Low Cloud Water Research Articles

Characteristic features of water ice clouds over Olympus and Arsia Mons using MOM and MRO observations

The present study used the observations from Mars Color Camera (MCC) onboard Mars Orbiter Mission, from the two instruments onboard Mars Reconnaissance Orbiter, i.e., Mars Climate Sounder (MCS) and Mars Color Imager (MARCI), for investigating the water ice cloud appearance over two Martian volcanos, Olympus Mons and Arsia Mons. Indeed, these regions offer an opportunity to explore the dynamical effects related to the orographic clouds and give an idea about their interactions with atmospheric transport. Also, the long-term MCS profile observations are much useful to emphasize the sub-seasonal, seasonal, and interannual variability of clouds at different altitudes. The MCC and MARCI images show a cloud patch over Olympus Mons within the aphelion and a thin cloud trails over Arsia Mons within the perihelion of Mars years. Night-time MCS observations suggest an appearance of thick low altitude clouds within 15–32 ​km height during LS ​= ​35–150° and thin high-altitude clouds within 30–50 ​km height during LS ​= ​225–315°. The profile observations over both the volcanic regions show that the atmospheric temperature variations more strongly control the water ice cloud distribution at lower altitudes during northern hemispheric spring and summer. In contrast, the high altitude thin clouds during southern hemispheric spring and summer are found to be more associated with the elevated dustiness and vertical advection of dust-laden mountain induced regional circulation, as suggested by MarsWRF simulated wind. The appearance of the consistent thick followed by thin clouds during aphelion and perihelion season shows a nearly constant cloud water content of ~0.2 and ~0.05 pr μm. The high or low magnitude of cloud water content could be distinguished from the peak water ice extinction within the column, as observed in the inter-annual variations. However, irrespective of low cloud water content during the second half of the year, the presence of high altitude thin clouds leads to an overall cloud vertical depth (≥30 ​km) higher (mostly visible during the night) than the first half of the year. Comparison of the nighttime appearance of thick clouds (centered at ~20 ​km) during the first half and thin clouds (centered at ~40 ​km) during the second half of the year over two volcanos indicates the suitable atmospheric conditions in the case of the Olympus Mons for the former type clouds and over Arsia Mons for the later. Also, the appearance of thick clouds mostly resembles the consistent and stable aphelion cloud cycle. In contrast, the thin high altitude clouds are more variable and influenced by the vertical advection during the perihelion season. Besides, these high altitude clouds drive the more prominent east-west asymmetry in the cloud abundance over the Arsia Mons region during the perihelion period.

Population exposure to concurrent daytime and nighttime heatwaves in Huai River Basin, China

Heatwaves are likely to increase over different regions of the world as the climate warms, which poses potential risk to the environment, society, and public health. Concurrent daytime and nighttime heatwaves have a more significant impact on human health than individually occurring heatwaves, especially in regions with high population density. The Huai River Basin (HRB) is taken as a case in this study to explore the characteristics of concurrent daytime and nighttime heatwaves in a high-population-density area, including quantification of the population’s exposure to concurrent heatwaves. Nighttime hot events are found to increase to greater extent than daytime hot events from 1961 to 2017. A single daytime or nighttime hot event can provide 40–60 % capacity for the concurrent daytime and nighttime hot events. The concurrent daytime and nighttime heatwaves show an obvious southeast-northwest gradient with high values in the southeast and low values in the northwest of the HRB. Daytime hot events, nighttime hot events, and concurrent daytime and nighttime heatwave events all show significant upward trends from 1987 to 2017. The population exposed to the heat extremes increased significantly over this period, especially in regards to concurrent daytime and nighttime heatwaves. The population exposure in terms of the quantity, duration, and magnitude of concurrent daytime and nighttime heatwaves increased by around 5-, 9-, and 35-fold from 1984 to 2017. Atmospheric circulation analysis of the 2013 concurrent daytime and nighttime heatwaves in the HRB shows a long-lasting anomalous circulation background (e.g., anomalous high pressure system and low cloud water content) leading to severe concurrent daytime and nighttime heatwaves.

Riming Electrification in Hokuriku Winter Clouds and Comparison with Laboratory Observations

Abstract Riming electrification is the main charge separation mechanism of thunderstorms, occurring mainly during graupel particle–ice crystal collisions. Laboratory experiments have found that charge separation polarity and magnitude depend critically on cloud water content and temperature. Several groups have mapped this dependence, but there are substantial differences between their results. These conflicting laboratory-derived riming electrification topographies can be tested by comparing them to field observations. Here, direct and simultaneous sonde-based measurement of both precipitation particle type and charge (videosonde) and cloud water content [hydrometeor videosonde (HYVIS)] in lightning-active Hokuriku winter clouds at Kashiwazaki, Niigata Prefecture, Japan, are reported. With decreasing height, summed graupel charge transitioned from negative to positive at a mean temperature of −11°C, and the mean peak cloud water content in the positive graupel domain was 0.4 g m−3. Thus, in cloud regions of relatively high temperature (≥−11°C) and low cloud water content (CWC; ≤0.4 g m−3), graupel particles were mainly positively charged. This result can be compared with those of laboratory riming experiments; for example, in this temperature/cloud water content domain, graupel electrification has been reported to be positive by Takahashi, largely negative in early reports using the Manchester cloud chamber, positive in later reports using the Cordoba and Manchester modified cloud chambers, and partially positive in a more recent report using the Cordoba cloud chamber.

Fogwater deposition modeling for terrestrial ecosystems: A review of developments and measurements

Recent progress in modeling fogwater (and low cloud water) deposition over terrestrial ecosystems during fogwater droplet interception by vegetative surfaces is reviewed. Several types of models and parameterizations for fogwater deposition are discussed with comparing assumptions, input parameter requirements, and modeled processes. The relationships among deposition velocity of fogwater (Vd) in model results, wind speed, and plant species structures associated with literature values are gathered for model validation. Quantitative comparisons between model results and observations in forest environments revealed differences as large as 2 orders of magnitude, which are likely caused by uncertainties in measurement techniques over heterogeneous landscapes. Results from the literature review show that Vd values ranged from 2.1 to 8.0 cm s−1 for short vegetation, whereas Vd = 7.7–92 cm s−1 and 0–20 cm s−1 for forests measured by throughfall-based methods and the eddy covariance method, respectively. This review also discusses the current understanding of the impacts of fogwater deposition on atmosphere-land interactions and over complex terrain based on results from numerical studies. Lastly, future research priorities in innovative modeling and observational approaches for model validation are outlined.

Microphysical implications of cloud‐precipitation covariance derived from satellite remote sensing

AbstractCovariance between cloud and precipitation water in shallow marine boundary layer clouds is assessed using collocated satellite observations from CloudSat and the moderate resolution imaging spectroradiometer (MODIS) at spatial scales typical of global models. An analytic construct is presented, which suggests that global models that do not take subgrid scale cloud‐precipitation covariance into account in their microphysical parameterizations may significantly underestimate grid mean microphysical process rates in warm clouds. The proposed framework indicates a mean bias in autoconversion rates of 129% when subgrid scale cloud water variability is neglected and bias in accretion rates of 60% when subgrid cloud‐precipitation covariability is neglected at a model grid resolution of 141 km. The bias in accretion rate is dependent on the significant correlation (ρ) found between cloud and precipitation, which in the global mean is found to be ρ = 0.44. The regional distribution of the process rate biases is largely governed by the spatial pattern of cloud water variance. Specific areas of low cloud water variance are found in the subtropical eastern ocean basins and the high latitudes, whereas much of the tropics display relatively larger cloud water variance. These regional distinctions in cloud water variance are associated with commensurate regionality in the process rate biases. The magnitude of the bias has a scale dependence that is governed by the spatial scaling behavior of the cloud and precipitation variances, which follow a power law scaling with exponent of 2/3 at scales below about 10 km and decreasing exponent above this length scale. While the parametric framework reduces biases in the accretion rate estimated from the grid‐mean values of cloud and precipitation water, it is shown that it still undercorrects the accretion rate because it neglects the fact that the precipitation fractional area is less than the cloud fractional area and is preferentially colocated with the highest cloud water concentrations. These results imply that (1) predicting the appropriate balance of autoconversion to accretion in global models requires not only the subgrid scale cloud water variability but also the subgrid scale covariability of cloud and precipitation water and (2) the ability of a global model to calculate the correct regional variation in process rates depends crucially on the fidelity of that model to predict or diagnose the spatial distribution of the variance in cloud water.

Detecting the Ratio of Rain and Cloud Water in Low-Latitude Shallow Marine Clouds

Abstract Satellite observations are used to deduce the relationship between cloud water and precipitation water for low-latitude shallow marine clouds. The specific sensors that facilitate the analysis are the collocated CloudSat profiling radar and the Moderate Resolution Imaging Spectroradiometer (MODIS). The separation of the cloud water and precipitation water signals relies on the relative insensitivity of MODIS to the presence of precipitation water in conjunction with estimates of the path-integrated attenuation of the CloudSat radar beam while explicitly accounting for the effect of precipitation water on the observed MODIS optical depth. Variations in the precipitation water path are shown to be associated with both the cloud water path and the cloud effective radius, suggesting both macrophysical and microphysical controls on the production of precipitation water. The method outlined here is used to place broad bounds on the mean relationship between the precipitation water path and the cloud water path in shallow marine clouds, given certain clearly stated assumptions. The ratio of precipitation water to cloud water is shown to increase from zero at low cloud water path values to roughly 0.5 at 500 g m−2 of cloud water. The retrieval results further show that the median influence of precipitation on the observed optical depth increases monotonically with optical depth varying between 1% and 5% at 500 g m−2 of cloud water with the source of the uncertainty deriving from the assumption of the nature of the precipitation drop size distribution.

A comparison of AMIP II model cloud layer properties with ISCCP D2 estimates

The cloud amounts and liquid and ice water paths as a function of height in five Atmospheric Model Intercomparison Project (AMIP) II models have been compared to International Satellite Cloud Climatology Project (ISCCP) d2 observations. The model layer data have been transformed to the ISCCP low, mid and height cloud amount and vertically integrated water values. In addition a simple radiative transfer model has been used to transform both model output and ISCCP cloud amount and water contents into top of atmosphere albedos for the low, mid and high cloud fractions. Overall, most models represent moderately well the spatial, seasonal and interannual variability of total cloud albedo, which is largely a function of the total cloud amount. The models also tend to predict moderately well the spatial, seasonal, and interannual variability of cloud fraction, but fail to display the observed spatial, and especially, seasonal and interannual variability in cloud water path. In particular nearly all models have mid and low cloud water path variabilities, which are much larger than those observed in the ISCCP observations. This increased cloud water path variability seems to compensate partially for smaller underestimates of cloud fraction variability in most models. Furthermore, variations in cloud amount and cloud water path are much more often negatively correlated in models than in the observations. A simple estimate of the influence of cloud overlap suggests that monthly mean model cloud layers are less stacked in the vertical in models than in an observational estimate based upon a combination of satellite and ground-based observations.

Pollutant concentrations in fog and low cloud water at selected sites of the Czech Republic

In this paper, the basic composition of fog and low cloud water are presented, resulting from the analyses of water samples from 111 fog/cloud events. The samples were collected at five sites located in various regions of the Czech Republic. Two sampling sites are in mountainous regions and three sites represent various urban areas. The mountain stations are located in two regions of the Czech Republic with different industry types. At all the sites, active fog collectors were employed. In the water samples, the conductivity, acidity (pH), cations (H +, Na +, K +, NH 4 +, Mg 2+, Ca 2+) and anions (F −, Cl −, NO 3 −, SO 4 2−) were determined. A mean pH value of about 4.5 was obtained at mountain sites whereas the measurements in urban areas showed mean pH values from 4.9 to 6.4. The mean conductivity values in the samples from the two mountain stations were 137 and 191.5 μS cm −1. The samples from urban sites showed mean values between 127.7 and 654.4 μS cm −1. The maximum concentration means for the three dominant pollutants (expressed by the ratio mountain sites/urban sites) are 32.9/99.6 mg l −1 for NO 3 −, 32.5/192.9 mg l −1 for SO 4 2− and 18.5/52.7 mg l −1 for NH 4 +. As expected, we found higher ion concentrations in the northern part of the Czech Republic where larger numbers of lignite-burning power plants, chemical factories and opencast lignite mines are located. A decrease in ion concentrations was observed at higher altitude sites, probably reflecting at least in part higher liquid water contents at these locations.

Comparison between Pollutant Concentration in the Samples of Fog and Rime Water Collected at Mt. Milešovka

The article aims at showing the differences in concentration of pollutants that are contained in the samples of fog and/or low cloud water in comparison with the water from rime. The results follow from the fog and rime measurements made at Mt. Milesovka (Ceske Středohoři Mountains). They are compared with the results of other studies that also report the differences in fog and rime chemistry.

Density and surface temperature of graupel and the charge separation during ice crystal interactions

Significant amounts of charge are separated when vapor‐grown ice crystals interact with riming graupel a mechanism widely believed to be responsible for the generation of electric charge in thunderstorms. This study shows that the density of graupel, by itself, is not an effective parameter in determining the sign of the separated charge. The well‐known charge sign reversal temperature is found to be strongly influenced by the surface temperature of the rime at low cloud water content (CWC). At a cloud temperature of −10°C and a CWC of 0.4 g m−3 the charging current to the graupel reversed from positive to negative as the surface temperature of the graupel exceeded −6°C. The maximum CWC at which the sign could thus be reversed increased as the cloud temperature decreased, ranging from less than 0.7 g m−3 at −10°C to less than 0.9 g m−3 at −20°C. At higher cloud water contents the rime surface temperature had no effect on the charging sign. The observations are shown to be broadly explicable in terms of the hypothesis where, during an interaction between two ice particles, the particle that is growing faster from the vapor acquires the positive charge.

A field study of the cloud chemistry and cloud microphysics at Great Dun Fell

A comprehensive series of experiments designed to investigate the oxidation of SO 2 to sulphate by H 2O 2 and O 3 are being performed in the cap cloud at Great Dun Fell. In this paper results of the first set of experiments are presented. These took place during November 1985. The aim of these experiments was chiefly to monitor the H 2O 2 oxidation process by measuring its depletion with time within the cloud in the presence of SO 2. Increases in sulphate content of the cloud water were not observed during this experiment because H 2O 2 levels were too low and oxidation by O 3 was inhibited by the low cloud water pH. The concentrations of aqueous phase H 2O 2 measured were typically 100 nmol and O 3 gas phase concentrations 20 ppbv. It was found that, in the presence of SO 2, the concentration of H 2O 2 declined much more rapidly with height above cloud base than predicted by simple dilution by liquid water. Assuming this to indicate reaction with SO 2, a comparison was made with the predictions of the model of Hill, Choularton and Penkett (1986, Atmospheric Environment 20, 1763–1771). It was found that the rate of reaction was consistent with a value for the second order rate constant κ H 2 O 2 of 2 ± 1 × 10 5 s −1 where κ H 2 O 2 is defined by: d[Svl] dt = H kH 2O 2[H 2O 2][SO 2·H 2O] 0.1+[H + (Martin and Damschen, 1981, Atmospheric Environment 15, 1615–1621). This was determined with a cloud temperature of 8.5°C and a pH of 4.8. This is about three times larger than the laboratory determined rate constant found in Martin and Damschen (1981) when corrected to the same temperature and pH. On some occasions microphysical measurements in the cap cloud indicated that tropospheric air from above the cloud top was being entrained into the cloud. Increases in H 2O 2 concentrations with altitude within the cap cloud on these occasions showed that extra H 2O 2 was being simultaneously introduced to the cloud system. It is suggested that this entrainment process may play a very important role in SO 2 oxidation in clouds when the reaction is oxidant limited.