Cloud Bottom Research Articles

Effects of aerosol on evaporation, freezing and precipitation in a multiple cloud system

Aerosol effects on clouds and precipitation account for a large portion of uncertainties in the prediction of the future course of global hydrologic circulations and climate. As a process of a better understanding of interactions between aerosol, clouds and precipitation, simulations are performed for a mixed-phase convective multiple-cloud system over the tropics. Studies on single-cloud systems have shown that aerosol-induced increases in freezing, associated increases in parcel buoyancy and thus the intensity of clouds (or updrafts) are a main mechanism which controls aerosol–cloud–precipitation interactions in convective clouds. However, in the multiple-cloud system that plays much more important roles in global hydrologic circulations and thus climate than single-cloud systems, aerosol effects on condensation play the most important role in aerosol-induced changes in the intensity of clouds and the effects on freezing play a negligible role in those changes. Aerosol-induced enhancement in evaporation intensifies gust fronts and increases the number of subsequently developing clouds, which leads to the substantial increases in condensation and associated intensity of convection. Although aerosol-induced enhancement in freezing takes part in the increases in condensation by inducing stronger convergence around cloud bottom, the increases in condensation are ~one order of magnitude larger than those in freezing. It is found that while aerosol-induced increases in freezing create intermittent extremely heavy precipitation, aerosol-induced increases in evaporation enhance light and medium precipitation in the multiple-cloud system here. This increase in light and medium precipitation makes it possible that cumulative precipitation increases with increasing aerosol concentration, although the increase is small. It is interesting that the altitude of the maximum of the time- and domain-averaged hydrometeor mass densities is quite robust to increases in aerosol concentration. This is because locations of gust fronts and homogeneous freezing do not vary significantly with changing aerosol concentration and this outweighs aerosol effects on hydrometeor size.

Studies on Low Altitude Clouds Over New Delhi, India Using Lidar

This study reports the altitude distribution of physical and optical properties of clouds in the lower troposphere over the urban tropical region Delhi using an UV (355 nm) lidar which is capable of operating in both day and night time. Most of the low altitude clouds are observed above the planetary boundary layer during the observation period. The low altitude cloud bottom and top height varies between 0.58 ± 0.21 and 1.5 ± 0.61 km respectively during the observation period. The depolarization ratio of the observed clouds varies from 0.18 ± 0.01 to 1.2 ± 0.58. The role of the atmospheric region below the cloud in the growth process of the cloud cell is studied. Cloud turbulence is derived to show its role in maintaining the strength of the cloud.

The role of gravitational instabilities in deposition of volcanic ash

Volcanic ash is a significant hazard for areas close to volcanoes and for aviation. Gravitational instabilities forming at the bottom of spreading volcanic clouds have been observed in many explosive eruptions. Here we present the first quantitative description of the dynamics of such instabilities, and correlate this with the characteristics of the fall deposit from observations of the 4 May 2010 Eyjafjallajokull (Iceland) eruption. Gravitational instabilities initially took the form of downward-propagating fingers that formed continuously at the base of the cloud, and appeared to be advected passively at the crosswind speed. Measurements of finger propagation are consistent with initial conditions inferred from previous studies of ash cloud dynamics. Dedicated laboratory analogue experiments confirmed that finger downward propagation significantly exceeded the settling speed of individual particles, demonstrating that gravitational instabilities provide a possible mechanism for enhanced sedimentation of fine ash. Our observations challenge the view that aggregation is the primary explanation of proximal fine ash sedimentation, and give direct support for the role of gravitational instabilities in providing regions of high particle concentration that can promote aggregation.

Biological ice nucleation initiates hailstone formation

AbstractCloud condensation and ice nuclei in the troposphere are required precursors to cloud and precipitation formation, both of which influence the radiative balance of Earth. The initial stage of hailstone formation (i.e., the embryo) and the subsequent layered growth allow hail to be used as a model for the study of nucleation processes in precipitation. By virtue of the preserved particle and isotopic record captured by hailstones, they represent a unique form of precipitation that allows direct characterization of the particles present during atmospheric ice nucleation. Despite the ecological and economic consequences of hail storms, the dynamics of hailstone nucleation, and thus their formation, are not well understood. Our experiments show that hailstone embryos from three Rocky Mountain storms contained biological ice nuclei capable of freezing water at warm, subzero (°C) temperatures, indicating that biological particles can act as nucleation sites for hailstone formation. These results are corroborated by analysis of δD and δ18O from melted hailstone embryos, which show that the hailstones formed at similarly warm temperatures in situ. Low densities of ice nucleation active abiotic particles were also present in hailstone embryos, but their low concentration indicates they were not likely to have catalyzed ice formation at the warm temperatures determined from water stable isotope analysis. Our study provides new data on ice nucleation occurring at the bottom of clouds, an atmospheric region whose processes are critical to global climate models but which has challenged instrument‐based measurements.

Vertical structure of ice cloud layers from CloudSat and CALIPSO measurements and comparison to NICAM simulations

AbstractThe shape of the vertical profile of ice cloud layers is examined using 4 months of CloudSat and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) global measurements taken on January, April, July, and October 2007. Ice clouds are selected using temperature profiles when the cloud base is located above the 253 K temperature level. The obtained ice water content (IWC), effective radius, or extinction coefficient profiles are normalized by their layer mean values and are expressed in the normalized vertical coordinate, which is defined as 0 and 1 at the cloud base and top heights, respectively. Both CloudSat and CALIPSO observations show that the maximum in the IWC and extinction profiles shifts toward the cloud bottom, as the cloud depth increases. In addition, clouds with a base reaching the surface in a high‐latitude region show that the maximum peak of the IWC and extinction profiles occurs near the surface, which is presumably due to snow precipitation. CloudSat measurements show that the seasonal difference in normalized cloud vertical profiles is not significant, whereas the normalized cloud vertical profile significantly varies depending on the cloud type and the presence of precipitation. It is further examined if the 7 day Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulation results from 25 December 2006 to 1 January 2007 generate similar cloud profile shapes. NICAM IWC profiles also show maximum peaks near the cloud bottom for thick cloud layers and maximum peaks at the cloud bottom for low‐level clouds near the surface. It is inferred that oversized snow particles in the NICAM cloud scheme produce a more vertically inhomogeneous IWC profile than observations due to quick sedimentation.

Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals

Abstract. Remote sensing of cloud effective particle size with passive sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) is an important tool for cloud microphysical studies. As a measure of the radiatively relevant droplet size, effective radius can be retrieved with different combinations of visible through shortwave and midwave infrared channels. In practice, retrieved effective radii from these combinations can be quite different. This difference is perhaps indicative of different penetration depths and path lengths for the spectral reflectances used. In addition, operational liquid water cloud retrievals are based on the assumption of a relatively narrow distribution of droplet sizes; the role of larger precipitation particles in these distributions is neglected. Therefore, possible explanations for the discrepancy in some MODIS spectral size retrievals could include 3-D radiative transport effects, including sub-pixel cloud inhomogeneity, and/or the impact of drizzle formation. For three cloud cases the possible factors of influence are isolated and investigated in detail by the use of simulated cloud scenes and synthetic satellite data: marine boundary layer cloud scenes from large eddy simulations (LES) with detailed microphysics are combined with Monte Carlo radiative transfer calculations that explicitly account for the detailed droplet size distributions as well as 3-D radiative transfer to simulate MODIS observations. The operational MODIS optical thickness and effective radius retrieval algorithm is applied to these and the results are compared to the given LES microphysics. We investigate two types of marine cloud situations each with and without drizzle from LES simulations: (1) a typical daytime stratocumulus deck at two times in the diurnal cycle and (2) one scene with scattered cumulus. Only small impact of drizzle formation on the retrieved domain average and on the differences between the three effective radius retrievals is noticed for both cloud scene types for different reasons. For our, presumably typical, overcast stratocumulus scenes with an optical thickness of 8 to 9 and rain rates at cloud bottom up to 0.05 mm/h clear drizzle impact on the retrievals can be excluded. The cumulus scene does not show much drizzle sensitivity either despite extended drizzle areas being directly visible from above (locally >1 mm/h), which is mainly due to technical characteristics of the standard retrieval approach. 3-D effects, on the other hand, produce large discrepancies between the 1.6 and 2.1 μm channel observations compared to 3.7 μm retrievals in the latter case. A general sensitivity of MODIS particle size data to drizzle formation is not corroborated by our case studies.

Particle Growth and Drop Collection Efficiency of Warm Clouds as Inferred from Joint CloudSat and MODIS Observations

Abstract This study describes an approach for combining CloudSat and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations to investigate the microphysical processes of warm clouds on the global scale. MODIS column optical thickness is vertically distributed between the cloud top and cloud bottom according to adiabatic and condensational growth assumptions and used as a vertical coordinate system to analyze profiles of CloudSat-observed radar reflectivity. The reflectivity profiles thus rescaled as a function of the in-cloud optical depth clearly depict how the cloud-to-rain particle growth processes take place within the cloud layer and how these processes vary systematically with variations in MODIS-derived effective particle radius. It is also found that the effective radii retrieved using two different wavelengths of MODIS tend to trace the microphysical change of reflectivity profiles in a different way because of the difference in the layer depth that characterizes these two effective radii. The reflectivity profiles as a function of optical depth are also interpreted in terms of drop collection processes based on the continuous collection model. The slope of the reflectivity change with optical depth provides a gross measure of the collection efficiency factor. The systematic changes of reflectivity profiles with MODIS-derived particle sizes are then interpreted as demonstrating a strong dependency of the collection efficiency on particle size. These results provide a quantitative insight into the drop collection process of warm clouds in the real atmosphere.

Influence of CO2 line profiles on radiative and radiative‐convective equilibrium states of the Venus lower atmosphere

Influence of CO2 line profiles on vertical temperature distributions in the radiative and radiative‐convective equilibria is examined in the Venus atmosphere. The CO2 opacity obtained by the Voigt (Lorentz) profile without the line cutoff is shown to be excessive since this opacity gives surface temperatures of about 860–1020 K in the radiative‐convective equilibrium. On the other hand, the opacity obtained by the extremely sub‐Lorentzian profiles of Pollack et al. (1993) and Tonkov et al. (1996) are underestimated; the surface temperature obtained with this opacity remains 600 K even in the radiative equilibrium. In this case, convection does not take place below the cloud layer because of the cloud opacity. It is also shown that Fukabori et al.'s (1986) and Meadows and Crisp's (1996) profiles, both of which have intermediate absorption coefficients, give temperature distributions close to the observed one in the radiative‐convective equilibrium. In these cases, the convection layer extends from the surface to 30–50 km altitudes. Then, the temperature distribution below the cloud layer is determined by a dry adiabatic lapse rate and the temperature near the cloud bottom. The surface temperature in the radiative‐convective equilibrium is strongly affected by the temperature near the cloud bottom in this situation. The detailed structure of the H2SO4 cloud must be taken into account to construct a realistic radiative transfer model.

구름-장파복사 상호작용이 아시아 몬순에 미치는 영향

Three-dimensional distributions of longwave radiation flux for the April-September 1998 period are generated from radiative transfer calculations using the GEWEX Asian Monsoon Experiment (GAME) reanalysis temperature and humidity profiles and International Satellite Cloud Climatology Project (ISCCP) cloudiness as inputs to understand the effect of cloud radiative forcing in the monsoon season. By subtracting the heating of the clear atmosphere from the cloudy radiative heating, cloud-induced atmospheric radiative heating has been obtained. Emphasis is placed on the impact of horizontal gradients of the cloud-generated radiative heating on the Asian monsoon. Cloud-induced heating exhibits its maximum heating areas within the Indian Ocean and minimum heating over the Tibetan Plateau, which establishes the north-south oriented differential heating gradient. Considering that the differential heating is a ultimate source generating the atmospheric circulation, the cloud-induced heating gradient established between the Indian Ocean and the Plateau can enhance the strength of the north-south Hadley-type monsoon circulation. Cooling at cloud top and warming at cloud bottom, which are the vertical distributions of cloud-induced heating, can exert on the monsoon circulation by altering the atmospheric stability.

Sensitivity of satellite observations for freshly produced lightning NO<sub>x</sub>

Abstract. In this study, we analyse the sensitivity of nadir viewing satellite observations in the visible range to freshly produced lightning NOx. This is a particular challenge due to the complex and highly variable conditions of meteorology, (photo-) chemistry, and radiative transfer in and around cumulonimbus clouds. For the first time, such a study is performed accounting for photo-chemistry, dynamics, and radiative transfer in a consistent way: A one week episode in the TOGA COARE/CEPEX region (Pacific) in December 1992 is simulated with a 3-D cloud resolving chemistry model. The simulated hydrometeor mixing ratios are fed into a Monte Carlo radiative transfer model to calculate box-Air Mass Factors (box-AMFs) for NO2. From these box-AMFs, together with model NOx profiles, slant columns of NO2 (SNO2), i.e. synthetic satellite measurements, are calculated and set in relation to the actual model NOx vertical column (VNOx), yielding the "sensitivity" SNO2/VNOx. From this study, we find a mean sensitivity of 0.46. NOx below the cloud bottom is mostly present as NO2, but shielded from the satellites' view, whereas NOx at the cloud top or above is shifted to NO due to high photolysis and low temperature, and hence not detectable from space. However, a significant fraction of the lightning produced NOx in the middle part of the cloud is present as NO2 and has a good visibility from space. Due to the resulting total sensitivity being quite high, nadir viewing satellites provide a valuable additional platform to quantify NOx production by lightning; strong lightning events over "clean" regions should be clearly detectable in satellite observations. Since the observed enhancement of NO2 column densities over mesoscale convective systems are lower than expected for current estimates of NOx production per flash, satellite measurements can in particular constrain the upper bound of lightning NOx production estimates.

Aerosol retrieval from two‐wavelength backscatter and one‐wavelength polarization lidar measurement taken during the MR01K02 cruise of the R/V Mirai and evaluation of a global aerosol transport model

Using two‐wavelength lidar with one‐wavelength depolarization measurement installed on the research vessel Mirai, we retrieved vertical distributions of extinction coefficients of water‐soluble, sea‐salt, and dust particles at 532 nm. In the retrieval, the mode radii, standard deviations, and refractive indexes for each aerosol component are prescribed; the retrieval uncertainties due to spherical assumption in our dust model are estimated to be 30–50%. The ship‐based measurements were conducted in the western Pacific Ocean near Japan from 14 to 27 May 2001. For the analysis, we applied two‐wavelength lidar algorithms to the three‐channel lidar data, i.e., for signal strengths at 532 and 1064 nm and the total depolarization ratio at 532 nm. Water‐soluble and sea‐salt particles occurred below 1 km, whereas air masses dominated by water‐soluble and dust particles were sometimes found above 1 km. We also investigated the correlation of sea‐salt extinction coefficient with surface wind velocity for various altitudes. A positive correlation was found at low altitude, but no correlation was indicated at high altitude. We also compared the extinction coefficients of water‐soluble and sea‐salt particles directly under cloud bottom with those in clear‐sky. Below clouds, the extinction coefficients of water‐soluble and sea‐salt particles were 1.6 and 1.4 times larger, respectively, than those in clear sky; this could be explained by hygroscopic growth using Hänel theory. Finally, we evaluated the global aerosol transport model SPRINTARS using the retrieved aerosol properties and measured lidar signals. The model underestimated sea salt and overestimated dust, although the general patterns agreed with the observed patterns.

An algorithm that retrieves aerosol properties from dual‐wavelength polarized lidar measurements

We developed two types of algorithms, backward and forward, to estimate vertical profiles of extinction coefficient at a wavelength of λ = 532 nm for each aerosol type, using information from three‐channel Mie‐lidar measurements, i.e., copolarization and cross‐polarization components at λ = 532 nm and total component (copolarization and cross polarization) at λ = 1064 nm. The mode radii, standard deviations, and refractive index for each aerosol type are assumed in the algorithms. The algorithms have the following main features: (1) Extinction coefficient of total aerosols at a far end is estimated in the backward algorithm, while the value at a far end is prescribed as the boundary condition in the Fernald method. (2) They determine aerosol types, i.e., water soluble, sea salt, or dust, for each layer. (3) The vertical profiles of microphysical properties of aerosols such as lidar ratio are also estimated. The backward algorithm is first applied to the lidar signals only calibrated for the spectral ratio to the total components to derive the calibration constant and aerosol properties under clear‐sky condition. Next, the forward algorithm is used to retrieve aerosol properties under cloud bottom. We performed intensive error analyses. The errors for each aerosol component in the extinction coefficient are found to be smaller than 20% (50%) for the backward (forward) algorithms, respectively, when measurement errors are ±5%. The validation of the algorithms from the comparison against sky radiometer measurements over ocean shows that the optical thickness agrees within 2% (10%) for the backward (forward) algorithms.

Dynamical effect of thermal tides in the lower Venus atmosphere

The thermal tides in the lower atmosphere of Venus are examined. It is shown that both the diurnal and semidiurnal tides excited in the cloud layer propagate to the ground, and the existence of thermal tides below the cloud bottom can be attributed almost to the solar heating in the cloud layer. At altitudes of 0–10 km, the atmospheric superrotation is accelerated and decelerated by the momentum transport associated with the diurnal and semidiurnal tides, respectively. The effect of diurnal tide is much smaller than that of the semidiurnal tide, so that the mean zonal flow is accelerated in the direction opposite to the Venus rotation there. It is argued that this momentum transport by the semidiurnal tide is balanced with the surface friction in the atmospheric layer adjacent to the ground, and the net momentum is supplied from the solid Venus to maintain the atmospheric superrotation.

Those Moments

Ihuse_Muojents Richard Garcia The sky this evening over Charleston, just dramatic enough to say, it's good— strolling across the parking lot while the sun lights up the bottom of clouds as you are on your way to Taco Bell. Those clouds remind you of a tromp-l'oeil crépuscule in Las Vegas that makes you believe you are sitting by a canal in Venice in the early evening. The gondoliers in their striped shirts drift along the shore. O Sole Mio. The sky nostalgic, wisps of cirrus seem to drift across it. You ask to sit outside, because this is one of those moments, even if this is not Venice, even if you're really indoors. 96 ...

Vertical variation of NLC particle sizes retrieved from Odin/OSIRIS limb scattering observations

The vertical variation of noctilucent cloud (NLC) particle sizes is derived from limb scattering observations in the UV‐B spectral range performed with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The particle sizes are determined under the assumptions that NLC particles can be treated as Mie particles with a known refractive index, and that their size distribution can be approximated by a log‐normal distribution. A sample data set of 16 bright NLCs observed during the 2003 Northern Hemisphere NLC season and the 2003/2004 Southern Hemisphere NLC season was selected for this study. In almost all the cases the derived NLC particle sizes increase with decreasing altitudes. This is in agreement with the standard assumption of larger particles sedimenting out of the cloud bottom. The derived particle radii are between about 10–50 nm.

An algorithm for retrieval of cloud microphysics using 95‐GHz cloud radar and lidar

We develop a new forward algorithm for the retrieval of effective size and ice water content (IWC) of ice crystals in clouds by using collocated 95‐GHz (3.16 mm) cloud radar and lidar with the wavelength of 0.532 μm. Use of radar or lidar alone has a fundamental difficulty to obtain cloud microphysics because of the wide variety of size distributions of cloud particles, though it is effective to obtain cloud macrophysical information such as cloud boundaries. The combined use of radar and lidar can overcome this problem. One unique feature of the algorithm is the attenuation‐correction to the lidar signals according to the cloud microphysics determined by look‐up tables of backscattering and extinction for the radar and lidar signals. Consequently, the combined system enables the retrieval of vertical profiles of the effective radius (reff) and ice water content (IWC). We perform several numerical analyses of the retrieved values for potential sources of errors, that is, the shape of the size distribution, biases in the radar and lidar signals, and the effect of multiple scattering. Then we provide the formulations that describe the retrieval errors as a function of the given bias and optical thickness. Finally, we demonstrate retrieval of microphysics for ground‐based observation of cirrus clouds in February 2000 in Kashima, Japan. We examine the vertical distributions of reff, IWC, fall velocity, and depolarization ratio as well as the interrelationships between them. The radius of the particles turns out to be the largest near the cloud bottom, and the fall velocity also shows a trend consistent with the reff tendency. There are no in situ measurements to validate the retrieved parameters for the observations. Instead, supporting arguments are given on the basis of information about the expected behavior of the relationships between the cloud microphysical parameters from the literature.

Microphysical modeling of ethane ice clouds in titan’s atmosphere

A time-dependent microphysical model is used to study the evolution of ethane ice clouds in Titan’s atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles. For a critical saturation of 1.15 (a lower limit, determined by laboratory experiments), we find that ethane clouds can be sustained between altitudes of 8 and 50 km. Growth due to coalescence is inefficient, limiting the peak in the size distribution (by number) to 10 μm. These clouds vary with a period of about 20 days. This periodicity disappears for higher critical saturation values where clouds remain subvisible. Rainout of ethane due to methane cloud formation raises the altitude of the ethane cloud bottom to near the tropopause and may eliminate ethane clouds entirely if methane cloud formation occurs up to 30 km. However, clouds formed above the troposphere from other gases in Titan’s atmosphere could be sustained even with rainout up to 30 km. Although the optical depth of ethane clouds above 20 km is typically low, short-lived clouds with optical depths of order 0.1–1 can be created sporadically by dynamically driven atmospheric cooling. Ethane cloud particles larger than 25 μm can fall to the surface before total evaporation. However, ethane clouds remain only a small sink for tholin particles. At the peak of their cycle, the optical depth of ethane clouds could be comparable to that of tholin in the near-infrared, resulting in a 5% increase in Titan’s albedo for wavelengths between 1 and 2 μm. A number of factors limit our ablility to predict the ethane cloud properties. These factors include the mixing time in the troposphere, the critical saturation ratio for ethane ice, the existence of a surface reservoir of ethane, the magnitude and timing of dynamically driven temperature perturbations, and the abundance and life cycle of methane clouds.

Estimating the vertical variation of cloud droplet effective radius using multispectral near‐infrared satellite measurements

This paper presents a satellite‐based retrieval method for inferring the vertical variation of cloud droplet effective radius (DER) by utilizing multispectral near‐infrared (NIR) measurements at 1.25, 1.65, 2.15, and 3.75 μm, available from the Moderate Resolution Imaging Spectrometer (MODIS) satellite observations. The method is based on the principle that these multispectral NIR measurements convey DER information from different heights within a cloud, which is sufficient to allow for the retrieval of a linear DER vertical profile. The method is applicable to low‐level, nonprecipitating, stratiform clouds as their DER often increases monotonically from cloud bottom to cloud top. As such, an optimum linear DER profile can be derived by comparing multispectral NIR measurements to corresponding model values generated for a large set of linear DER profiles. The retrieval method was evaluated and compared to the conventional 3.7‐μm retrieval method by applying both methods to some marine stratocumulus clouds with in situ observations of microphysical profiles. Capable of capturing the DER variation trend, the retrieved linear DER profiles showed large improvement over the conventional 3.75‐μm retrievals. Mean differences between the linear DER retrievals and observed profiles were generally small for both cloud top and bottom (<1.0 μm), whereas the conventional retrievals are prone to systematic overestimation near cloud bottom. The sensitivities of the linear DER retrieval to various parameters, as well as the error analyses, were also investigated extensively.

Physical Basis for the Temperature-Based Melt-Index Method

The close relationship between air temperature measured at standard screen level and the rate of melt on snow and ice has been widely used to estimate the rate of melt. The parameterization of the melt rate using air temperature usually takes a simple form as a function of either the mean temperature for the relevant period or positive degree-day statistics. The computation provides the melt rate with sufficient accuracy for most practical purposes. Because of its simplicity, it is often called a crude method and is rated as inferior to other more sophisticated methods such as the energy balance method. The method is often used with the justification that temperature data are easily available or that obtaining energy balance fluxes is difficult. The physical process responsible for the temperature effect on the melt rate is often attributed to the sensible heat conduction from the atmosphere. The simulation capacity of the temperature-based melt-index method, however, is too good to be called crude and inferior. The author investigated physical processes that make the air temperature so effective a parameter for melt rate. Air temperature has a more profound influence on melt than previously has been acknowledged. The influence of air temperature through the turbulent sensible heat flux is limited, however. The air temperature information is transferred to the surface mainly through longwave atmospheric radiation, which is by far the most important heat source for melt. Under cloudless-sky conditions, as much as 60% of the atmospheric emission is derived from within the first 100 m and 90% from the first 1 km of the atmosphere. When the sky is overcast with the cloud bottom within the first 1 km, more than 90% originates within this layer between the surface and the bottom of the cloud. When the sky is overcast with the cloud bottom higher than 1 km, the first 1 km of the atmosphere still makes up about 70% of the longwave irradiance at the surface, for which the air temperature measured at standard screen level is the single most influential factor. Wind speed is only weakly correlated with melt rate, because the main energy source for melting is longwave atmospheric radiation, followed by the absorbed global radiation, both of which are independent of the movement of the atmosphere.

Modeling of lightning generated electric field transitional processes

A new quasi‐electrostatic model of high altitude electric field generation due to the lightning‐induced change of a thundercloud electric structure is presented. The key point of the model is the assumption that a highly conducting channel arises due to a cloud‐to‐ground (CG) discharge, which brings the ground potential to a region near the cloud bottom in some tens of milliseconds after the discharge initiation. Substantial increase in the electric field strength above the thundercloud at this moment is found. A horizontal extension of the lightning channel is taken into account in the framework of the bidirectional model of the channel propagation. This geometry provides a substantially bigger electric field perturbation than the simplest geometry of the vertical lightning channel does.