DOE/SC-ARM-14-030 ARM Cloud Aerosol Precipitation Experiment (ACAPEX) Science Plan LR Leung, Principal Investigator K Prather S Hagos M Ralph M Hughes D Rosenfeld C Long R Spackman S Rutledge P DeMott D Waliser C Fairall H Wang J Fan September 2014 DISCLAIMER This report was prepared as an account of work sponsored by the U.S. Government. Neither the United States nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof. DOE/SC-ARM-14-030 ARM Cloud Aerosol Precipitation Experiment (ACAPEX) Science Plan Principal Investigator LR Leung, Pacific Northwest National Laboratory Co-Principal Investigators K Prather, University of California at San Diego, Scripps Institution of Oceanography M Ralph, NOAA Earth System Research Laboratory D Rosenfeld, Hebrew University of Jerusalem R Spackman, NOAA Earth System Research Laboratory, Science & Technology Corporation (STC) Co-Investigators P DeMott, Colorado State University C Fairall, NOAA Earth System Research Laboratory J Fan, Pacific Northwest National Laboratory S Hagos, Pacific Northwest National Laboratory M Hughes, NOAA Earth System Research Laboratory, CIRES C Long, Pacific Northwest National Laboratory S Rutledge, Colorado State University D Waliser, NASA Jet Propulsion Laboratory H Wang, Pacific Northwest National Laboratory September 2014 Work supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research LR Leung, September 2014, DOE/SC-ARM-14-030 Summary The western U.S. receives precipitation predominantly during the cold season when storms approach from the Pacific Ocean. The snowpack that accumulates during winter storms provides about 70-90% of water supply for the region. Understanding and modeling the fundamental processes that govern the large precipitation variability and extremes in the western U.S. is a critical test for the ability of climate models to predict the regional water cycle, including floods and droughts. Two elements of significant importance in predicting precipitation variability in the western U.S. are atmospheric rivers and aerosols. Atmospheric rivers (ARs) are narrow bands of enhanced water vapor associated with the warm sector of extratropical cyclones over the Pacific and Atlantic oceans. Because of the large lower-tropospheric water vapor content, strong atmospheric winds and neutral moist static stability, some ARs can produce heavy precipitation by orographic enhancement during landfall on the U.S. West Coast. While ARs are responsible for a large fraction of heavy precipitation in that region during winter, much of the rest of the orographic precipitation occurs in post-frontal clouds, which are typically quite shallow, with tops just high enough to pass the mountain barrier. Such clouds are inherently quite susceptible to aerosol effects on both warm rain and ice precipitation-forming processes. The Atmospheric Radiation Measurement (ARM) Cloud Aerosol Precipitation Experiment (ACAPEX) will deploy the DOE ARM Mobile Facility 2 (AMF2) and the ARM Aircraft Facility (AAF) G1 in January – March 2015 in conjunction with CalWater 2 – a NOAA field campaign. The joint field campaign aims to improve understanding and modeling of large-scale dynamics and cloud and precipitation processes associated with ARs and aerosol-cloud interactions that influence precipitation variability and extremes in the western U.S. Our observational strategy consists of the use of land and offshore assets to monitor (1) the evolution and structure of ARs and their moisture sources from near their regions of development, (2) long-range transport of aerosols in eastern North Pacific and potential interactions with ARs, and (3) how aerosols from long-range transport and local sources influence cloud and precipitation in the U.S. West Coast where ARs make landfall and post-frontal clouds are frequent. Deployed onboard the NOAA R/V Ron Brown, AMF2 will provide critical measurements to quantify the moisture budget and cloud and precipitation processes associated with ARs, and to characterize aerosols and aerosol-cloud-precipitation interactions associated with aerosols from long-range transport in the Pacific Ocean. The G1 aircraft will probe the clouds that form over the ocean and their transformations upon landfall as well as the orographic effects over the coastal range and the Sierra Nevada. The G1 flights will provide critical information needed for comparing the simulated and observed processes of the vertical profiles of cloud microstructure, and the resultant precipitation initiation and glaciation. This will allow the development and validation of more realistic simulations that will replicate the aircraft measurements and thus quantify more reliably the entities that cannot be obtained directly by the aircraft measurements to improve understanding and modeling of aerosol-cloud-precipitation interactions. iii LR Leung, September 2014, DOE/SC-ARM-14-030 Acronyms and Abbreviations ACAPEX ARM Cloud Aerosol Precipitation Experiment AR Atmospheric Rivers ARM Atmospheric Radiation Measurement BC Black Carbon CALIPSO Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation CAS Cloud Aerosol Spectrometer CCN Cloud Condensation Nuclei CIP Cloud Imaging Probe CPC Condensation Particle Counter CSPHOT Cimel sunphotometer CVI Counter-flow Virtual Impactor DOE U.S. Department of Energy EFREP Enhanced Flood Response and Emergency Preparedness FRSR Fast Rotating Shadow Band Radiometer HIAPER High-performance Instrumented Airborne Platform for Environmental Research HIPPO HIAPER Pole-to-Pole Observations HMT Hydrometeorological Testbed IN Ice Nuclei INS Inertial Navigation System MAERI Marine Atmospheric Emitted Radiance Interferometer MET Marine Meteorological Instruments MFRSR Multi-filter Rotating Shadowband Radiometer MISR Multi-angle Imaging Spectro Radiometer MJO Madden-Julian Oscillation MPL Micropulse Lidar NOAA National Oceanic and Atmospheric Administration PIR Precision Infrared Radiometer PNNL Pacific Northwest National Laboratory PRP Portable Radiation Package PSD Particle Size Distributions PSP Precision Spectral Pyranometer RASS Radio Acoustic Sounding System RPH Roll, Pitch and Heave SBJ Sierra Barrier Jet iv LR Leung, September 2014, DOE/SC-ARM-14-030 SEANAV Sea-borne Navigation System SWE Snow Water Equivalent TDR Tail Doppler Radar WISPAR Winter Storms and Pacific Atmospheric Rivers WV Water Vapor v LR Leung, September 2014, DOE/SC-ARM-14-030 Contents Summary ...................................................................................................................................................... iii  Acronyms and Abbreviations ...................................................................................................................... iv  1.0  Introduction .......................................................................................................................................... 1  2.0  Objectives and Science Questions ........................................................................................................ 3  3.0  Observations ......................................................................................................................................... 4  3.1  Overarching Strategy for the Joint CalWater 2/ACAPEX ........................................................... 4  3.2  Objectives and Strategies for ACAPEX ....................................................................................... 8  3.2.1  AMF2 Deployment ........................................................................................................... 8  3.2.2  AAF G1 Deployment ...................................................................................................... 10  4.0  Science ................................................................................................................................................ 13  4.1  Evolution and Structure of Atmospheric Rivers ........................................................................ 13  4.2  Aerosol Effects on Cloud and Precipitation ............................................................................... 15  5.0  Relevance to DOE Mission ................................................................................................................ 17  6.0  References .......................................................................................................................................... 18  vi LR Leung, September 2014, DOE/SC-ARM-14-030 Figures 1.  Left: Special Sensor Microwave Imager retrieved vertically integrated water vapor on February 16, 2004 in which an AR was detected. ................................................................................ 1  2.  Conceptual framework for CalWater 2 / ACAPEX science objectivest. .............................................. 4  3.  The CalWater 2/ACAPEX observational strategy using high- and low-altitude aircraft platforms. .............................................................................................................................................. 5  4.  A combined >100-site network of state-of-the-art hydrometeorological observations from NOAA’s HMT and EFREP. ................................................................................................................. 6  5.  The main orographic flight plan for CalWater in the coastal and foothill areas of central California. ........................................................................................................................................... 13  6.  Cross section showing the dropsonde data through the AR in the inset figure for the first Global Hawk science flight. ................................................................................................................ 15  Tables 1.  Aircraft observations ............................................................................................................................. 7  2.  AMF2 instruments and measurements. ................................................................................................. 9  3.  G1 instruments and measurements. .................................................................................................... 11  vii LR Leung, September 2014, DOE/SC-ARM-14-030 1.0 Introduction The western U.S. receives precipitation predominantly during the cold season when storms approach from the Pacific Ocean. The snowpack that accumulates during winter storms provides about 70-90% of water supply for hydropower generation, irrigation, and other uses. Understanding and modeling the fundamental processes that govern the large variability of precipitation in the western U.S. is a critical test for the ability of climate models to simulate clouds and precipitation and to predict the regional water cycle and extremes from intraseasonal to century time scales. Two elements of significant importance in predicting precipitation variability in the western U.S. are atmospheric rivers (AR) and aerosols. ARs are narrow bands of enhanced water vapor associated with the warm sector of extratropical cyclones over the Pacific and Atlantic oceans (Zhu and Newell 1998; Ralph et al. 2004; Bao et al. 2006). Because of the large lower-tropospheric water vapor content, strong atmospheric winds and neutral moist static stability (Figure 1), some ARs can produce heavy precipitation by orographic enhancement during landfall on the U.S. West Coast (Ralph et al. 2005, 2006; Neiman et al. 2008). Figure 1. Left: Special Sensor Microwave Imager (SSM/I) retrieved vertically integrated water vapor on February 16, 2004 in which an AR was detected. Right: A schematic showing the vertical profiles of atmospheric moisture flux, moist stability, and wind speed associated with an AR and heavy precipitation as the AR makes landfall on the mountainous west coast (Source: Ralph et al. 2005). While ARs are responsible for a large fraction of heavy precipitation in the western U.S. during winter, much of the rest of the orographic precipitation occurs in post-frontal clouds, which are typically quite shallow, with tops just high enough to pass the mountain barrier. In such conditions supercooled cloud water was documented to occur quite regularly in the western side of the orographic clouds over the topographic barrier when the cloud tops were >-15°C to -20°C (Heggli et al. 1983, Reynolds and Dennis 1986). Such clouds are inherently quite susceptible to aerosol effects on both warm rain and ice precipitation-forming processes. Measurements from the Suppression of Precipitation (SUPRECIP) field campaigns (Rosenfeld et al. 2008) suggest that aerosols that are incorporated in orographic clouds can efficiently slow down cloud-drop coalescence and riming on ice precipitation and delay the conversion of cloud water into precipitation. As a result, precipitation is redistributed with significant reductions on the upwind slopes and small compensation on the lee side, resulting in a net loss of precipitation and winter snowpack in the mountains. In an effort to advance scientific understanding, numerical modeling, and measurements of critical physical processes underlying future changes in water supply and flood risks, a multi-year field 1 LR Leung, September 2014, DOE/SC-ARM-14-030 experiment CalWater has been formulated to study the AR and aerosol effects on precipitation (http://www.esrl.noaa.gov/psd/CalWater/). Field experiments were carried out in Jan – Feb 2009 and Jan – Mar 2010 at Sierra Nevada sites that include ground-based aerosol and hydrometeorological measurements. In the Dec 2010 – Mar 2011 experiment, the Pacific Northwest National Laboratory (PNNL) G1 research aircraft flew between 2 February and 7 March 2011 and documented meteorology, cloud microphysics, and aerosol size and sources/composition in the Sierra Nevada and Central Valley. The CalWater field experiments have documented important cloud and precipitation processes associated with the ARs and the significant role of the Sierra Barrier Jet (SBJ) in orographic enhancement of precipitation (Neiman et al. 2010; Lundquist et al. 2010). However, much remains unanswered as to the development of ARs and the amount and origin of moisture that is transported by the AR to feed the heavy precipitation in the west coast of the U.S. Previous studies by Mo (1999) and Bond and Vecchi (2003) have linked tropical variability including the Madden-Julian Oscillation (MJO) to precipitation in the western U.S. Based on a detailed case study, Ralph et al. (2011) found that the phasing of several major planetary-scale phenomena including the MJO and extratropical wave activities led to the direct entrainment of tropical water vapor into the AR that subsequently produced heavy precipitation over the coastal mountain ranges. Guan et al. (2011) showed that AR timing and frequency and snow water equivalent (SWE) in the Sierra Nevada are significantly augmented when MJO is active over the far western tropical Pacific. However, to what extent tropical-extratropical interactions involving the MJO play a role in ARs and the importance of the tropical and other moisture sources to heavy precipitation as ARs make landfall on the west coast is not known. During the 2009 and 2010 CalWater field experiments, comprehensive aerosol chemistry and meteorological measurements documented the potential role of long-range (Asian) dust transport to precipitation in the Sierra Nevada. Comparing two storms with enhanced water vapor associated with AR conditions, Ault et al. (2011) hypothesized that Asian dust transported across the Pacific and incorporated into the upper altitudes of precipitation-producing clouds of a storm increased snowpack compared to the other storm with similar meteorological conditions but lower dust content in precipitation. Augmented by data collected on the G1 aircraft, the 2011 CalWater field experiment further provided important evidence of Asian dust on snowfall in the Sierra Nevada. In addition, the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) field campaigns measured a comprehensive suite of tracers of the carbon cycle and related species using the NSF/NCAR G-V aircraft during 2009-2011. From several meridional cross sections over the mid-Pacific, the HIPPO data showed episodes of high concentrations of black carbon (BC) from Asian sources. How Asian aerosols including dust and BC influence precipitation in the western U.S. depends on their composition and concentrations as well as their ability to serve as ice nuclei (IN) and cloud condensation nuclei (CCN) as they are transported across the Pacific. To fill the above gaps in our understanding and ability to simulate and predict AR and aerosol effects that influence cloud and precipitation, the Atmospheric Radiation Measurement (ARM) Cloud Aerosol Precipitation Experiment (ACAPEX) will deploy the DOE ARM Mobile Facility 2 (AMF2) and the ARM Aircraft Facility (AAF) G1 in January – March 2015 in conjunction with the NOAA CalWater 2 observational assets to improve understanding and modeling of large-scale dynamics and cloud and precipitation processes associated with AR and aerosol-cloud interactions that influence precipitation variability and extremes in the western U.S. AMF2 will be deployed on NOAA R/V Ron Brown, together with the NOAA G-IV and P-3 aircrafts to quantify the atmospheric water budget in ARs and characterize aerosols from long-range transport over the Pacific Ocean, while the G1 aircraft will document the 2