temperature and the saturation ratio has been developed by modifying traditional Kohler model for the equilibrium state. The modification of Kohler model is based on the mass and the heat conservation laws for the water (vapor and liquid) in the air parcel. Variations of size, temperature and saturation ratio with number density of CCN are simulated numerically. The results of simulations show that the temperature variation of the cloud droplet and air parcel is so small to change the values of physical properties but large enough to make the air parcel unstable (or convective) in the atmosphere.

Approximately 400 Automated Surface Observing System (ASOS) observations of convective cloud-base heights at 2300 UTC were collected from April through August of 2001. These observations were compared with lifting condensation level (LCL) heights above ground level determined by 0000 UTC rawinsonde soundings from collocated upper-air sites. The LCL heights were calculated using both surface-based parcels (SBLCL) and mean-layer parcels (MLLCL—using mean temperature and dewpoint in lowest 100 hPa). The results show that the mean error for the MLLCL heights was substantially less than for SBLCL heights, with SBLCL heights consistently lower than observed cloud bases. These findings suggest that the mean-layer parcel is likely more representative of the actual parcel associated with convective cloud development, which has implications for calculations of thermodynamic parameters such as convective available potential energy (CAPE) and convective inhibition. In addition, the median value of surface-based CAPE (SBCAPE) was more than 2 times that of the mean-layer CAPE (MLCAPE). Thus, caution is advised when considering surface-based thermodynamic indices, despite the assumed presence of a well-mixed afternoon boundary layer.

In the present paper, the effects of inlet pressure on the heat and mass transfer rates of an air cooler are numerically predicted by a local analysis method. The pressures of the moist air vary from 2 to 4 bars. The psychrometric properties such as dew point temperature, relative humidity and humidity ratio are employed to treat the condensing water vapor in the moist air when the surface temperatures are dropped below the dew point. The effects of the inlet pressures on the heat transfer rate, the dew point temperature, the rate of condensed water, the outlet temperature of air and cooling water are calculated. The condensation process of water vapor is discussed in detail. The results of present calculations are compared with the test data and shows good agreements.

Cloud droplets grow on cloud condensation nuclei (CCN) with condensing the ambient water vapor in the atmospheric air parcel. The size of the cloud droplet in the equilibrium state is estimated usually by Kohler Equation. However, Kohler Equation is based on the assumption that the cloud droplet grows in an infinitely large reservoir of water vapor at constant pressure, i. e., at constant saturation ratio. This assumption causes some erroneous results in the estimation of real size in the air parcel, although the assumption makes the calculation easy. There are two apparent deficiencies in Kohler Equation. One is that the equilibrium size cannot be decided in case of the larger saturation ratio than the critical ratio. The other is that the variation of the size with the number density of CCN, i. e., the competitive growth, cannot be considered in the estimation of the size. In order to improve these deficiencies, the conventional Kohler Equation has been modified using the mass conservation of the water contained in the air parcel.

Acoustic oscillations of a gas parcel in a wet stack of a thermoacoustic prime-mover are numerically simulated with a Lagrangian approach taking into account Rott equations and the effect of non-equilibrium evaporation and condensation of water vapor at the stack surface. A gas parcel gradually drifts to higher and lower temperature side of a stack in traveling-wave and standing-wave prime-movers, respectively, owing to a shift of phase angle of particle’s velocity with a movement of a parcel according to Rott equations even without evaporation and condensation. It is an alternative acoustic streaming in a narrow tube with temperature gradient along a tube. In a traveling-wave prime-mover, the volume oscillation amplitude of a gas parcel always increases by evaporation and condensation, and pV work done by a gas parcel is accordingly enhanced. On the other hand, in a standing-wave prime-mover, the volume oscillation amplitude sometimes decreases by evaporation and condensation, and pV work is suppressed. ...