Cloud Base Temperature Research Articles

Cloud contribution to the daily and annual radiation budget in a mountainous valley

An automated-ventilated radiation station has been set up in a mountainous valley at the Logan Airport in northern Utah, USA, since mid-1995, to evaluate the daily and annual radiation budget components, and develop an algorithm to study cloudiness and its contribution to the daily and annual radiation. This radiation station (composed of pyranometers, pyrgeometers and a net radiometer) provides continuous measurements of downward and upward shortwave, longwave and net radiation throughout the year. The surface temperature and pressure, the 2-m air temperature and humidity, precipitation, and wind at this station were also measured. A heated rain gauge provided precipitation information. Using air temperature and moisture and measured downward longwave (atmospheric) radiation, appropriate formula (among four approaches) was chosen for computation of cloudless-skies atmospheric emissivity. Considering the additional longwave radiation during the cloudy skies coming from the cloud in the waveband which the gaseous emission lacks (from 8–13 μm), an algorithm was developed which provides continuous 20-min cloud information (cloud base height, cloud base temperature, percent of skies covered by cloud, and cloud contribution to the radiation budget) over the area during day and night. On the partly-cloudy day of 3 February, 2003, for instance, cloud contributed 1.34 MJ m − 2 d − 1 out of 26.92 MJ m − 2 d − 1 to the daily atmospheric radiation. On the overcast day of 18 December, 2003, this contribution was 5.77 MJ m − 2 d − 1 out of 29.38 MJ m − 2 d − 1 . The same contribution for the year 2003 amounted to 402.85 MJ m − 2 y − 1 out of 9976.08 MJ m − 2 y − 1 . Observations (fog which yielded a zero cloud base height and satellite cloud imaging data) throughout the year confirmed the validity of the computed data. The nearby Bowen ratio station provided the downward radiation and net radiation data. If necessary, these data could be substituted for the missing data at the radiation station. While the automated surface observing systems (ASOS) ceilometer at the Logan airport provides only the overhead cloud information, the proposed algorithm provides this information over the valley. The proposed algorithm is a promising approach for evaluation of the cloud base temperature, cloud base height, percent of skies covered by cloud, and cloud contribution to the daily and annual radiation budget at local and regional scales.

The Motion of Simulated Convective Storms as a Function of Basic Environmental Parameters

Abstract 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.

SUNYA Regional Climate Model Simulations of East Asia Summer Monsoon: Effects of Cloud Vertical Structure on the Surface Energy Balance

We used the State University of New York at Albany (SUNYA) regional climate model to study the effect of cloud vertical distribution in affecting the surface energy balance of the East Asia summer monsoon (EASM). Simulations were conducted for the summers of 1988 and 1989, during which large contrast in the intra-seasonal cloud radiative forcing (CRF) was observed at the top of the atmosphere. The model results indicate that both the high and low clouds are persistent throughout the summer months in both years. Because of large cloud water, low clouds significantly reduce the solar radiation flux reaching the surface, which nevertheless still dominate the surface energy balance, accounting for more than 50% of the surface heating. The low clouds also contribute significantly the downward longwave radiation to the surface with values strongly dependent on the cloud base temperature. The presence of low clouds effectively decreases the temperature and moisture gradients near surface, resulting in a substantial decrease in the sensible and latent heat fluxes from surface, which partially compensate the decrease of the net radiative cooling of the surface. For example, in the two days, May 8 and July 11 of 1988, the total cloud cover of 80% is simulated, but the respective low cloud cover (water) was 63% (114 gm^(-2)) and 22% (21 gm^(-2)). As a result, the downward solar radiation is smaller by 161 Wm-2 in May 8. On the other hand, the cloud temperature was 10℃ lower, yielding 56 Wm^(-2) smaller downward longwave radiation. The near surface temperature and gradient is more than 1℃ smaller (and moisture gradient), leading to 21 and 81 Wm^(-2) smaller sensible heat and latent heat fluxes. It is also demonstrated that the model is capable to reproduce the intra-seasonal variation of shortwave CRF, and catches the relationship between total cloud cover and SW CRF. The model results show the dominance of high cloud on the regional mean longwave CRF and low cloud on the intra-seasonal variation of shortwave CRF, indicating the importance of cloud vertical structure. The strong negative feedbacks from the responses of latent and sensible heat flux tend to limit the effects of low clouds on the surface temperature simulations, as evidently the surface air temperatures bias of only 0.34℃ in the EASM simulations while the variances of the surface radiative fluxes and heat fluxes are, respectively, in the ranges of 100-200 and 60-110 Wm^(-2) when total cloud cover are all near 80%. Therefore, it is also concluded that surface air temperature, precipitation, and total cloud cover, which are three frequently examined variables for climate models, are not sufficient for model evaluation, but instead the cloud vertical structure needs to be examined.

Infrared Thermal Imagery of Cloud Base in Tornadic Supercells

AbstractDuring the spring seasons of 2003 and 2004, an infrared thermal camera was deployed in and around supercell thunderstorms in an attempt to retrieve the temperature at the cloud base of a mesocyclone prior to tornadogenesis. The motivation for this exercise was to obtain temperature information that might indicate the thermal structure, timing, and extent of the rear-flank downdraft (RFD) and possibly elucidate its relationship to tornadogenesis.An atmospheric transmissivity study was conducted to account for the effects of atmospheric transmission on the measured temperatures, and to determine an ideal range of distances from which infrared images of a wall cloud or a tornado could be safely captured while still retrieving accurate cloud temperatures. This range was found to be 1.5–3 km.Two case days are highlighted in which the infrared camera was deployed within 1.5–3 km of a tornado; the visible and infrared images are shown side by side for comparison. On the single occasion on which the tornadogenesis phase was captured, the infrared images show no strong horizontal temperature gradients. From the infrared images taken of tornadoes, it can be inferred that the infrared signal from the tornado consisted primarily of infrared emissions from lofted dust particles or cloud droplets, and that the infrared signal from the tornado condensation funnel was easily obscured by infrared emissions from lofted dust particles or intervening precipitation curtains.The deployment of the infrared camera near supercell thunderstorms and the analysis of the resulting images proved challenging. It is concluded that the infrared camera is a useful tool for measuring cloud-base temperature gradients provided that distance and viewing angle constraints are met and that the cloud base is unobscured by rain or other intervening infrared emission sources. When these restrictions were met, the infrared camera successfully retrieved horizontal temperature gradients along the cloud base and vertical temperature gradients (close to the moist adiabatic lapse rate) along the tornado funnel.

Observed Relationships between Arctic Longwave Cloud Forcing and Cloud Parameters Using a Neural Network

Abstract A neural network technique is used to quantify relationships involved in cloud–radiation feedbacks based on observations from the Surface Heat Budget of the Arctic (SHEBA) project. Sensitivities of longwave cloud forcing (CFL) to cloud parameters indicate that a bimodal distribution pattern dominates the histogram of each sensitivity. Although the mean states of the relationships agree well with those derived in a previous study, they do not often exist in reality. The sensitivity of CFL to cloud cover increases as the cloudiness increases with a range of 0.1–0.9 W m−2 %−1. There is a saturation effect of liquid water path (LWP) on CFL. The highest sensitivity of CFL to LWP corresponds to clouds with low LWP, and sensitivity decreases as LWP increases. The sensitivity of CFL to cloud-base height (CBH) depends on whether the clouds are below or above an inversion layer. The relationship is negative for clouds higher than 0.8 km at the SHEBA site. The strongest positive relationship corresponds to clouds with low CBH. The dominant mode of the sensitivity of CFL to cloud-base temperature (CBT) is near zero and corresponds to warm clouds with base temperatures higher than −9°C. The low and high sensitivity regimes correspond to the summer and winter seasons, respectively, especially for LWP and CBT. Overall, the neural network technique is able to separate two distinct regimes of clouds that correspond to different sensitivities; that is, it captures the nonlinear behavior in the relationships. This study demonstrates a new method for evaluating nonlinear relationships between climate variables. It could also be used as an effective tool for evaluating feedback processes in climate models.

Computational assessment of a proposed technique for global warming mitigation via albedo-enhancement of marine stratocumulus clouds

A simplified version of the model of marine stratocumulus clouds developed by Bower, Jones and Choularton [Bower, K.N., Jones, A., and Choularton, T.W., 1999. A modeling study of aerosol processing by stratocumulus clouds and its impact on GCM parameterisations of cloud and aerosol. Atmospheric Research, Vol. 50, Nos. 3–4, The Great Dun Fell Experiment, 1995-special issue, 317–344.] was used to examine the sensitivity of the albedo-enhancement global warming mitigation scheme proposed by Latham [Latham, J., 1990. Control of global warming? Nature 347, 339–340; Latham, J., 2002. Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds. Atmos. Sci. Letters (doi:10.1006/Asle.2002.0048).] to the cloud and environmental aerosol characteristics, as well as those of the seawater aerosol of salt-mass m s and number concentration Δ N, which–under the scheme–are advertently introduced into the clouds. Values of albedo-change Δ A and droplet number concentration N d were calculated for a wide range of values of m s, Δ N, updraught speed W, cloud thickness Δ Z and cloud-base temperature T B: for three measured aerosol spectra, corresponding to ambient air of negligible, moderate and high levels of pollution. Our choices of parameter value ranges were determined by the extent of their applicability to the mitigation scheme, whose current formulation is still somewhat preliminary, thus rendering unwarranted in this study the utilisation of refinements incorporated into other stratocumulus models. In agreement with earlier studies: (1) Δ A was found to be very sensitive to Δ N and (within certain constraints) insensitive to changes in m s, W, Δ Z and T B; (2) Δ A was greatest for clouds formed in pure air and least for highly polluted air. In many situations considered to be within the ambit of the mitigation scheme, the calculated Δ A values exceeded those estimated by earlier workers as being necessary to produce a cooling sufficient to compensate, globally, for the warming resulting from a doubling of the atmospheric carbon dioxide concentration. Our calculations provide quantitative support for the physical viability of the mitigation scheme and offer new insights into its technological requirements.

Cloud base temperature measurements using a simple longwave infrared cloud detection system

We have studied the 10 μm temperature of overcast skies, as measured from the ground, in a coastal region of South Australia. The long‐term aim of this work is to be able to use measurements of the sky temperature, plus other readily measured ground‐based parameters, to recognize the presence, or otherwise, of clouds. Comparisons have been made between the 10 μm sky temperature and cloud height in overcast conditions, measured both with a ceilometer and radiosondes. A low‐altitude cloud exhibits a wide range of emissivities, such that clouds with base heights up to 1200 m have emissivities ranging from below 0.2 to close those expected for black bodies at those wavelengths. However, higher clouds exhibit different properties, as their temperatures approach those to be expected for a clear sky with water vapor.

On the temperature correlation of δ 18O in modern precipitation

Reevaluation of modern precipitation, temperature, and isotope data permits reconciliation of previous disparate values for the correlation between δ 18O of modern precipitation and surface temperature. Past analysis has used the mean surface temperature over the time interval of sample collection (e.g., mean weekly, monthly, or annual temperature) to calculate temperature coefficients, and different approaches at mid-latitudes yield different temperature coefficients (Δδ 18O/ΔT): spatial correlations among geographically distinct sites yield ∼0.55‰/K; seasonal variations at single sites yield 0.2–0.4‰/K; and 12 month running averages yield 0.5–1‰/K. However, there are systematic differences in temperature during precipitation events vs. time-averaged surface temperature means. Correction for this bias using hourly weather and monthly isotope data from U.S. sites yields a single value of ∼0.55‰/K for all three approaches. Revised temperature coefficients based on surface observations are also commensurate with coefficients obtained using cloud base temperatures and with theoretical distillation models (0.5–0.7‰/K). These coefficients provide a consistent basis for validation of general circulation models that incorporate stable isotopes of precipitation, and for comparison to independent estimators of the isotopic response to climate change.

On predicting maximum snowfall amounts in Alberta

Snowfall in excess of 10 cm per day in Alberta is usually associated with large‐scale ascent within a wave cyclone and the maximum amount of snowfall depends on the maximum amount of vapour available for deposition. Using the principle of conservation of water, the maximum snowfall amount is related to the saturated vapour mixing ratio at cloud base and cloud top levels. The inherent uncertainties of the input data allow for numerical approximations that lead to a linear relationship between maximum snowfall and cloud base temperature. To test the validity of the linear snowfall–temperature relationship, the correlation of 24‐hour snowfall measurements with temperature observations from upwind soundings was analysed. The dataset covered all of Alberta (with the exception of the mountainous west) for the period October 1990 to April 1993. The data suggest that the snow amounts have an approximately linear dependence on the 850‐mb temperature with a correlation coefficient of 0.62. We further investigated whether the snowfall–temperature relationship can be used to complement the Quantitative Precipitation Forecast (QPF) available for a Numerical Weather Predicition (NWP) model. Specifically, the snowfall–temperature relationship offered value above that available from the output of the European Centre for Medium‐Range Weather Forecasts (ECMWF) model. The implications of this finding on predicting heavy snowfall for Alberta are discussed.

Thermodynamic phase of maritime Antarctic clouds from FTIR and supplementary radiometric data

A Fourier Transform Infrared (FTIR) spectroradiometer was deployed at Palmer Station, Antarctica, from 1 September to 17 November 1991. This instrument is similar to the Atmospheric Emitted Radiance Interferometer (AERI) deployed with the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program. The instrument measured downwelling zenith radiance in the spectral interval 400–2000 cm−1, at a resolution of 1 cm−1. The spectral radiance measurements, which can be expressed as spectral brightness temperature Tb(ν), contain information about cloud optical properties in the middle infrared window (800–1200 cm−1, 8.3–12.5 μm). In this study, this information is exploited to (1) provide additional characterization of Antarctic cloud radiative properties, and (2) demonstrate how multisensor analysis of ARM data can potentially retrieve cloud thermodynamic phase. Radiative transfer simulations demonstrate how Tb(ν) is a function of cloud optical depth τ, effective particle radius re, and thermodynamic phase. For typical values of τ and re, the effect of increasing the ice fraction of the total optical depth is to flatten the slope of Tb(ν) between 800–1000 cm−1. For optically thin clouds (τ ∼ 3) and larger ice particles (re (ice) > 50 μm) the behavior of Tb(ν) in this interval switches from a decrease with increasing wavenumber ν to an increase with ν, once the ice fraction of the total optical depth exceeds ∼0.7. The FTIR spectra alone cannot be interpreted to obtain thermodynamic phase, because a relatively small slope in Tb(ν) between 800–1000 cm−1 could represent either an optically thin cloud in the ice or mixed phase, or an optically thick cloud radiating as a blackbody. Sky observations and ancillary radiometric data are needed to sort the FTIR spectra into categories of small cloud optical depth, where the mid‐IR window data can be interpreted; and larger cloud optical depth, where the FTIR data contain information only about cloud base temperature. Spectral solar ultraviolet (UV) irradiance measurements from the U.S. National Science Foundation's UV Monitor at Palmer Station are used to estimate area‐averaged effective cloud optical depth τsw, and these estimates are used to sort the FTIR data. FTIR measurements with colocated τsw < 16 are interpreted to estimate cloud thermodynamic phase. Precipitating cloud decks generally show flatter slopes in Tb(ν), consistent with the ice or mixed phase. Altostratus decks show a larger range in Tb(ν) slope than low cloud decks, including increasing slopes with ν, suggesting a more likely occurrence of the ice phase. This study illustrates how cloud thermodynamic phase can be defensibly retrieved from FTIR data if high quality shortwave radiometric data are also available to sort the FTIR measurements by cloud opacity, and both data types are available at the ARM sites.

Accuracy of cloud liquid water path from ground‐based microwave radiometry 2. Sensor accuracy and synergy

The influence of microwave radiometer accuracy on retrieved cloud liquid water path (LWP) was investigated. Sensor accuracy was assumed to be the sum of the relative (i.e., Gaussian noise) and the absolute accuracies of brightness temperatures. When statistical algorithms are developed the assumed noise should be as close as possible to the real measurements in order to avoid artifacts in the retrieved LWP distribution. Typical offset errors of 1 K in brightness temperatures can produce mean LWP errors of more than 30 g m−2 for a two‐channel radiometer retrieval, although positively correlated brightness temperature offsets in both channels reduce this error to 16 g m−2. Large improvements in LWP retrieval accuracy of about 50% can be achieved by adding a 90‐GHz channel to the two‐channel retrieval. The inclusion of additional measurements, like cloud base height from a lidar ceilometer and cloud base temperature from an infrared radiometer, is invaluable in detecting cloud free scenes allowing an indirect evaluation of LWP accuracy in clear sky cases. This method was used to evaluate LWP retrieval algorithms based on different gas absorption models. Using two months of measurements, the Liebe 93 model provided the best results when the 90‐GHz channel was incorporated into the standard two‐channel retrievals.

The glaciation of a cumulus cloud over New Mexico

The Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi-thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud. The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett-Mossop (H-M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud-base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H-M splinter production and auto-conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm-rain process in the EMM is fundamental to the production of graupel, H-M splinters and precipitation. Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H-M process, with ice production being dominated by the freezing of supercooled raindrops in the H-M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.

Ground-Based Infrared Remote Sensing of Cloud Properties over the Antarctic Plateau. Part II: Cloud Optical Depths and Particle Sizes

Abstract One full year of twice-daily longwave atmospheric emission spectra measured from the surface at 1-cm−1 resolution are used to infer optical thicknesses and ice crystal sizes in tropospheric clouds over the Antarctic Plateau. The method makes use of the cloud's emissivity at 10- and 11-μm wavelength and the cloud's transmittance of stratospheric ozone emission in the 9.6-μm band. Knowledge of the cloud-base temperature and the vertical distributions of ozone and temperature is required; these are available at South Pole Station from radiosondes and ozonesondes. The difference in emissivity between 10 and 11 μm is sensitive to ice particle size because the absorption coefficient of ice varies greatly between these two wavelengths. The retrieval of optical depth (expressed as its value in the geometric-optics limit τg) is limited to τg < 5, and the effective particle radii reff are distinguished only for reff < 25 μm, but 80% of the clouds observed have τg and reff in the retrievable range. These cl...

Long-Term Observations of Cloud Liquid, Water Vapor, and Cloud-Base Temperature in the North Atlantic Ocean

Abstract Measurements of precipitable water vapor, cloud liquid, and cloud-base temperature made at Porto Santo Island, Madeira Archipelago, are presented. The observations were made continuously over a 1-yr period from July 1992 to June 1993. Instrumentation consisted of a 20- and 31-GHz ground-based microwave radiometer for measurement of water vapor and cloud liquid, and a 10.7-μm infrared radiometer for measurement of cloud-base temperature. A statistical summary of the data is presented for clear and cloudy conditions, considered both separately and combined. The mean and variance of precipitable water vapor are smaller during clear than during cloudy periods. Values of water vapor during the winter are 26% smaller than during summer months. The mean decorrelation time of precipitable water vapor is generally found to be less than 12 h.

Integration of radar with analysis fields — Bringing distant radar observations “down to earth”

Normal refraction of radar waves through the atmosphere prevents radar data at distant ranges from being quantitatively representative for the Earth's surface unless precipitation-related processes taking place between the radar echo and the surface are taken into account. A one-dimensional model is presented which uses a physical description of the processes involved in obtaining surface precipitation rate from a radar echo aloft, comprising local production, coalescence and evaporation. One or two cloud layers are assumed depending on the cloud base height. Input data are operationally analysed cloud base height, temperature, and humidity along with radar observations and their altitudes. At the moment, only precipitation as liquid water is assumed in the model. The method and results using it will be presented and discussed.

Ground based passive remote sensing of ice clouds with scattered solar radiation in the near infrared

Abstract The feasibility of ground based passive remote sensing in the near infrared for the estimation of optical properties of thin ice clouds is examined and confirmed. The radiative transfer through ice clouds is simulated as a function of crystal size, crystal shape and optical depth under the assumption of single scattering. A method is derived to estimate the effective radius r eff for optically thin ice clouds from radiances at wavelengths of 1046 and 1550 nm . The results are not dramatically different if one assumes hexagonal crystals or surface equivalent spheres. The estimation of the ice cloud optical depth depends strongly on particle shape, leading to a less reliable estimate. Ground based measurements of scattered near infrared radiances with the array spectrometer OVID have been used to verify and to apply the derivation technique. The evaluation of measurements for different ice cloud types and contrails mostly yields particle sizes between 30 and 200 μ m . Occasionally they are as small as 1.6 μ m or as large as 600 μ m . The known increase of particle size with cloud base temperature was confirmed. Contrails prove to contain small crystal sizes that increase with age to the values found in the surrounding cirrus clouds. A systematic size reduction in contrail clouds older than 30 minutes is indicated but cannot b e conclusively proven by the present data.

A ground based multi-sensor system for the remote sensing of clouds

Abstract A ground based system combining active and passive remote sensing instruments from different spectral ranges has been set up at the Meteorological Institute in Bonne to investigate cloud processes. Currently, the sensor package includes a laser ceilometer, an infrared radiometer, an X-Band Radar, and a 4-channel microwave radiometer. Cloud base height and temperature, precipitation, and the integrated quantities of water vapor and cloud liquid water can be measured on small time scales (≤ 15 s). The system will be extended in fall 1998 with a 22-channel microwave to derive profiles of humidity, temperature and liquid water content. A case study using measurements from April 1, 1998 demonstrates the ability of the system to characterize various types of clouds.

The sublimation of falling snow over the Mackenzie River Basin

The sublimation of falling snow may be an important component of the atmospheric water budget of the Mackenzie River Basin and many parts of the Arctic. To investigate this issue, a simple sublimation model is used along with surface precipitation observations and sonde data obtained during the autumn 1994 Beaufort and Arctic Storms Experiment (BASE). Model results are then compared with actual precipitation measurements at Inuvik and Tuktoyaktuk, sites in Northern Canada, to approximate mass loss due to sublimation. The sublimation results are found to vary in concert with cloud base height, precipitation intensity aloft and the nature of the precipitation. Atmospheric conditions are furthermore examined over a wide range of the Arctic, especially the Mackenzie River Basin, to assess to what degree the results can be generalized. The presence of a relatively dry near-surface layer, a favourable environment for sublimation, is a key feature of most sites during the early autumn storm period. Estimates of sublimational mass losses are found over Inuvik and Tuktoyaktuk using sonde derived cloud base heights and temperature and humidity profiles. Sublimation losses for such sites are found to be of the order of 40–60%, which shows that sublimation is indeed a significant process over the Mackenzie Basin and needs to be well handled in climate models. However, increasing the vertical resolution of the sublimation model to that of climate scales can dramatically affect predicted sublimation amounts; how to properly account for sublimation then remains a difficult task.

Evaluation of effective atmospheric emissivity and parameterization of cloud at local scale

We have measured the radiation components continuously over a partially cheatgrass-covered area since October 1995 and compared the measured incoming longwave (atmospheric) radiation with that computed by Brutsaert's formula. Data analysis showed very good agreement between the measured and computed atmospheric radiation during cloudless periods. During cloudy conditions the measured atmospheric radiation is greater than that computed by Brutsaert's formula. The additional longwave radiation during the cloudy skies comes from clouds in the waveband which the gaseous emission lacks, i.e. from 8 to 13 μm (the so-called atmospheric window). We proposed an approach which uses the thermodynamic concepts and some climatic parameters to parameterize cloud at local scale. Based upon the proposed method, the cloud base height, cloud base temperature, and percent of sky covered by cloud can be evaluated continuously for any location without introducing any empirical and local constants.

Assessing the Potential for Rain Augmentation–The Nelspruit Randomized Convective Cloud Seeding Experiment

Abstract The experimental design, analyses, and results of the first Nelspruit randomized cloud seeding experiment are described. The experiment ran for three years, commencing in October 1984, and involved the on-top seeding of new cloud turrets growing on the flanks of isolated multicellular storms using dry ice delivered from a Learjet at around the height of the −10°C isotherm. All storms were tracked by a radar operating in computer-controlled volume scan mode. A total of 169 storms were examined, of which 94 passed the selection criteria. The most important criterion was based upon a microphysical classification scheme obtained from measurements made by the instrumented Learjet. This scheme, based upon a ratio of cloud-base temperature to potential buoyancy at 500 mb, rejected those storms in which the production of precipitation via coalescence was unlikely. A key element of the experiment was the ability to objectively track the storms using an automatic storm tracking algorithm. Storms were analy...