The Joint IPWG/GEWEX Precipitation Assessment

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    The Joint IPWG/GEWEX Precipitation Assessment
    (World Climate Research Programme, 2021) Roca, Rémy; Haddad, Ziad S.; Akimoto, Fumie F.; Alexander, Lisa; Behrangi, Ali; Huffman, George; Kato, Seiji; Kidd, Chris; Kirstetter, Pierre-Emmanuel; Kubota, Takuji; Kummerow, Chris; L’Ecuyer, Tristan; Levizzani, Vincenzo; Maggioni, Viviana; Massari, Christian; Masunaga, Hirohiko; Schröder, Marc; Tapiador, Francisco J.; Turk, Francis J.; Utsumi, Nobuyuki
    This report reviews the current state of precipitation products, and includes assessments of sub-daily global satellite precipitation products, discussions of various climate applications of precipitation products, and deliberations on emerging directions and the future of precipitation products.
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    Intrinsic uncertainty in the sub-daily satellite products at their native resolutions
    (World Climate Research Programme, 2021) Haddad, Ziad S.; Turk, Francis J.; Utsumi, Nobuyuki; Kirstetter, Pierre-Emmanuel
    Level-3 “sub-daily global merged satellite precipitation products” are typically reported on a fixed rectangular latitude-longitude grid at high spatial and temporal resolution (respectively 0.1º and ~0.5 hour). This section is specifically concerned with the uncertainties in these products at their reported resolution. The discussion reviews the uncertainties that are inherent in the retrieval and processing steps that are used to produce the Level-3 estimates. These include the detection error, the passive microwave (MW) and infrared (IR) estimation errors, and the error incurred when using frequent IR information to fill long revisit gaps between passive MW estimates. Advantages and disadvantages of Level-3 products are also summarized.
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    Validating the intrinsic uncertainty: Implications for hydrologic applications
    (World Climate Research Programme, 2021) Kirstetter, Pierre-Emmanuel
    The critical importance of accurate water flux estimates for science and applications explains the large body of verification analyses focusing on precipitation estimates. Very few are implemented at the relevant scales to address the intrinsic uncertainty of precipitation products. This endeavor requires the expert use of other precipitation sensors such as radar-gauge combinations. The intrinsic uncertainty structure of satellite-based quantitative precipitation estimates is still largely unknown at the spatiotemporal scales near the sensor measurement scale. Without relevant information on key uncertainty features, applications making use of satellite Level-3 precipitation products are impacted both in terms of outcomes and physical realism. Ultimately, it requires more than just one deterministic “best estimate” to adequately cope with the intermittent, highly-skewed distribution that characterizes precipitation. Advancing the use of uncertainty as an integral part of the relationship between sensor measurements and precipitation estimates has the potential to provide a framework for diagnosing intrinsic uncertainty.
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    Monitoring of satellite precipitation estimates through the IPWG validation studies
    (World Climate Research Programme, 2021) Kidd, Chris; Maggioni, Viviana
    A major activity of the International Precipitation Working Group (IPWG) is the verification, validation and intercomparison of precipitation products to enable product developers and users to continually monitor and assess the performance of the available products. This activity has developed an ongoing validation program, comparing surface reference datasets and satellite precipitation products to better inform product developers and the user community. This section gives an overview of the IPWG validation program. IPWG validation aims to improve satellite precipitation products, focusing upon statistical analysis over regions with existing reference data at moderate temporal/spatial resolutions.
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    Energy and water closure
    (World Climate Research Programme, 2021) Roca, Rémy; Kato, Seiji; L’Ecuyer, Tristan
    Energy and water budget closure has recently been extensively used to assess the consistency of precipitation products with various other terms of the water and energy budget. Most efforts are focused on water closure and explore regional scales. These studies reveal the difficulty of closing the regional water budget with the current generation of satellite and/or reanalysis products. The addition of energy constraints on the water budget allows us to enforce the closure and shows more consistent results of optimized observations. At the global scale, fewer studies have been conducted. They tend to show that only a handful of precipitation products are consistent with the radiation budget at this scale. The major recommendation of this chapter is to elaborate on the characterization of uncertainty of the precipitation product to better serve the energy and water closure study.
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    Climate variability and trends
    (World Climate Research Programme, 2021) Tapiador, Francisco J.
    This chapter describes the role of satellite precipitation datasets in the analysis of climate variability and trends. Two major applications are explored, namely the validation of the variability in the statistical moments of the climatology (mean, variance, kurtosis, etc.), and the analysis of the model-derived modes of variability (PDO, AMO, IOD, MJO and ENSO). It is shown that precipitation may be seen as the privileged field to reveal, diagnose and quantify the nonlinear relationship between the variability in the climate system and changes in mean state. The fingerprints of climate variability and trends in precipitation are highly revealing of model performances, and thus arises the need for continually improving precipitation datasets based on satellite retrievals.
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    Climate model validation
    (World Climate Research Programme, 2021) Tapiador, Francisco; Levizzani, Vincenzo
    This chapter reviews the use of satellite-derived precipitation datasets to validate climate models, with an emphasis in quality control and normalization. Two checklists are provided: one with the facts regarding the major issues that must be considered in the use of precipitation for validation, and another with recommendations for a proper use of precipitation datasets in model validation, on the line of the ISO normalization rules and quality-assurance of the whole process of validation. Validation of latent heat release, microphysics of precipitation and extremes are considered, and some examples of extensive validations of simulations vs. observations are provided.
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    Intercomparison of products for climate applications
    (World Climate Research Programme, 2021) Masunaga, Hirohiko; Akimoto, Fumie F.; Kubota, Takuji; Kummerow, Chris; Schröder, Marc
    Observational datasets of global precipitation are widely used for a range of climate applications. The precipitation products, however, are not strictly a “true” representation of nature, but have their own uncertainties related to issues such as sampling errors and algorithmic assumptions. We present here an intercomparison of 11 global precipitation datasets. Major conclusions are: - While the overall geographical pattern of precipitation is coherent among products, the magnitude varies from one dataset to the other. The agreement is poor particularly at high latitudes, since light and/or solid precipitation typical of high latitudes is difficult to estimate accurately from satellite microwave radiometry. - A systematic bias is present between gridded gauge products, which is presumably partially responsible for the spread in merged multi-satellite datasets adjusted to the gauge products. - The bias characteristics in the annual/monthly mean precipitation are a poor predictor of those in extreme precipitation.
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    Extreme and intense precipitation
    (World Climate Research Programme, 2021) Roca, Rémy; Masunaga, Hirohiko; Alexander, Lisa
    The availability of numerous gridded precipitation products and the importance of the question of extreme precipitation have led to a number of new findings. Over land, this intercomparison generally emphasizes that global space-based precipitation products show the potential for climate-scale analyses of extremes to serve as a complementary source to in situ gridded data, while reanalysis should be used with caution. Over the ocean, the spread among the products of extreme intensity is larger than over land. The global characteristics of the representation of precipitation extremes are found at the regional scale as well. Yet, the magnitude and the behavior among the products show significant regional disparities that are more and more carefully documented in the literature. With these caveats, the new generation of satellite products are successfully used for process-oriented studies and climate models evaluation.
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    Toward the new generation of products
    (World Climate Research Programme, 2021) Huffman, George; Kirstetter, Pierre-Emmanuel
    Modern precipitation products provide fine resolution precipitation estimates by combining estimates from many individual high-quality satellite sensors and by using approximations to fill numerous gaps in the mosaics of short-interval segments from the various sensors. Challenges include (1) the need for consistent estimates within the mosaics and across generations of sensors with differing capabilities, and (2) increasing the information content to better resolve uncertainty and extremes at full resolution, spanning from retrievals to merged estimates. These challenges are accentuated by the diversity of current and new sources of satellite and surface observations. The future directions of global observationally-based precipitation products involve improvements in the individual retrievals, improvements and operationalization of additional sensor estimates, and innovations in assembling the merged products, including the intercalibration and homogenization of the data record. Seamless integration of ground networks, numerical models, new observations, and improved conditioning with precipitation processes are keys for future precipitation products.
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    Directions in error modeling
    (World Climate Research Programme, 2021) Maggioni, Viviana; Massari, Christian
    Although in the recent past numerous attempts have been made to develop error models of satellite precipitation products, several issues limit their use in applications. First off, the majority of these approaches is based on assumptions regarding the distribution of precipitation and/or associated errors. Second, simple error models may be preferable for some applications, but more complex solutions may be more appropriate for others. For instance, hydrological models used to simulate floods should be particularly sensitive to extreme precipitation events and the ability of detecting such events. Thus, an error model that account for missed precipitation cases and false alarms would be preferable. Third, precipitation errors and uncertainties depend on seasonality, rain rate, geophysical features, and the product’s temporal and spatial resolutions. Thus, the same error model would unlikely perform similarly everywhere in the world, at any time, for any precipitation event type, and for any application.
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    Emerging techniques for precipitation assessment and consistency studies
    (World Climate Research Programme, 2021) Behrangi, Ali
    Besides traditional sensors used to estimate precipitation amount and distribution, several other Earth-observing sensors can provide valuable insights about precipitation quantity. Here, we show how observation of mass change from GRACE and GRACE-FO satellites can help quantify snowfall accumulation over Antarctica and frozen surfaces in the Northern Hemisphere, two regions that precipitation estimation from typical precipitation measuring sensors are the most uncertain. Similarly, we discuss how snow depth observation from the Operation IceBridge (OIB) can help assess snow accumulation over sea ice, where almost no other reliable in situ data exists. These independent observations can also give us insights into the consistency of variables used in the water budget equation. Furthermore, we discuss how complementary observations from radars can help refine our estimate of precipitation over the ocean and serve as a reference to assess other precipitation products.
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    Requirements for a constellation of precipitation sensors
    (World Climate Research Programme, 2021) Kidd, Chris
    Satellites within the current precipitation constellation are old, with many missions beyond their designed operational lifetime. It is therefore crucial that there is a concerted program of new satellites and sensors to ensure continuity in satellite-based precipitation measurements. Planning the future constellation of precipitation sensors should include a long-term strategy for sensors that meet scientific and user requirements, support for current missions beyond their normal lifetime, new technology such as cubesats, and, fundamentally, a commitment to and support for current and planned precipitation-capable missions. Issues and considerations in designing this next generation of precipitation sensors are discussed in this subchapter, which also includes a concise inventory of missions planned for launch over the next decade.
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    List of Acronyms
    (World Climate Research Programme, 2021)
    This list contains the acronyms used in The Joint IPWG/GEWEX Precipitation Analysis along with their definitions.