The effects of anthropogenic aerosols on rainfall and snowfall: a science assessment of WMO and the International Union of Geodesy and Geophysics
Changes in rainfall and snowfall regimes and the frequency of extreme weather events are of great importance to life on our planet. Since atmospheric aerosols, especially soluble particles called cloud condensation nuclei (CCN) and other particles that nucleate ice, called ice nuclei, are responsible for the formation of drops and ice in clouds, respectively, any changes in their chemical composition and distribution could influence the amounts and distribution of precipitation. However, quantitative testing of the hypothesis that modification of atmospheric aerosol abundance and characteristics dominates the potential changes in precipitation has not yet been done.
Much of the work that was carried out over the years addressed the issues of the effects of aerosols on clouds. The fundamental scientific understanding of cloud microphysical processes achieved in those years was summarized in many textbooks (e.g. Mason, 1971; Pruppacher and Klett, 1978). Based on many measurements and model simulations the consensus emerged that, everything else being equal, the addition of more CCN to a cloud leads to the formation of smaller and more numerous cloud drops. It has also been observed that the addition of giant CCN (GCCN) to clouds can lead to the formation of larger cloud drops (e.g. Mather, 1991). Furthermore, recent work in shallow orographic clouds shows that, due to the smaller droplets in polluted clouds, riming efficiency is smaller, leading to smaller snow crystals (Borys et al., 2000; 2003). Considering these observations and related modelling studies, it is reasonable to hypothesize that, other things being the same, the consequence of pollution on clouds should be a reduction in precipitation.
Unfortunately, the connection between aerosol loading and the amount of precipitation on the ground is not yet clear. This is partly because feedbacks between the microphysical and dynamical processes exist and can sometimes lead to enhancement or suppression of precipitation via atmospheric dynamical rather than cloud microphysical effects of aerosol. Recent examples in support of this proposition may be found in the work on mesoscale storms by Van den Heever and Cotton (2007) and on the global scale by Rotstayn and Lohmann (2002), Rotstayn (2007) and Rotstayn et al. (2007). Over the years, there have been a number of attempts to shed light on this connection but the results vary widely between increases in rainfall, decreased rainfall and no connection at all.
WMO and the International Union of Geodesy and Geophysics (IUGG) recognized the importance of this issue and formed a group to assess knowledge of this subject and suggest directions for future research. The report concludes that there is no proof that a systematic reduction or increase in precipitation is necessarily correlated with increased particle pollution that enhances CCN levels in the atmosphere (Levin and Cotton, 2008).
Below, we summarize some of the major points in this report and outline some of the main conclusions and recommendations.
The effects of aerosols on clouds
Warm clouds are those that contain no ice. Measurements have shown that an increase in CCN from natural or anthropogenic sources increases cloud drop concentrations and reduces cloud drop size (the first indirect or “Twomey” effect). These ideas have been confirmed by many in situ measurements (e.g. Warner and Twomey, 1967; Garrett and Hobbs, 1995) and by analysis of ship tracks using satellite images (e.g. Coakley et al. 1987; Durkee et al., 2000, 2001).
Rosenfeld (2000) demonstrated that satellites could be used to determine the effective radius of cloud drops as a function of height. He showed that in clouds growing in a more polluted environment, the growth of cloud drops is slower, due to the increase in their concentrations and the decrease in their effective radius. Slow growth in the warm clouds may lead to the suppression of precipitation development. Radar echoes from the Tropical Rainfall Measuring Mission (TRMM) satellite were interpreted as showing that the development of precipitation in these clouds diminishes, although the number of such documented cases has been small and is controversial (Ayers, 2005; Rosenfeld et al., 2006).
Andreae et al. (2004) showed that, in the Amazon region, clean continental clouds with relatively low concentrations of aerosols, behaved similarly to marine clouds, namely, growth by coalescence occurs early and rapidly as the cloud develops. On the other hand, precipitation particles in clouds that developed in a smoke-filled atmosphere grew slower and at higher levels. Andreae et al. (2004) argued that the slow growth of the drops led to enhanced ice formation higher up in the clouds and to enhanced updrafts, due to the increased release of latent heat. Such clouds sometimes led to hail and lightning formation.
While observations of the effects of aerosols on the development of warm clouds seem to agree with our physical understanding, the effects on mixed-phase clouds are less clear. In fact, an understanding of the role of aerosols on the formation of the ice phase in clouds is critically missing. This is partly because ice can form in clouds in a number of ways (i.e. immersion freezing, deposition, contact and condensation freezing), all of which are not clearly understood. This lack of understanding stands out when we compare the number of ice crystals in clouds with the primary ice nuclei concentrations. Reports suggest that, in some clouds, the ice-crystal concentrations exceed the ice-nuclei concentrations by a few orders of magnitude. Some other observations suggest that the average ice-number concentration is independent of temperature. It is therefore clear that better understanding of the ice processes in clouds is mandatory if we are to be able to forecast the effect of aerosols on clouds.
While the effect of aerosols on clouds is at least partially understood, the effect of aerosols on precipitation reaching the ground is even less clear.
Aerosol pollution impact on rainfall on the ground
Warner and Twomey (1967) and Warner (1968) summarized the potential effects of smoke from sugarcane burning on rainfall by looking at multi-decadal rainfall records from stations upwind and downwind of these large aerosol sources. In spite of expectation that there would be direct correlation between increased pollution and rain suppression, they could not conclusively see any such correlation (Warner, 1971).
Rain enhancement of up to 30 per cent from warm clouds downwind of paper mills in Washington State were reported by Hobbs et al. (1970). Hindman et al. (1977) analysed the same case using a one-dimensional numerical model and concluded that the GCCN emitted from the paper mill could not by themselves account for the observed large increase in rainfall. On the other hand, the combined effects of heat, water vapour and CCN from the paper mill might be responsible for the observed increased precipitation.
Using Moderate-resolution Imaging Spectroradiometer (MODIS) and TRMM satellite data, Lin et al. (2006) analysed the effects of forest fires on precipitation in the dry season in the Amazon region. They report on increases in cloud height and precipitation with increases in aerosol optical depth. The increase in cloud height led to enhanced growth of ice crystals, which culminated in heavier precipitation. Despite the good correlation between these variables, however, the authors could not clearly establish causal links between aerosols and the observed changes in cloud height or with precipitation increases. The role of enhanced convection due to the heat from the fires and/ or from heat due to absorption of solar radiation by the smoke particles could not be ruled out.
Effects of urban pollution on rainfall
A large field experiment (METROMEX) was carried out around St Louis, Missouri, USA, motivated by the examination of historical data that revealed summer increases in the immediate downwind area of the city (Figure 1). The records show increases in: rainfall (10–17%); moderate rain days (11–23%); heavy rainstorms (80%); thunderstorms (21%); and hailstorms (30%) (Changnon et al., 1971). Braham (1974) reported that the CCN production from the city of St Louis was about 104 cm2/s, much higher than the surrounding rural areas, accompanied by an increase in cloud drop concentrations and a decrease in drop size. However, the radar echoes from these clouds usually occurred lower in the atmosphere than their counterparts in the rural surroundings. This seems to contradict our physical understanding of cloud growth, but it was concluded that one way to explain the observations is to assume that the urban area also emitted GCCN which were not detected by the CCN sampling methods in use, but could be responsible for the increased precipitation.
More recently, Van den Heever and Cotton (2007) simulated the effects of pollution on precipitation during the passage of a storm in the St Louis area. The data used were of a specific day. The simulation was carried out using two main cases; one with the city of St Louis without pollution and the second with the pollution containing both small CCN and GCCN. The results show that the temporal and spatial distribution of the rain changes due to the effects of pollution (see Figure 2, which shows the difference in rain amount and distribution between a simulation run without pollution and with pollution). At the beginning of the storm, early in the afternoon, polluted clouds produced much heavier precipitation. As the storm day progressed, however, the difference between the integrated amounts of rain from the beginning of the storms of the polluted minus the clean case diminished. After 1.5 hours, later in the afternoon, the integrated rain amount over the whole area was higher in the clean case. This work demonstrates the complexity of the interaction of aerosols and precipitation. Part of the complexity appears because the initial rain cleans the atmosphere of pollution, thus reducing its effects on further rainfall. In addition, downdrafts produced by the precipitation enhance the development of neighboring clouds, thus increasing the integrated rain amounts over the whole area. Van den Heever and Cotton (2007) concluded that the effect of pollution on precipitation in an urban setting strongly depends on the background aerosol loading. Adding more pollution to an already polluted atmosphere has very little effect on precipitation amounts. They further indicated that effects of land use play a dominant role in those precipitation anomalies.
Analysis of the effects of pollution on precipitation in two different urban environments (New York, representing a mid-latitude city; and Houston, representing a more tropical environment) was carried out by Jin et al. (2005). They analysed diurnal, weekly, seasonal, and interannual variations of urban aerosols with an emphasis on summer months using 4-years of the NASA-MODIS observations, in situ AERONET observations, and in situ EPA PM2.5 data. Their analysis of monthly mean aerosol optical thickness and rainfall did not show strong relationships between aerosol and rainfall in a climatological sense.
The lack of direct relationship between rainfall and urban aerosol optical thickness implies that urban rainfall anomalies are not fully related to changes in aerosol. This observation is consistent with the earlier conclusions from METROMEX (Ackerman et al., 1978).
The effects of urban environment (Tel Aviv, Israel) on rainfall distribution and amounts was carried out by Goldreich and Manes (1979) and Goldreich (2003). They showed that, with the development and expansion of the city, the rain amounts downwind increased. This was recently confirmed by Alpert et al. (2008) after analysing more than 50 years of rainfall record in Israel. The latter further showed that enhanced precipitation increased during the years as the size and population of Tel Aviv increased. They concluded that the increase in rainfall is more probably due to the urban land-use effects (e.g. urban heat island, changes in surface roughness, surface moisture, etc.) rather then the effects of aerosol pollution.
It is therefore clear that, in spite of many measurements, there is no conclusive evidence that aerosol pollution from urban regions affects precipitation in a consistent, systematic or repeatable manner.
Rain from orographic clouds
Orographic clouds are expected to exhibit the most consistent microphysical response to microphysical changes, primarily because the dynamical responses are thought to be minimal and their natural variability smaller. Borys et al. (2000) and Borys et al. (2003) provided some evidence that pollution can delay precipitation in shallow winter orographic clouds in the Rocky Mountains. Clouds growing in a polluted environment had more numerous and smaller drops. The reduced drop size leads to less efficient riming and therefore to smaller ice crystals (Figure 3), smaller fall velocities, and less snowfall.
Givati and Rosenfeld (2004) analysed about 100 years of orographic precipitation records in California and 50 years in Israel in regions downwind of pollution sources and compared them to precipitation in regions upwind of the mountain. In their study, they documented the precipitation trends in the orographic enhancement factor, Ro, which is defined as the ratio between precipitation over the hill with respect to the upwind lowland precipitation amount. The topography in both regions studied is similar, although the mountains in Israel are much lower than the Sierra Nevada (California). Their statistical results for both locations show that downwind of prevailing winds of pollution sources on the upslope of mountains and mountain tops, orographic precipitation is reduced by ~20 per cent and ~7 per cent, respectively. It was hypothesized that this decrease is due to an increase in droplet concentration and a decrease in droplet size. Farther downwind on the leeside of mountains, the amount of precipitation is increased by ~14 per cent. The authors postulated that this increase is due to smaller cloud particles taking longer to grow, allowing the winds aloft to carry them over the mountaintop (see earlier study of similar effects produced by deliberate overseeding with ice-producing particles, by Hobbs, 1975(a) and (b). However, they hypothesized that the integrated rainfall amount over the whole mountain range was reduced by the progressively increased pollution over the years. Subsequent studies show similar decreasing trends in Ro (ratio of precipitation at high altitude sites to that at upstream low altitude sites) over a few western states in the USA (Rosenfeld and Givati, 2006; Griffith et al., 2005) and the east slopes of the Colorado Rockies during upslope flow (Jirak and Cotton, 2006). They argue that, although absolute precipitation amounts and Ro are affected by fluctuations in the atmospheric circulation patterns, such as those associated with the Pacific Decadal Oscillation and the Southern Oscillation Index, these cannot explain the observed trends in Ro.
Recently, Alpert et al. (2008) re-analysed the data from Israel, using the orographic ratio method, taking the ratio between the stations in the Samaria mountains and the stations located upwind of the urban area along the seashore, as well as stations on the mountains in western Galilee and the seashore near the city of Haifa. Their results show the opposite effects from those reported by Givati and Rosenfeld (2004; 2005), namely, the orographic ratio actually increased (see Figure 4) over the years. They concluded that, at least in Israel, other factors besides aerosol pollution dominate the precipitation amount in orographic clouds. Alpert et al. (2008) and Paldor (2008) further argued that the orographic ratio is not an appropriate method to estimate the effect of pollution on rainfall. This is because the data are very noisy and Ro can decrease not only by decreasing the numerator but also by increasing the denominator. To make things worse, many of the stations upwind of the mountains used by Givati and Rosenfeld (2004 and 2005) were located in, or downwind of, urban pollution sources, where the rain actually increased over the years, as was pointed out above. Thus, these stations were affected not only by pollution but also by other probably much more important factors, such as urban land-use effects (e.g. urban heat island, changes in frictional velocity).
In summary, it is clear that, although pollution does affect clouds, the hypothesized effects on precipitation are still not clearly understood. It seems quite probable that other factors such as synoptic-scale processes, urban effects and dynamical factors on the mesoscale dominate the precipitation amounts over the microphysical processes, while long-term trends cannot be divorced from changing weather patterns driven by global warming.
The following key conclusions can be drawn:
Based on observations (e.g. Ayers et al., 1982; Changnon et al., 1971) it can be concluded beyond doubt that anthropogenic emissions of CN, CCN and GCCN in populated areas dominate over natural emissions.
It has been demonstrated that clouds growing in a more polluted environment contain, near the cloud base, higher concentrations of smaller droplets than clouds growing in less polluted air.
The role of pollution aerosols in the formation of the ice phase in clouds is less clear, primarily due to the various ways in which ice crystals are formed.
In general, the effects of how aerosol pollution can increase or decrease rainfall is still not known. This is because of the numerous feedbacks and interactions between cloud microphysical and dynamical processes. Where temporal and spatial trends in precipitation have been analysed, changes in atmospheric dynamical processes have offered the best explanations, rather than changes in cloud microphysics, all other things being equal.
The work done to date cannot clearly rule out microphysical forcing on precipitation efficiency as a part of the story in some circumstances. Although the observations suggest that the effect on total precipitation on the ground due to modification in cloud microphysics is relatively small.
In our view, no experiments have yet been carried out anywhere that measured, in an integrated way, all or most of the processes involved simultaneously. Such experiments are needed in order to demonstrate causality, with time-series studies to define precipitation trends, while covering, at the same time, both microphysical and dynamical forcing on precipitation processes (i.e. all confounders at once) and their interactions.
Due to its importance for water supplies in many regions of the world, it is recommended that comprehensive studies of orographic precipitation be carried out. It is possible that such studies would lead to a much clearer picture about the microphysical effects of pollution on precipitation under different types of synoptic conditions.
Since changes in precipitation due to aerosol pollution could be small, improvements in instrumentation for precipitation measurements are essential.
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2 Department of Atmospheric Science, Colorado State University, Ft Collins, Colorado, USA
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