A “climatology” of supercooled cloud tops is presented for southeastern Australia and the western United States, where historic glaciogenic cloud-seeding trials have been located. The climatology finds that supercooled cloud tops are common over the mountainous region of southeastern Australia and Tasmania (SEAT). Regions where cloud-seeding trials reported positive results coincide with a higher likelihood of observing supercooled cloud tops. Maximum absolute frequencies (AFs) occur ∼40% of the time during winter. There is a relationship between the underlying orography and the likelihood of observing supercooled liquid water (SLW)-topped clouds. Regions of the United States that have been the subject of cloud-seeding trials show lower AFs of SLW-topped clouds. The maximum is located over the Sierra Nevada and occurs ∼20% of the time during winter (Sierra Cooperative Pilot Project). These sites are on mountains with peaks higher than any found in SEAT (>3000 m). For the Sierra Nevada, the AF of SLW-topped clouds decreases as the elevation increases, with glaciation occurring at the higher elevations. The remote sensing of supercooled cloud tops is not proof of a region’s amenability for glaciogenic cloud seeding. This study simply highlights the significant environmental differences between historical cloud-seeding regions in the United States and Australia, suggesting that it is not reasonable to extrapolate results from one region to another. Without in situ cloud microphysical measurements, in-depth knowledge of the timing and duration of potentially seedable events, or knowledge of the synoptic forcing of such events, it is not possible to categorize a region’s potential for precipitation augmentation operations.
Intentional weather modification has been the subject of much scrutiny and skepticism. The U.S. National Research Council’s report entitled “Critical Issues in Weather Modification Research” (NRC 2003) concluded that there is “a lack of convincing scientific proof of the efficacy of intentional weather modification efforts.” The report identified numerous challenges for the field, including the need to better understand the natural environment in which the modification is attempted, and recommended capitalizing on “new remote and in-situ observation tools” to remedy this situation.
The Commonwealth Science and Industrial Research Organization (CSIRO), based in Australia, invested considerable resources in the field of glaciogenic cloud seeding from the 1940s through the 1990s (Ryan and King 1997). Only three field experiments were reported to have positive impacts: the Snowy Mountains during 1955–59 (Smith et al. 1963), Tasmania I during 1964–71 (Smith et al. 1979), and Tasmania II during 1979–83 (Ryan and King 1997). As explained in Ryan and King (1997), criticism over the design of Tasmania I, which found a 30% increase in precipitation during seeded events, ultimately led to the second field experiment, Tasmania II. Here, a 37% increase in precipitation was reported. The Victorian Alps were listed as a third potential location for glaciogenic cloud seeding (Ryan and King 1997) on the basis of measurements that observed relatively high concentrations of supercooled liquid water (Long and Huggins 1992).
The operational cloud-seeding program in Tasmania is the only Australian example of a cloud-seeding trial that has progressed from experimental to operational. The recent operational period is documented in Morrison et al. (2009) together with an analysis of the surface precipitation records over the 46-yr period of 1960–2005. This analysis suggests that during months of cloud-seeding activity (either operational or experimental), a 5%–13% increase in monthly rainfall is observed at the 5% significance level for 9 out of 10 independent tests.
Over the past decade there has been a renewed interest in glaciogenic cloud seeding in Australia. Snowy Hydro, Ltd., is currently undertaking a trial over the Snowy Mountains, the Snowy Precipitation Enhancement Research Project (Manton et al. 2011). Results for the first stage of this project (2004–09) indicate that a statistically significant 14% increase in surface precipitation was observed for seeded events when the analysis was constrained to seeding events that were well targeted (Manton and Warren 2011). The reported success of repeated cloud-seeding trials in southeastern Australia and Tasmania (SEAT) indicates that the environment of this region could be especially amenable to anthropogenic precipitation augmentation through glaciogenic seeding. It is therefore an ideal location to implement a climatological analysis to understand more fully the microphysical background state—a strategy suggested by the NRC report (NRC 2003).
Over the past decade there has been a dramatic increase in the ability to remotely observe clouds through passive and active sensors. Hu et al. (2010) employed the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIOP) instrument (Winkler et al. 2007) to highlight the frequent occurrence of clouds composed of supercooled liquid water (SLW) over water at higher latitudes (>50°). In particular, SLW-topped clouds were observed virtually year-round over the Southern Ocean. Morrison et al. (2011) employed the Moderate Resolution Imaging Spectroradiometer (MODIS) (Platnick et al. 2003) to similarly produce a climatological analysis for the storm tracks of the Southern Ocean and the North Pacific Ocean. This work revealed a substantial increase in the frequency of SLW-topped clouds at high latitudes, especially over the Southern Ocean (relative to the North Pacific). These storm systems account for much of the wintertime precipitation experienced across SEAT (Pook et al. 2006; Chubb et al. 2011), providing a further motivation to better understand the nature and causes of SLW in SEAT.
Huang et al. (2012) examined the thermodynamic phase of low-level clouds over the Southern Ocean using CALIOP and the so-called DARDAR mask, which is a combined radar–lidar product (Delanoe and Hogan 2010). Consistent with Morrison et al. (2011), SLW was observed to be the dominant cloud-top phase at temperatures between 0° and −20°C, rather than mixed phase or glaciation.
This paper presents a MODIS climatological analysis, or “climatology,” of supercooled cloud tops for the regions proposed to be likely candidates for precipitation enhancement through glaciogenic seeding by Ryan and King (1997): the mountainous regions of SEAT. This climatology is then contrasted against the historical glaciogenic cloud-seeding sites in the western United States to illustrate differences between the two regions.
2. Data preparation
MODIS instruments have been operating on the Terra and Aqua platforms since 1999 and 2002, respectively (Platnick et al. 2003). A wide range of data products are available. This study utilizes the level-2 (processed) products of cloud-top temperature (CTT) and cloud-top phase (CTP), as was done in Morrison et al. (2011). The resolution of the level-2 product varies from 5 km × 5 km at the nadir to approximately 5 km × 20 km as the instrument scans the surface.
The CTP product is derived from the inferred CTT and the brightness temperature difference between the 8.5- and 11-μm channels. The phase is split into four categories (ice, mixed phase, liquid, or uncertain) and is specified when the cloud-detection algorithm indicates the likely presence of a cloud within a pixel. For a detailed description of the MODIS cloud-detection and cloud-top phase-determination algorithms, see Platnick et al. (2003). The limitations of the cloud-detection, CTT, and CTP-determination algorithms are discussed in Morrison et al. (2011). Because the focus of this climatology is on supercooled clouds, an estimate of uncertainty in the MODIS instrument’s ability to confidently classify supercooled cloud tops is desirable. Cho et al. (2009) compare the MODIS CTP product with the CALIOP version-2 (V2) cloud-phase product and conclude that MODIS has a relative tendency to classify opaque midlevel clouds as unknown or mixed phase. MODIS estimates of supercooled cloud tops are therefore deemed conservative relative to the CALIOP V2 product.
An example MODIS level-2 CTP and CTT retrieval is shown in Fig. 1. An uncertain retrieval arises when the entire 5 × 5 block of pixels has mixed readings for either CTT or CTP. It also occurs as the default when the bispectral CTP algorithm fails to confidently define one of ice, liquid, or mixed phase. In this study, the liquid CTP is further split into two distinct phases: warm liquid water (≥0°C) and SLW (<0°C). Further discussion is limited to the prevalence of the latter.
The results presented herein cover the 5 yr from 2005 to 2009 with the austral winter (summer) being defined as the three months of June, July, and August (December, January, and February); vice versa for the boreal hemisphere. Approximately 5000 MODIS images from both the Terra and Aqua platforms were processed over southeastern mainland Australia, ∼4000 images were processed over the island of Tasmania (south of mainland Australia), and more than 15 000 images were processed over the continental United States. In broad terms, any 5° × 5° region of the earth’s surface is observed in part by a MODIS instrument ∼250 times in any given month. The observations cover both night- and daytime overpasses.
The construction of the climatology involved assigning each individual MODIS level-2 pixel to a 0.25° × 0.25° region on the earth’s surface. In general, a region of this size within the midlatitudes is observed by two–three flyovers per day; this number can be as low as one, however. If a MODIS level-2 pixel is classified as cloudy, both the CTT and CTP for this pixel are stored. The final dataset is simply the absolute frequency (AF) with which a given CTP occurs in each individual 0.25° × 0.25° region. This is the same method as was implemented in Morrison et al. (2011). The frequency is with respect to all observations, cloudy and clear, so if 1000 MODIS pixels observed a location somewhere in a given 0.25° × 0.25° region (including both cloudy and clear) and 100 of these observations were classified as containing supercooled clouds then it would yield an AF of 0.1.
A subset of this climatology was constructed for MODIS images taken during local daylight hours only (1000–1400 local standard time). The purpose here was to investigate the possibility that the MODIS cloud-detection algorithm was biased in low-light conditions over snow-covered terrain. Differences between the full climatology and the daylight-only climatology were not found to be particularly noteworthy. Only the full climatology is considered hereinafter.
For the purposes of glaciogenic seeding, it may be readily argued that a temperature threshold of colder than 0°C would be more appropriate. Figure 2 illustrates the cloud-top SLW climatology over Tasmania for thresholds of 0°, −5°, and −10°C, respectively. The AF of supercooled cloud tops peaks at approximately 40% for the 0°C threshold and drops to a peak of 5% at the −10°C threshold. When moving to the colder temperatures with lower AFs, the climatology displays a greater element of noise that weakens the relationship between the AF of SLW-topped clouds and the underlying topography. In an effort to minimize the element of noise within the climatology, the 0°C threshold is used for the rest of this paper.
The AF values reported herein cannot be directly interpreted as the percentage of time that cloud seeding would be viable at a given location, even if a colder threshold were to be employed. Without further observations, it is not possible to extrapolate SLW at cloud top to liquid water through the entire cloud layer (to infer seedability). For example, Rauber and Tokay (1991) illustrated the common occurrence of a thin layer of SLW overlying a glaciated cloud. Conversely, overlying cirrus that may be quite removed from any orographic clouds have the potential to partially or completely mask any underlying orographic clouds. These multilayer cloud structures are known to be particularly difficult for a radiometer like MODIS, for which values of CTT and CTP may not be clearly defined (Platnick et al. 2003). Nasiri and Kahn (2008) examined the limitations of the MODIS CTP algorithm, finding that more than 75% of cloudy retrievals between −8° and −23°C are classified as either mixed phase or uncertain. They further find that even though mixed-phase clouds are not uncommon, especially within this temperature range, multilayer clouds can have spectral signatures that imitate midlevel or mixed-phase clouds. Ultimately, the MODIS CTP algorithm is, by design, conservative in the identification of liquid water, with poorly defined retrievals commonly classified as uncertain. The CTP climatology of Morrison et al. (2011) highlights the common occurrence of uncertain retrievals between −10° and −20°C over the Southern Ocean. Huang et al. (2012) find that within this temperature range both CALIOP and the DARDAR mask predominantly record SLW.
It is not possible to employ active remote sensors like CALIOP or a combined lidar–radar product to construct a climatology over a given mountain. One-dimensional horizontal paths preclude the study of specific orographic effects. For example, the A-Train orbit has gaps of over 100 km above the midlatitudes. Moreover, the track is repeated only once every 16 days, which means that over the course of five winters there are only 29 repeated granules. It is even unclear that a lidar alone would be a better tool for such a climatology given that the instrument cannot distinguish SLW from mixed phase and would be highly sensitive to thin layers of SLW overlying thicker clouds (Hogan et al. 2004). Chan and Comiso (2011) have further highlighted that CALIOP has the potential to miss low-elevation clouds that are picked up by MODIS.
3. Mainland southeastern Australia and Tasmania
Focused exclusively on clouds with tops composed of SLW (CTP = liquid and CTT < 0°C), Fig. 3 shows the AF with which MODIS detects such clouds during the austral winter (June–August) for the 5 yr spanning 2005–09 over southeastern Australia. Figure 2a is for the same time period but over the island of Tasmania. During the winter months, MODIS observes clouds with tops composed of SLW over many regions of the Australian Great Dividing Range and the western Central Highlands of Tasmania. Over some 0.25 × 0.25° regions, ∼40% of all observations are of supercooled cloud tops. There appears to be a strong relationship between the underlying elevation and the AF of supercooled cloud tops. A similar climatology for the uncertain CTP (not shown) shows a similar pattern, with peak AFs reaching ∼30% over the highest peaks across SEAT.
The immediate impression is that this MODIS climatology of supercooled cloud tops supports the historical conclusions of Ryan and King (1997) that portions of SEAT may be especially amenable to glaciogenic cold cloud seeding. In particular, they show that these alpine regions have an environment in which the occurrence of a supercooled cloud top is a relatively common event. It is noted that unsuccessful experiments conducted by CSIRO (e.g., western Victoria, the New England district of New South Wales, and southern Australia) show absolute frequencies of supercooled cloud tops to be less than 0.2. While this climatology may be argued to support the historical fieldwork, this is by no means a complete scientific argument. It is entirely possible that these MODIS-inferred SLW-topped clouds are not actually viable for cloud seeding. As stated earlier, thin layers of SLW overlying ice (Rauber and Tokay 1991) are not suitable for cloud seeding. In the region of SEAT, however, there are observations that support a connection between SLW-topped clouds and clouds suitable for seeding. Morrison et al. (2010) examined two case studies over Tasmania with in situ airborne cloud observations and compared these with the nearest available MODIS images.
The elevations in this region are relatively modest in comparison with other dividing ranges around the world. The peak elevation of the Australian Great Dividing Range is 2228 m, at Mount Kosciuszko in the Snowy Mountains. Tasmania has a peak elevation of only 1617 m at Mount Ossa. As one moves away from the regions of orographic enhancement, the AF of SLW-topped clouds drops to a background level of less than 0.1. The background AF increases to the south of Tasmania, consistent with Morrison et al. (2011), who found that the peak AF of supercooled cloud tops resides over the remote Southern Ocean near 60°S (>0.40).
The climatology of the ice CTP (not shown) displays a very modest orographic signature. This presumably is a consequence of the Great Dividing Range and Central Highlands being at such modest elevations. It appears that orographic lifting over these regions produces little glaciation.
Similar 5-yr climatologies have been made for all months of the year, including the summer months of December–February (not shown). These images suggest a lack of SLW during the summer period. The peak AF over the Great Dividing Range drops to 0.05, and the peak AF over Tasmania drops to 0.20 over the southwestern corner. It is noted that during Tasmania I cloud seeding was attempted year-round (Smith et al. 1979); this practice was discontinued in Tasmania II because of a lack of suitable events during the summer months (Ryan and King 1997). This decrease in the AF of supercooled cloud tops is unsurprising and is likely due to the poleward displacement of the Southern Ocean storm tracks during the summer months (Simmonds and Keay 2000).
4. Western United States
As a point of comparison, a climatology of supercooled cloud tops is produced for the western United States (Fig. 4), where a number of winter orographic glaciogenic cloud-seeding trials were held. NRC (2003) lists examples of glaciogenic winter orographic experiments including the Sierra Cooperative Pilot Project (SCPP; Deshler et al. 1990), Climax I and II (Mielke et al. 1981), and the Bridger Range experiment (Super and Heimbach 1983). Figure 5 portrays the AF of SLW-topped clouds, as seen by MODIS, for the locations of these three experiments. The images are for the boreal winter months (December–February) for the 5 yr of study (2005–09).
There are relatively low AFs of supercooled cloud tops in the three regions shown in Fig. 5, relative to SEAT. Strictly on the basis of these images, the best potential location for cloud seeding would be over the Sierra Nevada where a clear orographic enhancement is observed on the upwind side of the ridge. Of interest is that the AF decreases farther toward the peak. This remotely sensed MODIS observation is consistent with the in situ measurements of Deshler and Reynolds (1990), where SLW was found on the upwind side of the mountains and ice was observed at the top. Glaciation is commonly occurring as part of the natural process, and in such events glaciogenic cloud seeding will presumably increase the rate of this process. This situation is not evident across SEAT, where little glaciation is observed and peak likelihoods for observing supercooled cloud tops are located over the highest regions of the terrain.
It is interesting to note that the peak AF of SLW-topped clouds across the western United States occurs over the Glen Canyon region of southern Utah, which is an unlikely region for glaciogenic cloud seeding and is actually in the lee of some of the mountains previously explored for cloud seeding in Utah (Rauber and Grant 1987). Without in situ observations it is not possible to assess whether such clouds are actually appropriate for seeding or are inappropriate (e.g., a thin layer of SLW overlying ice). Another region of potential interest is along the coast of western Washington and Oregon where SLW has been observed (e.g., Stoelinga et al. 2003). Indeed, the elevation of the Olympic Mountains is not all that much higher than that over SEAT.
5. Concluding remarks
This paper has demonstrated the value of utilizing satellite observations to identify potential regions for glaciogenic cloud seeding/weather modification, as suggested by NRC (2003). By utilizing the MODIS instrument on the Terra and Aqua platforms, a climatology comprising the absolute frequency of occurrence of supercooled cloud tops has been presented. The analysis presented herein focused on locations where glaciogenic cloud seeding has taken place in southeastern Australia and Tasmania and in the western United States. The climatologies illustrate the differences between the two regions with respect to the presence of supercooled liquid water.
The satellite observations show that supercooled cloud tops are common over the alpine regions of SEAT. The likelihood of observing SLW at cloud top over this region is as high as 40% of the total time during the winter months. These observations are consistent with the high concentrations of SLW as measured by aircraft over Tasmania; both Ryan and King (1997) and Morrison et al. (2010) report events with concentrations of supercooled liquid water of greater than 0.3 g kg−1 (5-min average) at temperatures between −6° and −8°C. This value is about 3 times the typical concentrations in the Canadian freezing-drizzle experiments reported by Guan et al. (2001, 2002) and Vaillancourt et al. (2003) and is substantially larger than was reported over the Sierra Nevada, where the most common peak SLW concentration (per flight track) was ∼0.1 g m−3 (Deshler and Reynolds 1990).
The likelihood of observing SLW-topped clouds over regions of the western United States that have previously been the subject of cloud-seeding trials is much lower—less than 20%. One hypothesis for this difference is that peak elevations in this region are much higher than those of SEAT, leading to colder temperatures at the mountaintops and therefore to natural glaciation. The differences in the climatologies of SLW between SEAT and the western United States are not proof either that cloud seeding is viable over Australia or that it is not viable over the western United States. Indeed, the observations presented in Geerts et al. (2010), who used a Doppler radar to study microphysical changes in clouds undergoing anthropogenic glaciogenic seeding, provide physical evidence of the impact of glaciogenic seeding in the western United States that is not available for SEAT. The climatologies do, however, suggest that these environments are fundamentally different and that it is not possible to extrapolate conclusions from one region to the other.
Despite the observed levels of SLW and the positive field results of Smith et al. (1963, 1979), Ryan and King (1997), Morrison et al. (2009), and Manton and Warren (2011), the physical understanding of the response of these clouds to glaciogenic seeding remains to be fully documented. This fact may be particularly relevant given the unique environment of the Southern Ocean. These clouds are where the Hallett–Mossop ice-multiplication process was first discovered (Mossop et al. 1970) and, according to CloudSat, precipitate most of the time (Mace et al. 2007). Further, they exist in one of the most pristine environments on Earth with cloud condensation nuclei concentrations that are an order of magnitude lower than in other maritime environments and are two orders of magnitude lower than a typical continental air mass (Yum and Hudson 2005; Bennartz 2007). In addition, they commonly exist in an environment of high winds/high wind shear (Russell et al. 1998; Wang et al. 1999; Jensen et al. 2000). Nevertheless, this MODIS-based climatology of supercooled clouds is compelling and provides another independent basis for the conclusions of Ryan and King (1997) that glaciogenic cloud seeding may be viable over the three Australian regions of Tasmania, the Snowy Mountains, and the Victorian Alps.
This research has been supported by the Australian Research Council through Linkage Grant LP0562358. The authors are particularly indebted to Simon Caine and Thomas Chubb. This research has also been supported by the Victorian Department of Sustainability and Environment, Snowy Hydro, Ltd., and Hydro Tasmania.