Revisit science priorities for CalWater 2015 – ACAPEX

L.R. Leung, PNNL

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AR Science ConsiderationsMeasurement + modeling:

Landfall, overland and topography considerations (G-1, HMT)Water Budgets over ocean and land (G-IV, Ship, HMT)Microphysical considerations (G-1, HMT) Modeling Improvements (microphysical to multi-scale) (G-1, ship, HMT)Synoptic Meteorology & Vapor Sources (G-IV, HMT, Ship)Air Sea Interaction (Ship)Prediction skill (G-IV, Ship, HMT)

Modeling Studies:Energy and Momentum budgets Multi-scale interactions (e.g. ENSO to mesoscale)Climate change implications Predictability

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AR science goals and priorities

What influences the evolution and structure of AR and its associated cloud and precipitation? To what extent does water vapor in ARs originate from the

tropics? What role does tropical convection play in this? What are the characteristics and variability of water vapor fluxes

associated with AR? (water budget, vapor sources) What are the roles of air-sea fluxes and ocean mixed layer

processes in AR evolution? (air-sea interactions, vapor sources) What are the key dynamical and microphysical processes that

modulate cloud and precipitation from landfalling ARs? (landfall, topography, microphysics)

How well can models predict AR characteristics? What influence model prediction skill? (prediction skill, model improvements)

(Note: one synoptic meteorology question (cyclogenesis) in ACP)

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AR science goals and priorities

What influences the evolution and structure of AR and its associated cloud and precipitation?

How much water vapor is entrained directly from the tropics and how much of this makes it to the coast and falls as precipitation? What role does tropical convection play in the development of AR and its moisture budget?What fraction of rainfall in landfalling ARs results from air-sea fluxes of moisture from the ocean’s surface and how much is from horizontal convergence of pre-existing atmospheric water vapor?How much rainout occurs in ARs over the ocean (and are the cloud and precipitation processes sensitive to possible influences of Asian aerosols)?Does “recycling” of atmospheric water via evaporation in virga play a significant role in the AR water vapor transport budget?

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AR science goals and prioritiesWhat influences the evolution and structure of AR and its associated cloud and precipitation?

Can mesoscale frontal waves associated with the parent cold front of an AR be detected and if so, can this aid in predictions of AR duration at coastal sites - a critical factor controlling how heavy the precipitation will be and where it will occur?How does the Sierra Barrier Jet modulate the mesoscale distribution of precipitation (and aerosols and their impacts in the mountains near the north end of the Central valley)?What global weather patterns (e.g., MJO, ENSO, Western Pacific decaying typhoons) affected by tropical convection most influence AR evolution, structure and impacts on the U.S. West Coast?Do ARs transport other key atmospheric gases or aerosols besides moisture?

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ACP science goals and priorities

Characterize aerosols and microphysical properties and examine aerosol transport and removal processes over the Pacific Ocean

Improve understanding and modeling of aerosol-cloud-precipitation interactions in clouds transitioning from the maritime regime to the orographic regime

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Overarching science questions

How do aerosols affect the amount and phase of precipitation?o How frequent are aerosols transported across the Pacific and what

characteristics make them effective CCN and/or IN? How does cloud processing of aerosols influence the aerosol properties and cloud forming ability?

o How do aerosols from long-range transport and local sources influence cloud and precipitation over California, in both AR and non-AR conditions?

o How do aerosols influence cyclogenesis and the thermodynamic development of extratropical cyclones and the coupled atmospheric rivers associated with these storms?

o How are the aged and new aerosols from local California sources such as those in the Bay Area, Central Valley, and Sacramento eventually removed in the different precipitation systems?

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More detailed science questions

How do different aerosol types change the cloud and precipitation forming processes in maritime convective clouds, deep marine clouds associated with ARs, orographic clouds, and laminar layer cap clouds over the mountain ridges?

What are the composition of CCN and IN in different cloud types? Where do they originate (local vs long-range transported)?How do CCN and IN affect precipitation forming processes in different cloud types?How important is primary ice nucleation in different cloud types?How can supercooled water be sustained in marine clouds? How important is supercooled rain in different cloud types?

How does wet scavenging (transformation and removal) of aerosols from long-range transport influence their ability to serve as IN and change how aerosols interact with clouds and precipitation over land?How well do current cloud microphysical parameterizations capture aerosol-cloud interactions in mixed-phase clouds? What is the role of the Sierra Barrier Jet in aerosol transport and how does this influence cloud and precipitation?

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CCN in marine convective clouds

Laboratory measurements show that sea spray particles have large fraction of organics, and more so in smaller particles.

What fraction of the CCN are of sea spray, and what constitutes the rest? How does this translate into size resolved CCN, including GCCN?How do the CCN and cloud base updrafts determine drop size distribution (DSD) at cloud base and the DSD evolution with height?How does the vertical evolution of DSD determine the height for precipitation initiation? How does it depend on wind speed that cause breaking waves?Do most raindrops start by accretion on large drops that were nucleated on GCCN, or by coalescence of the main mode of the DSD?

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IN in marine convective clouds

Laboratory measurements show that some sea spray particles can act as IN, probably by biological matter.

What is the composition of the initial ice particles that form in convective clouds? Where do these aerosols originate? Can we get hints from their isotopic composition?How does it differ from the composition of ice particles in mature glaciated cloud at the same top temperature?Does the initiation of ice in the convective clouds change the precipitation amounts, or does it form well after warm rain has already advanced and precipitated much of the cloud water?What is the relative importance of marine and Asian IN in convective precipitation forming processes?

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CCN in atmospheric rivers

Deep marine clouds are expected to precipitate very efficiently by warm rain processes. To what extent CCN matter in their precipitation efficiency?

What fraction of the CCN are of sea spray, and what constitutes the rest? How dose this translates into size resolved CCN, including GCCN?How do the CCN and cloud base updrafts determine drop size distribution (DSD) at cloud base and the DSD evolution with height?Do the rain and snow from above accrete most cloud water, thus overriding coalescence process? How do these processes differ between the warm and cold sides of the AR?

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IN in atmospheric rivers

Cloud tops of ARs are very cold and long lasting, and therefore fully glaciated. Under such circumstance, where ice has much time to develop and proliferate, to what extent IN affect the ice precipitation forming processes?

What are the IN? Can we know their origin from their elemental a and isotopic composition? What are the differences between the warm and cold side of the AR?What is the composition of initial ice particles that form in the clouds? Can we find primary ice at all, or is it all formed by seeder feeder mechanism?

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CCN in orographic clouds

How do the CCN of the orographic precipitation that form above the boundary layer differ from the CCN feeding into the convective cloud base over ocean?Can we detect the source of the CCN by the chemical and isotopic composition using direct ATOFMS, or captured cloud water, warm rain and graupel-melt?Can we detect long range transported CCN from Asia, and how do they affect the cloud and precipitation properties?Are there GCCN left above the boundary layer and inland in an amount that can affect rain initiation?How do CCN affect warm rain initiation in the convective elements that are triggered at the foothills by orographic lifting? The same question for cap clouds over the crest.How do CCN and their impacts change from the coastal range, Sierra Nevada, and the mountain ranges further east in the desert? The latter was never addressed so far.

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IN in orographic cloudsHow do the IN of the orographic precipitation that form above the boundary layer differ from the IN feeding into the convective cloud base over ocean?Can we detect the source of the IN by the chemical and isotopic composition using direct ATOFMS, or captured unrimed ice?How can we obtain sustained highly supercooled water clouds, as found in the previous campaign? What is the aerosols source in such cases, and how does it affect the precipitation processes?How do IN and their impacts change from the coastal range, Sierra Nevada, and the mountain ranges further east in the desert? The latter was never addressed so far.Can we use orographic cap clouds over the eastern ranges for documenting primary ice nucleation?What is the relative importance of marine and continental IN in orographic precipitation forming processes?