Decrease In Cloud Cover Research Articles

Effect of Antarctic solar radiation on sewage bacteria viability

The majority of coastal Antarctic research stations discard untreated sewage waste into the near-shore marine environment. However, Antarctic solar conditions are unique, with ozone depletion increasing the proportion of potentially damaging ultraviolet-B (UV-B) radiation reaching the marine environment. This study assessed the influence of Antarctic solar radiation on the viability of Escherichia coli and sewage microorganisms at Rothera Research Station, Adelaide Island, Antarctic Peninsula. Cell viability decreased with increased exposure time and with exposure to shorter wavelengths of solar radiation. Cell survival also declined with decreasing cloud cover, solar zenith angle and ozone column depth. However, particulates in sewage increased the persistence of viable bacteria. Ultraviolet radiation doses over Rothera Point were highest during the austral summer. During this time, solar radiation may act to partially reduce the number of viable sewage-derived microorganisms in the surface seawater around Antarctic outfalls. Nevertheless, this effect is not reliable and every effort should be made to fully treat sewage before release into the Antarctic marine environment.

Retrieval of Cloud Properties for Partly Cloudy Imager Pixels

Abstract Retrievals of cloud properties from satellite imagery often invoke the assumption that the fields of view are overcast when cloud-contaminated, even though a significant fraction are only partially cloud-covered. The overcast assumption leads to biases in the retrieved cloud properties: cloud amounts and droplet effective radii are typically overestimated, while visible optical depths, cloud altitudes, cloud liquid water amounts, and column droplet number concentrations are typically underestimated. In order to estimate these biases, a retrieval scheme was developed to obtain the properties of clouds for partially covered imager fields of view. The partly cloudy pixel retrieval scheme is applicable to single-layered cloud systems and invokes the assumption that clouds that only partially cover a field of view are at the same altitude as nearby clouds from the same layer that completely cover imager pixels. The properties of the retrieval are illustrated through its application to 2-km Visible and Infrared Scanner (VIRS) data from the Tropical Rainfall Measuring Mission (TRMM) for a marine stratocumulus scene. The scene was chosen because the cloud properties are typical of such systems based on an analysis of VIRS data for February and March 1998. Comparisons of properties for clouds in partly cloudy pixels and those for clouds in nearby overcast pixels reveal that the optical depths and droplet effective radii are generally smaller for the clouds in the partly cloudy pixels. In addition, for pixel-scale cloud fractions between 0.2 and 0.8, optical depth, droplet effective radius, and column droplet number concentration decrease slowly with decreasing cloud cover fraction. The changes are only about 20%–30%, while cloud cover fraction changes by 80%. For comparison, changes in optical depth and column number concentration retrieved using a threshold method decrease by 80%–90%. As long as the cloud cover in partly cloudy pixels is greater than about 0.1, uncertainties in the estimates of the cloud altitudes and of the radiances for the cloud-free portions of the fields of view give rise to uncertainties in the retrieved cloud properties that are comparable to the uncertainties in the properties retrieved for overcast pixels.

Taking China's Temperature: Daily Range, Warming Trends, and Regional Variations, 1955–2000

Abstract In analyzing daily climate data from 305 weather stations in China for the period from 1955 to 2000, the authors found that surface air temperatures are increasing with an accelerating trend after 1990. They also found that the daily maximum (Tmax) and minimum (Tmin) air temperature increased at a rate of 1.27° and 3.23°C (100 yr)−1 between 1955 and 2000. Both temperature trends were faster than those reported for the Northern Hemisphere, where Tmax and Tmin increased by 0.87° and 1.84°C (100 yr)−1 between 1950 and 1993. The daily temperature range (DTR) decreased rapidly by −2.5°C (100 yr)−1 from 1960 to 1990; during that time, minimum temperature increased while maximum temperature decreased slightly. Since 1990, the decline in DTR has halted because Tmax and Tmin increased at a similar pace during the 1990s. Increased minimum and maximum temperatures were most pronounced in northeast China and were lowest in the southwest. Cloud cover and precipitation correlated poorly with the decreasing temperature range. It is argued that a decline in solar irradiance better explains the decreasing range of daily temperatures through its influence on maximum temperature. With declining solar irradiance even on clear days, and with decreases in cloud cover, it is posited that atmospheric aerosols may be contributing to the changing solar irradiance and trends of daily temperatures observed in China.

Climate-driven increases in global terrestrial net primary production from 1982 to 1999.

Recent climatic changes have enhanced plant growth in northern mid-latitudes and high latitudes. However, a comprehensive analysis of the impact of global climatic changes on vegetation productivity has not before been expressed in the context of variable limiting factors to plant growth. We present a global investigation of vegetation responses to climatic changes by analyzing 18 years (1982 to 1999) of both climatic data and satellite observations of vegetation activity. Our results indicate that global changes in climate have eased several critical climatic constraints to plant growth, such that net primary production increased 6% (3.4 petagrams of carbon over 18 years) globally. The largest increase was in tropical ecosystems. Amazon rain forests accounted for 42% of the global increase in net primary production, owing mainly to decreased cloud cover and the resulting increase in solar radiation.

Climatic implications of Late Quaternary plant macrofossil assemblages from the Taymyr Peninsula, Siberia

Plant macrofossil investigations were carried out on Late-Quaternary deposits at Cape Sabler (NW shore of Lake Taymyr, Siberia). Four dated samples (three Late Pleistocene, one Holocene) rich in plant macrofossils were analysed with respect to their floristic composition. The flora of the Holocene sample is similar to the recent plant cover in the study area and is mainly characterized by subarctic and high arctic dwarf-shrubs and hydrophytes. The Pleistocene samples, radiocarbon dated to ca. 30, 27 and 18 ka BP, reflect a species-rich vegetation. The high proportion of pioneer plants within these samples indicates frequent disturbances of the vegetation cover. Furthermore, an almost complete absence of hydrophytes and swamp plants and the dominance of xerophytes, respectively, are typical for the Pleistocene vegetation from the Cape Sabler section. The presence of the tundra–steppe Crassulaceae Orostachys spinosa gives evidence for extremely cryoxeric climatic conditions in the Central Taymyr Peninsula during the Late Pleistocene. The species composition of the Pleistocene samples thus proves the predominance of a continental climate during the Weichselian, which was characterized by a decreased cloud cover, and thus increased direct solar radiation reaching the ground and extreme annual temperature variations. That would have entailed higher summer temperatures, lower precipitation, and longer vegetation periods (due to shorter snowmelt), thus, better conditions than today for the growth of plants in the study area. The higher species diversity in the Weichselian samples confirms this assumption. All these factors caused mainly aerobic soil environments and favourable conditions for seasonal heating of soils.

A framework for understanding the relationship between environment and vegetation based on the surface area to volume ratio of leaves

Abstract1. Existing data show that the surface area to volume ratio (Λ) of leaves generally decreases along both aridity and altitudinal gradients. That results in the relations: Λ decreases as it gets drier and hotter, as well as wetter and colder. Thus variations in rainfall and temperature do not explain the gross trends in Λ in a consistent manner. We hypothesized that Λ should vary in a regular manner with variations in light, water, nutrients and CO2, because recent work has shown regular relationships between Λ and the carbon, nitrogen and water contents of leaves.2. To test the hypothesis we used existing measurements of leaves from various eucalypt species along aridity, altitudinal and fertility gradients in south‐east Australia. The measurements were converted from a dry mass to a volumetric basis using an empirical leaf model known as the RSBS‐model leaf.3. Variation in leaf composition and morphology generally followed that predicted by the RSBS‐model leaf.4. We found that Λ increased with site fertility. Analysis of data from the literature showed that increasing the supply rate of nitrogen also results in an increase in Λ. We conclude that increases in site fertility are associated with increases in Λ.5. Leaf thickness increased (hence Λ decreased) with light along the aridity and altitudinal gradients, in agreement with theoretical predictions. The increase in light along the aridity gradient is attributed to a decrease in cloud cover, while the increase in light with altitude is due to a decline in the optical thickness of the atmosphere.6. Along the altitudinal gradient, the soils became increasingly boggy and anoxic with elevation, leading to increased soil acidity and a consequent decline in oxygen and nutrient availability. In contrast, along the aridity gradient decreases in nutrient availability were related to dry soil.7. Both an excess and shortage of soil water can lead to reductions in the availability of oxygen and/or nutrients. When that relationship was considered, we found that a consistent framework linking Λ with the availability of light, water, nutrients and CO2/O2 could be established. In that framework, leaf area is controlled jointly by the availability of water and nutrients, while leaf thickness is controlled by the availability of carbon (i.e. light, CO2). Λ is the result of interactions between those two factors. The resulting framework should be useful in the development of models linking vegetation with environmental conditions.8. A method for extending these results to the modelling of vegetation communities is proposed.

Surface UV radiation over Australia, 1979–1992: Effects of ozone and cloud cover changes on variations of UV radiation

Time series of daily erythemal ultraviolet radiation (UVR) exposure, ozone, and cloud cover were analyzed over the Australian continent using data sets from the NASA Total Ozone Mapping Spectrometer (TOMS) from 1979–1992. The TOMS UVR exposures showed good agreement with data from surface observations. Using a relationship derived from comparisons of the TOMS partial cloud reflectivities with surface cloud cover observations, the TOMS reflectivities were converted into estimates of cloud cover for several Australian regions. It was shown that the deseasonalized time series of UVR exposures can be statistically described as a linear function of ozone and cloud cover anomalies. Results of a trend analysis indicated statistically significant increases in UVR exposures of 10% decade−1 in the summer months in the tropics. These were associated with a simultaneous depletion of ozone and a decrease in cloud cover. Midlatitudinal regions showed no significant trends of UVR. It was found that variations of ozone and UVR over Australia were significantly influenced by the quasi‐biennial oscillation (QBO). An increase in zonal wind strength of 20 m s−1 was correlative with reductions of ozone of 1.7% and enhancements of UVR exposures of 2.2%. An increase in solar radio flux of 100×10−22 W m−2 (Hz)−1 was associated with significant reductions of UVR of 5–10% in the tropical and subtropical regions in summer. The results suggested that enhancements in summer UVR exposures of about 10–20% above the climatological average might be expected in years in which the QBO is in its westerly phase and the solar cycle is at its minimum.

A Land–Atmosphere Interaction Theory for the Tropical Deforestation Problem

A theoretical framework is developed in understanding the mechanisms and processes determining the response of the land–atmosphere system to tropical deforestation. The analytical approach is made possible by simplifications in the vertical from the quasi-equilibrium moist convective closure, and in the horizontal from the dynamical temperature homogenization process. The theory emphasizes the energy and water balance. It highlights the interaction among processes of moist convection, cloud, radiation, and surface hydrology while each individual process is simplified. The zero surface energy flux condition, due to the small heat capacity of land, makes land–atmosphere interaction distinctly different from ocean–atmosphere interaction. This imposes a constraint on the sensitivity to the details of surface energy partitioning. Consequently, land surface temperature is largely a response to the energy and water balance, rather than a forcing as in the case of sea surface temperature. Results from a wet-season surface albedo change case compare well with a recent RCCM2/BATS simulation, with the theory depicting the mechanisms and the roles of the intertwining processes. The precipitation has a significant decrease, initiated by ground radiative forcing as increased surface albedo reflects more solar radiation into space. A positive feedback by moisture convergence is essential for this tendency, with another positive feedback from reduced evaporation providing further enhancement. These are opposed by a negative feedback due to the reduced magnitude of negative cloud radiative forcing as cloud cover decreases. This sheds light on the higher sensitivity in some GCM studies with prescribed clouds. The cloud radiative forcing also has a negative feedback on the initial cooling tendency in ground temperature. Together with reduced evaporation, this leads to little change in the ground temperature. Sensitivities of precipitation and ground temperature changes to individual processes are found to depend on the reference state parameter values, implying a sensitivity of anomaly response to simulated climatology for GCMs. The analysis here also serves as an example of the tight coupling between convection, large-scale atmospheric dynamics, and land processes in the tropical land–atmosphere system.

Atmospheric and oceanic evidences of El Niño‐Southern Oscillation events in the south central Pacific Ocean from coral stable isotopic records over the last 137 years

We measured δ18O and δ13C in Porites lutea collected in the Moorea lagoon where the instrumental records show that an El Niño‐Southern Oscillation (ENSO) event implies both a cloud cover decrease and a weak sea surface temperature (SST) increase. Two proxies allow an ENSO record to be reconstructed at Moorea: the annual δ13C anomaly, associated with the South Pacific Convergence Zone motion, and an annual δ18O anomaly showing increased SST. The frequency bands exhibited by the singular spectral analysis (SSA) and the multitaper method of the annual δ18O and δ13C records are centered on 2.5 and 5.2 years and 2.4 and 3.2 years, respectively, which are the main ENSO modes. We used SSA to reconstruct ENSO events at Moorea over the past 137 years. Results indicate that climate variability at this site is strongly affected by ENSO events.

The impact of horizontal resolution on moist processes in the ECMWF model

Summer and winter climates simulated with the ECMWF (cycle 33) model at spectral scales T21, T42, T63 and T106 are analyzed to determine the impact of changes in horizontal resolution on atmospheric water vapor, clouds, convection, and precipitation. Qualitative changes in many moist processes occur in the transition from T21 to T42, especially in the tropics; at higher resolutions mostly incremental variations from patterns established at T42 result. Large-scale tropical moist processes are simulated more realistically at T21 than at finer resolutions, possibly reflecting a mismatch between the finer-scale dynamics and the scales at which the underlying assumptions of the physical parameterizations apply. Global precipitation increases monotonically with resolution, as a consequence of increasing convection. Global cloud cover, however, decreases in the transition from T21 to T42 due to drying of the tropics, but then increases slightly at finer resolutions. These small global increases are an outcome of compensating changes in different regions: decreases in cloud cover due to drying of the atmosphere at low latitudes are offset by high-latitude increases resulting from enhanced relative humidity associated with an intensifying atmospheric cold bias at finer resolutions.

Response of a general circulation model to a change in cloud solar forcing: Model feedbacks and comparison with satellite data

The response of a general circulation model to a change in its treatment of cloud solar forcing is investigated. Radiation field data from the forecast model of the Navy Operational Global Atmospheric Prediction System for five Julys (1979–1983) are presented in an investigation of the effect of a change from grid cell averaged clouds to maximally overlapping clouds in the model's solar radiation scheme. The model results are compared with Nimbus 7 Earth Radiation Budget top of the atmosphere (TOA) solar and longwave irradiances and with derived surface solar irradiance data. Although the maximal overlap scheme performs considerably better than the grid cell averaging scheme (reducing maximum deficiencies in TOA and surface solar irradiance by over 100 W m−2), significant errors remain. The simulated correlation between TOA net solar and longwave irradiance improves at low latitudes in the northern hemisphere, with little change at higher latitudes. This improved correlation is consistent with the greater consistency between the treatments of solar and longwave cloud radiative forcing brought to the model by the new solar radiation scheme. The change in the radiation treatment is shown to have the greatest direct effect on solar radiation over convective regions, a consequence of the scarcity of optically thick clouds produced by the model's cloud parameterization in other regions. The model responds with an increase in convective activity over land and an increase in the flux of moisture from sea to land. Planetary cooling over the oceans increases because of a decrease in cloud cover. From mid to high latitudes in the northern hemisphere, there are scattered regions of increased cloud water content associated with increased tropospheric temperatures. Over land the model response in terms of TOA downwelling solar irradiance tends to counter the increase in solar irradiance caused by the model change in all latitudinal zones in the northern hemisphere. This response is caused primarily by changes in the cloud fields, which thus act as a negative feedback following the change in cloud solar forcing. The significance of this response is examined with respect to the perturbation in solar irradiance represented by the model change. An estimate of this perturbation is obtained by taking the difference in solar irradiance diagnosed by the two cloud solar forcing treatments for simulations employing the grid cell averaging scheme. The response is significantly greater in magnitude in the tropics than at midlatitudes, both in an absolute sense and as a percentage of this perturbation. Because TOA longwave irradiance exhibits a positive response in the tropics, and a negative response at midlatitudes, however, the percentage response in net TOA downwelling irradiance is actually greater in magnitude at midlatitudes. In a number of regions the cloud feedback is very large, showing the importance for cloud field prediction of improvements in the treatment of cloud solar forcing. Such cloud feedback also explains the small improvement seen here in the prediction of TOA solar irradiance in certain regions/Increases in surface sensible heating and longwave cooling are generally considerably less than increases in surface latent heating, though a notable exception occurs in arid central Asia. A large ground temperature increase in that region is strongly correlated at low levels with the atmospheric temperature increase observed at midlatitudes in the northern hemisphere.

An assessment of air quality variations in the south‐western USA using an upper air synoptic climatology

AbstractDaily particulate concentrations in the Desert South‐west from 1984 to 1988 are analysed using a synoptic climatology based upon tropospheric thermal, moisture, and flow conditions. Using a combination of principal components analysis and cluster analysis, 13 distinct synoptic types (clusters) are identified that correspond to seasonal variations in the atmosphere's general circulation. These include summer clusters, which reflect monsoonal and non‐monsoonal conditions, winter clusters defined by the positions of the polar and subtropical jets and shortwave ridges and troughs, and clusters that predominate in the transition seasons.Particulate concentrations vary significantly by synoptic type. Three clusters have poor air quality. ‘Summer monsoon’ days have high humidity and cloud cover, conditions that encourage aerosol growth. ‘Continental high’ days have a slow‐moving anticyclone centred over the Great Basin that traps particulates within the boundary layer. ‘Rockies ridge’ days are associated with strong south‐westerly flow and the advection of regional haze from southern California into the Desert South‐west. In all of these clusters, high humidity or cloud cover enhance particulate growth. Good air quality in the summer occurs on monsoonal ‘break’ days when relative humidity and cloud cover decrease. In the winter, good air quality occurs when Arctic or continental polar air masses are present, or when either the polar or subtropical jets are strong and positioned over the South‐west. In some of these latter situations, widespread precipitation removes particulates through wet deposition.These results suggest that the Navajo Generating Station (NGS), a large coal‐fired power plant on the Arizona‐Utah border, does not contribute significantly to air quality degradation in the Grand Canyon National Park, Arizona. Most of the days with high particulate levels are associated with transport from southern California or unstable summer days with high humidity and contributions from many point and regional sources. There may be some contribution from the NGS on ‘continental high’ days, but when this cluster occurs on consecutive days, air quality improves rather than declines. This research provides a useful framework for future mesoscale modelling studies of air quality in south‐western USA.

Greenhouse Gas–induced Climate Change Simulated with the CCC Second-Generation General Circulation Model

The Canadian Climate Centre second-generation atmospheric general circulation model coupled to a mixed-layer ocean incorporating thermodynamic sea ice is used to simulate the equilibrium climate response to a doubling of C02. Features of the simulation include the use of higher model resolution than previously for studies of this kind, specification of ocean heat transport for the open ocean and under sea ice, incorporation of information on vegetation and soil type in the treatment of land surface processes, and the inclusion of a parameterization of variable cloud optical properties. The results of the simulation indicate a global annual warming of 3.5°C with enhanced warming found over land and at higher latitudes. Precipitation and evaporation rates increase by about 4%, and cloud cover decreases by 2.2%. Soil moisture decreases over continental Northern Hemisphere land areas in summer. The frozen component of soil moisture decreases and the liquid component increases in association with the increase of temperature at higher latitudes. The simulated accumulation rate of permanent snow cover decreases markedly over Greenland and increases slightly over Antarctica. Seasonal snow and sea ice boundaries retreat, but local decreases in planetary albedo are counteracted by tropical increases, so there is little change in the global average. Large-scale patterns of change are found in mean sea level pressure accompanied by a general decrease in short-term variability.

Cloud parametrization at different horizontal resolutions

As faster computers emerge, resolution continues to improve in weather-prediction and climate models. How does this affect the problems that we are dealing with in the models? In this study we have concentrated on one of these problems, namely the treatment of clouds. Among the questions addressed are: How do cloud-related processes (condensation, precipitation) depend on grid resolution? How should we take this into account? Can we simulate orographic clouds when we have a grid resolution of hundreds of kilometres (climate models)? To tackle these questions simulations are performed with a limited-area model that is run with horizontal grid resolutions of 150,50 and 25 km. The model has a sophisticated treatment of condensation and clouds. For verification, use is made of satellite-derived cloud water content and cloud cover, and surface observations of precipitation. It is found that when resolution is enhanced, precipitation systematically increases while cloud cover decreases. The main reason is that much stronger vertical motions develop at high resolution, yielding dense, narrow clouds, with clear air between. The consequences of these results for cloud parametrization are discussed. It is further shown that cloud-cover simulations over mountainous terrain can be improved by paying regard to the sub-grid-scale orography.

Cloud parametrization at different horizontal resolutions

As faster computers emerge, resolution continues to improve in weather-prediction and climate models. How does this affect the problems that we are dealing with in the models? In this study we have concentrated on one of these problems, namely the treatment of clouds. Among the questions addressed are: How do cloud-related processes (condensation, precipitation) depend on grid resolution? How should we take this into account? Can we simulate orographic clouds when we have a grid resolution of hundreds of kilometres (climate models)? To tackle these questions simulations are performed with a limited-area model that is run with horizontal grid resolutions of 150,50 and 25 km. The model has a sophisticated treatment of condensation and clouds. For verification, use is made of satellite-derived cloud water content and cloud cover, and surface observations of precipitation. It is found that when resolution is enhanced, precipitation systematically increases while cloud cover decreases. The main reason is that much stronger vertical motions develop at high resolution, yielding dense, narrow clouds, with clear air between. The consequences of these results for cloud parametrization are discussed. It is further shown that cloud-cover simulations over mountainous terrain can be improved by paying regard to the sub-grid-scale orography.

LIGHT RELATIONS OF TERRESTRIAL PLANT COMMUNITIES AND THEIR MEASUREMENT

Summary1. ‘Light’ covers a variety of related measures of radiant energy: the particular usage must be defined for each investigation.2. Light in the open varies in intensity and spectral composition, both with time and over space. These variations can usually be formulated mathematically and used to predict conditions in plant communities.3. Light may be measured directly with radiometric instruments, responding equally to all wavelengths within a certain waveband, or with photometric instruments, whose response approximates to that of the human eye. The use of the former is desirable whenever possible, as it simplifies subsequent interpretation of the results.4. Photographs covering an entire hemisphere can be used to estimate the contribution of diffuse and direct light separately under many circumstances. They are a great help with the interpretation of many forms of measurement.5. If the percentage of light transmitted by a plant community is to be treated as constant, diffuse and direct light must be treated separately. Even so, it is only the average transmission over considerable periods that is reasonably constant. The pattern of diffuse light distribution in the community is very different on clear and on cloudy days.6. In general, most diffuse light penetrates the plant community near the zenith. However, on clear days the aureole of light near the sun shining through a low‐altitude canopy gap may entirely alter this relationship.7. The distribution of direct light can be analysed from tracing the path of the sun across the canopy. The sun shines unobstructed through gaps of an angular width greater than 1/2.8. In some herbaceous communities the logarithm of relative intensity of diffuse light inside and outside the community is approximately proportional to the cumulative leaf area per unit area of ground above the level investigated. The relation varies with the angle of incidence of direct sunlight. The relation is not valid in all communities, and the errors involved in the assumption of its validity require further investigation.9. The leafy plant community reflects and transmits light selectively. It is important to allow for the interaction of changed spectral composition on the spectral sensitivity of the instrument when making estimates of albedo or percentage light transmission.10. Although the spectral characteristics of reflexion and transmission of individual leaves are nearly identical, this is not so for plant communities with a highly inhomo‐geneous structure.11. The percentage of total light transmitted usually increases with increasing cloud cover of the sky. The absolute intensity of diffuse light in the stand is usually higher when the sky is partly overcast than when the sky is entirely covered or clear. This is due to the increase of total light in the open with decrease of cloud cover, while the proportion of diffuse light in the total decreases with decrease in cloud cover. Provided the stand transmits more diffuse than direct light, the percentage transmission of the total will therefore fall as the total increases.12. Daily variation of diffuse light generally follows that in the open.13. In deciduous woods a light phase before leaf expansion and a shade phase after it can be distinguished. Each phase is characterized by a fairly constant percentage of diffuse light: that of direct light depends on daily and seasonal variation of solar altitude, increasing with increasing altitude.14. Light measurements given as percentages of that in the open may disguise important features of the variation of absolute amounts of light. Wherever possible these absolute quantities should be calculated. Where no direct meteorological data exist, it is often possible to make reasonable assumption from other data. The precise time, place and cloud conditions should be given for each measurement,15. Because of the great variety of instruments and techniques used, often inadequately described, comparison of results is nearly impossible. A thorough examination of the magnitude of the errors of each technique is badly needed.