Revision 05
01.10.2015
A Cloud and Precipitation Process Mission (CaPPM)
1. Introduction
The 21st century poses extreme challenges for the sustainable management of water resources at all levels from the local to the global scale. Water is a fundamental requirement for life and effective water management is needed to provide society’s most basic needs. However, demand for water resources is increasing, due to population growth and economic development, while water resources are under pressure globally from over-extractionand pollution. This is increasingly leading to competition for water, at local, regional and international levels. Environmental change is adding to these pressures as anthropogenic influences changeland, precipitation, drainage basins and groundwater aquifers that supply the bulk of the renewable freshwater supply to society.
The international climate community through the World Climate Research Programme (WCRP) has identified the issues underlying water availability as one on the grand challenges facing both our understanding of and ability to adapt to climate change. Underpinning this challenge is the need to predict precipitation and to understand the role of clouds and their transition to precipitation withinthe evolving climate system. Joint cloud and precipitation observations that can advance the prediction of evolving characteristics of major precipitation bearing weather systems is central to progress on such an important challenge.
The importance of the water cycle and precipitation in particular has led to significant investments by NASA, JAXA and ESA over the last two decades. TRMM, in particular, was a highly successful mission that first mapped the tropical rainfall and its associated latent heating from radar and radiometers and has been used extensively for assessing climate models. Its success was the basis for NASA/JAXA’s current GPM mission that was designed not only to extend the accuracy and coverage of precipitation mapping by radar/radiometer combinations to higher latitudes, but also to take the high quality precipitation profile information provided by GPM’s core satellite and use that to unify an existing constellation of passive microwave radiometers. Cloud properties have been advanced in parallel by the current CloudSat mission that has not only defined cloud vertical structures needed to improve our understanding of the Earth’s radiative balance, but has also complemented the precipitation radars flown in TRMM and GPM by providing the first ever statistics of drizzle and light precipitation that is associated with shallow clouds around the world. Its success will besustainedby EarthCare, a joint JAXA/ESA mission to be launched in 2016,that will add important information on vertical motions through the first ever use of Doppler measurements. GPM, CloudSat, and EarthCare are also capable of measuring falling snow, an important contribution to society’s water resources. Together, these missions have been foundational for characterizing the evolving nature of clouds and precipitation on Earth over a few decades.
In order to predict future climate, what is needed is not only the characterization of cloud and precipitationproperties in the current climate, but elucidating the processes operating within the atmosphere to create these clouds and precipitation. By processes, we mean the fundamental mechanisms governing the evolution of embryonic cloud droplets and ice crystals to precipitation sized particles, mechanisms that should be valid no matter what changes in the Earth’s circulation are predicted. These nucleation and growth processes from cloud droplets and ice crystals to raindrops, snow flakes, graupel and hail, ultimately determine the distribution of precipitation at the surface. Latent heating, and the vertical velocities associated with it are critical to enhancing our understanding of these microphysical processes, and hence our understanding of hydrometeor nucleation and growth processes, the vertical transport and distribution of hydrometeors, and the dynamical-microphysical feedbacks that occur through phase changes.Such understanding is critical if we are to advance cloud/precipitation processes in Cloud Resolving Models (CRMs) as well as parameterizations in weather and climate models as these numerical models approach cloud-permitting scales.
While progress has been made in representing both warm rain and ice processes inmodels, significant challenges remain. It is apparent from comparisons of simulated warm clouds with CloudSat data that rain is typically produced too quickly in models (Suzuki et al 2011; King et al 2014). Furthermore, the multiple pathways by which ice nucleates and grows in the atmosphere, and the various ice species that may result, is still not well known, and hence are often poorly parameterized in CRMs. Small changes to the manner in which ice processes are parameterized, such as varying the density of graupel or hail, have significant implications for the prediction of precipitation rates, characteristics and accumulations, the vertical distribution of energy, the strength and location of the strongest updrafts, and the timing and intensity of surface wind fields (Gilmore et al 2004; van den Heever and Cotton, 2004; Adams-Selin et al. 2013; Bryan and Morrison, 2013; Varble et al 2014). Measured statistics are desperately needed that couple in-cloud vertical velocities to resulting hydrometeor types and their profiles in order to improve the dynamical and microphysical parameterizations, and the feedbacks between them, in CRMs.
2. Specific Questions
While the skill of weather forecast and climate models is increasing as we better understand atmospheric processes and increase the spatial and temporal resolution of these models in order to capture physics at the appropriate horizontal and vertical scales, there are also some issues, particularly related to convective clouds, that remain far from solved and thus form the backbone of the CaPPM mission. Two overarching questions related to processes that govern precipitationrequired for modeling future climate scenarios are: “What cloud processes lead to the onset of precipitation?” and “What cloud processes determine the distribution of precipitation rates, including extremes?”
2.1 What cloud processes lead to the onset of precipitation?
Precipitation sized particles cannot form sufficiently fast through condensation and deposition processes alone. Studies show that larger particles form either through the warm rain collision-coalescence process, or the cold rain process that involves ice-phase particles colliding with and collecting super-cooled water (riming) or other ice particles (aggregation). A number of in-cloud and environmental factors impact the collision-coalescence processes necessary for warm rain production, and hence for the conversion from cloud droplets to rain drops. The number concentration and size spectra of cloud condensation nuclei (CCN) are important factors determining the initial cloud droplet size distribution. Broader size distributions tend to enhance droplet collisions as particles of different sizes fall at different speeds.There is thus a great deal of synergy between this concept and the ACE mission concept that focuses on observations of both aerosols and clouds. In addition to aerosols, the strength of the updraft determines the rate at which supersaturation is produced, and hence the cloud droplet nucleation and growth rates within different regions of the cloud, which in turn produces droplet populations varying in size and fall speeds. In-cloud turbulence assists in droplet growth by increasing collision opportunities between drops, while turbulence at cloud boundaries enhances collision coalescence processes through enhanced evaporation of smaller droplets and growth of larger droplets. The schemes representing the conversion of cloud water to rain vary in sophistication from simple, computationally cheap schemes that convert cloud water into rain water when certain cloud liquid water contents are available, to more complex, computationally expensive schemes that represent the interactions between droplets. Nature, however, is more complex and there are clear differences between the onset of precipitation and cloud liquid water contents under different environmental conditions. This needs to be better understood so that statistical parameterizations that better represent real cloud evolution can be introduced in numerical models. While more field experiments have focused on liquid rather than ice drop formation, the basic processes are not well understood for either.
2.2What cloud processes determine the distribution of precipitation rates, including extremes?”
The distribution, or Probability Distribution Function (PDF) of precipitation is an extremely important characteristic in connecting atmospheric processes to water availability. The same 20 mm of precipitation, for instance, will yield completely different run-off and stream levels if it falls over the course of a day as steady rain as opposed to a few minutes in a torrential downpour. Water availability is thus not only linked to total rainfall which at least globally is not likely to change substantially from present levels, but tightly coupled to regional variations and to the PDF of this rainfall. The primary factor governing likely regional redistributions of precipitation is related to the global circulation while the future PDFs of precipitation are governed by cloud microphysics and vertical air motions in particular. Locally intense updrafts (~10’s of m s–1) are responsible for locally heavy rain, graupel or hail. But just as important as instantaneous vertical air velocity is the vertical transport of mass, which can occur in large amounts as a result of numerous moderate updrafts (~1–10 m s–1) or steady weaker upward movement of air (<1 m s–1).
Latent heating associated with the condensation and deposition of water vapor onto cloud water and ice particles enhances the strength of the updraft. The microphysical processes involved in extreme precipitation in turn depend on the strength of the upward motions. Understanding the feedbacks between the storm dynamics and the cloud microphysics is thus critical. When the updrafts are moderate-to-large, collection processes tend to dominate. Coalescence of liquid drops are extremely important below the 0°C level, and the riming of ice in the mixed-phase regions of the cloud is predominantin all forms of intense convection in which strong and moderate updrafts prevail. Aggregation of ice particles becomes more importantwhen the updrafts are weaker and occur over larger areas and longer time periods. When the extreme precipitation processes involve weaker vertical air motions in deep clouds, vapor deposition on ice particles prior to their undergoing riming and aggregation will be critical. The rate and vertical location in which these deposition and collection processes occur are still not well represented in numerical models. Furthermore, aspects influencing such processes such as the depth of the mixed layer are often poorly simulated. Errors in representing these processes leads to errors in the relative amounts of liquid water and ice within clouds, and ultimately to errors in precipitation rates and intensities. The better we can distinguish the processes of collision-coalescence, riming, aggregation and vapor deposition on ice from spaceborne platforms, the more effectively will we be able to identify the occurrence of extreme precipitation for both weather and climate considerations. There are very few in-situ measurements of these processes and feedback between vertical velocities and cloud microphysics is sufficiently complex that large scale improvements in the cloud model parameterizations is only possible through broad scale global statistical understanding of these relationships as envisioned and measured by CaPPM.
3. Measurement Concept
The science objectives outlined above require an observational strategy that significantly advances upon the current capabilities afforded individually by the TRMM, CloudSat ,GPM, and EarthCare missions, as well as the capabilities afforded jointly through these missions in convoys (A-Train concept) and constellations (GPM intercalibration concept). While measurement tradeoff studies are still underway, the core of the required observing system is envisioned to be a triple-frequency (Ku/Ka/W-band) Doppler radar with sufficient spatial resolution to resolve convective scalesat 1-5 km, together with precipitation imaging microwave radiometer capabilities. The combination of three radar frequencies on a single platform allows complete observation of all modes of precipitation from drizzle to deep convection and liquid to ice phase processes. Furthermore, the inclusion of Doppler measurements to this observational platform affords a direct link between the storm dynamics and cloud microphysics. Motivated by the focus of the mission on cloud and precipitation processes, the emphasis of the observations is on quality (i.e., sensitivity, resolution, and Doppler accuracy) as opposed to quantity (i.e., swath width, multi-beam); however a swath sufficient to capture the 3-D organization at the convective or meso-scale (~ XX km) is necessary to achieve the aforementioned quality of observations, and a wider swath would strengthen the representativeness of the radar observations for use in calibrating radiometric observations intended for precipitation mapping. Because of the complexities of simultaneous estimation of cloud and precipitation microphysics, the core radar sensor will benefit from any combination of additional observations including; (1) a simple imager with heritage channels in the visible, near-infrared, and infrared channels typical for cloud remote sensing, (2) a sub-millimeter ice cloud imager, and (3) a cloud lidar of any class including that currently operated in space. While all of these ancillary observations would provide unique and useful information, not all of the ancillary observations are required for CaPPM mission success. Furthermore it is expected that there will exist multiple opportunities for the formation of observing system convoys and constellations that will satisfy these ancillary measurement needs.
Radar technology specifically relevant to spaceborne cloud and precipitation radars has advanced significantly in the last 5 years through ongoing advancements in component technologies motivated by other application sectors: miniaturization, efficiency and maximum RF peak power of solid state power amplifiers, especially at Ka- and W-band; digital processing capacity and miniaturization; innovative machining and packaging techniques. All of the above have been at the focus of NASA-sponsored instrument development projects funded mainly by ESTO - but also by JPL and GSFC institutional funding, as well as other Programs such as ACE pre-formulation studies. The most significant evolution with respect to the first decade of this millennium is that now both Ka-band and W-band channels with electronic scanning, sufficient sensitivity and Doppler accuracy are within reach.
4. Synergies
CaPPM will not only advance our scientific knowledge of cloud and precipitation processesin the next decade, but in conjunction with other earth-observing missions will benefit many other areas of earth science as well.
Clouds could not exist without aerosols, and better understanding of aerosol indirect effects on cloud albedo, propensity to precipitate, and lifetime remain a source of uncertainty in climate change attribution, prediction, and geoengineering studies (IPCC 2013, Chapter 7). ACE is an aerosol-cloud and ocean ecosystem mission aimed at reducing the uncertainty in climate forcing in aerosol-cloud interactions and ocean ecosystem CO2 uptake (Decadal Survey pg. 4-4). One of the main goals of ACE is to quantify the role of aerosols in cloud formation, alteration of cloud properties and changes in precipitation. The complementary focus on precipitation processes envisioned for CaPPM with the careful documentation of aerosols and cloud objectives of ACE will provide a more in depth assessment of the aerosol impact on the earth’s hydrological science. Future aerosol-measuring satellite platforms will include high spectral resolution lidar and polarimeters which are needed to discriminate between natural and anthropogenic aerosols and their vertical distribution (Hair et al., 2001, Cairns 2003). Coincident overpasses of CaPPM, with its detailed cloud and precipitation profiles, and these aerosol measurements will provide the necessary context to understand the role of aerosol concentration and composition in cloud formation and precipitation.
CaPPM will continue and enhance the role that TRMM began and GPM currently performs as a detailed probe of cloud and precipitation profiles, which in turn are used to derive precipitation from a diverse constellation of research and operational passive microwave radiometers (Hou et al., 2014) at three-hourly temporal resolution, the minimum requirement for hydrological applications. The high-resolution, three-frequency radar measurements and multispectral passive microwave measurements will provide an even more accurate reference for the future constellation of microwave radiometers such as the METOP-SG Sentinels and continue the global climate record of precipitation that began with SSMI in 1987. Maintaining this record is critical for understanding changes in the distribution, intensity, and frequency of precipitation with respect to internal climate variability and long-term trends. It is also important for understanding of the feedbacks between the hydrological cycle and carbon cycle (Friedlingstein et al., 2006), and ocean circulations, which determine the heat and carbon fluxes into the ocean, as influenced by salinity gradients (Koblinksy et al., 2003).