A scientific focus area in DOE’s Earth and Environmental System Modeling (EESM) Program studies the roles of large-scale circulation and convection in the water cycle and the implications for variability and multidecadal changes of extreme events

Planet Earth harbors 1.5 trillion cubic kilometers of water, which is cycled continuously between ocean, atmosphere, and land. The water cycle makes all of life possible but unraveling its mysterious ways and understanding its past and future changes is challenging.

Some of the most consequential outcomes of global warming for human societies and ecosystems are extreme events related to changes in the water cycle. In the past 20 years, there has been a staggering rise in the number of extreme weather events globally. In the United States, 2010-2019 was a landmark decade for billion-dollar weather and climate disasters. 

To address these realities, the U.S. Department of Energy Water Cycle and Climate Extremes Modeling (WACCEM) scientific focus area, led by L. Ruby Leung at Pacific Northwest National Laboratory, works to improve understanding of the predictability of water availability and hydrologic extremes. WACCEM is organized around three overarching science questions:

1. How do large-scale circulation features modulate regional mean and extreme precipitation and their future changes?
2. What processes control mesoscale convection and how will such convection change in the future?
3. How do atmospheric circulations and water-cycle processes interact across scales and influence hydrologic extremes?

In this article, Leung provides a glimpse at some recent WACCEM research findings related to large-scale circulation (ITCZ and monsoons, for example); mesoscale convective systems and their hydrologic footprints; and the Madden-Julian Oscillation, atmospheric rivers, and tropical cyclones as they are connected to hydrologic extremes.

ITCZ: Recent and Future Changes

The Intertropical Convergence Zone (ITCZ) is a major circulation and precipitation feature in the tropics but even this global feature can be altered by human activities. Emphasizing the seasonal and regional characteristics of the ITCZ, Zhou et al. (2020a) provided important clarity on the observed and projected changes in the strength, width, and meridional position of the ITCZ. These changes are often obscured in the zonal and annual mean changes.

However, advancing understanding of the ITCZ and its response to warming requires improved modeling of clouds and convection and their interactions with radiation and circulation. Hagos et al. (2021) found that tropical precipitation operates in two distinct regimes: one primarily controlled by atmospheric moisture and the other by surface evaporation. Since climate models are unable to simulate the relative prevalence of these regimes, which are controlled by the vertical structure of convection, they produce double-ITCZ biases. That has consequences for projecting future precipitation changes even in faraway regions (Dong et al. 2021).

The WACCEM team finds that in a super-parameterized climate model that better resolves convection and simulates the ITCZ, radiation-cloud-convection-circulation interactions impact the ITCZ by increasing the heating contrasts between wet and dry regions (Lau et al. 2019). These processes are being further investigated using the Energy Exascale Earth System Model (E3SM) and the Model for Prediction Across Scales - Atmosphere (MPAS-A).

Teleconnected Changes: Monsoon Precipitation and More

Sixty percent of the Earth’s population relies on tropical rainfall for most of its water supply. The timing of the start of the rainy season has important implications for water resources and agriculture in monsoon regions. Two generations of climate models have robustly projected a delayed onset of tropical rainfall with future warming, but the mechanisms remained unclear.

To confront this knowledge gap, Song et al. (2018a) explained the mechanisms of the delay using the atmospheric energetic framework. More recently, Song et al. (2020) discovered that the delay only happens over land but it is uncertain over the ocean. (The authors attributed the latter to a competing feedback process over the ocean.)

Observations show that the delay over land has already emerged in recent decades (Song et al. in review), providing evidence of human influences on Earth’s hydrological cycle. To further understand tropical precipitation changes, Lu et al. (2021) developed an energy-balance model to predict atmospheric energy flow and Harrop et al. (in review) delved into the energy budgets and conservation in climate models.

Pivoting from the seasonal delay of tropical rainfall, Song et al. (2018a) found a robust seasonal dependence of the North Atlantic subtropical high in response to future warming. Changes in the subtropical high, combined with changes in the upper-level westerly jet, induced seasonally dependent changes in the U.S. Great Plains low-level jet. Those result in a distinct seasonal signature of precipitation changes in the central United States (Zhou et al. 2020b).

Mesoscale Convective Systems and Large-Scale Environments

Mesoscale convective systems (MCSs) are the largest form of deep convective storms. They contribute dominantly to extreme precipitation worldwide. That makes understanding and modeling MCSs important for predicting future changes in extreme precipitation and flooding. Earth system models are just now getting closer to the resolution at which MCSs can be accurately represented.

To start off, the WACCEM team first developed a novel algorithm (FLEXTRKR) for tracking MCSs (Feng et al. 2019). The MCS tracking data revealed an increasing trend in MCS rainfall in the central United States during the past decades, as reported in Hu et al. (2020a) and Feng et al. (2016). Combining the MCS tracking data with self-organizing map analysis of atmospheric circulation, Song et al. (2019) and Song et al. (in review) identified the MCS large-scale environments useful for understanding past and future changes of MCSs.

FLEXTRKR has been further developed to track MCSs across a range of spatial resolutions, from 4 to 50 kilometers, as in Feng et al. (2020). Metrics have been developed for evaluating MCSs and their environments in climate simulations.

FLEXTRKR has now been applied to satellite-derived cloud tops and surface precipitation to develop a near-global MCS tracking dataset at hourly and 10-km resolution (Feng et al. in review). These data can be combined with other datasets to understand the MCS characteristics that play a crucial role in the global energy and water cycles, such as their latent heating (Liu et al. in review).

Mesoscale Convective Systems: Hydrologic Footprints and Land-Atmosphere Interactions

In the central United States, MCSs produce rainfall seven times more intense than is generated by non-MCS storms, as demonstrated in Hu et al. (2020a). Such disparity in rainfall intensity could lead to different hydrologic footprints of rainfall over land.

In Hu et al. (2020b), researchers numerically tagged rainfall produced by MCS and non-MCS storms. In a land model, they tracked how the tagged water moves through the surface and showed that MCS rainfall disproportionately contributes to runoff.

In related work, Hu et al. 2021a (in review) combined high-resolution (4-km) MCS track data with a storm events database. MCSs, they show, account for most low-rising and hybrid floods in the central United States.

Besides the impacts on runoff and floods, MCS rainfall can generate spatially extensive and deep soil moisture anomalies. Hu et al. 2021b (in review) demonstrate that early warm-season MCS rainfall may provide predictability for summer precipitation in the central United States through soil moisture-precipitation feedback.

Predictability of the Madden-Julian Oscillation and Connection with Atmospheric Rivers

Lasting between 30 to 60 days, the Madden-Julian Oscillation (MJO), which is characterized by eastward traveling anomalies of tropical convection, is linked to extreme events worldwide. However, predicting the MJO is limited by how little we know about its variability.

Hagos et al. (2019) revealed factors that modulate the MJO, including moisture convergence in monsoons. In related work, Hagos et al. (2020) shows how MJO amplitude and precipitation are modulated by insolation and soil moisture in the Maritime Continent islands. The two studies hint at why some MJO events stall while others travel across the Maritime Continent.

In South Asia and the Indian Ocean, where MJO events originate, MCSs play a crucial role in the vertical transport of mass, energy, and moisture (Chen et al. in review; Chen et al. 2020). The WACCEM team is poised to significantly contribute to understanding MCS-MJO interactions for improving MJO prediction by using the modeling and analysis tools developed for (or improved by) the project. 

The MJO has remote influence on extreme events, such as atmospheric rivers (ARs), narrow corridors of intense atmospheric moisture transport. AR intensity and landfall are influenced by background divergent moisture flux. Hagos et al. (in review) found that during the different phases of the MJO, the background divergent moisture transport changes direction. Eastward moisture transport in the western Pacific during MJO phases 7 and 8 favor AR landfall in western North America, increasing the risk of flooding and other hazards. Such risk could increase with warming that prompts a lengthening of landfalling ARs over the California coast (Lu et al. 2018).

Tropical Cyclones and the Role of the Ocean

In the recent past, researchers on WACCEM have published extensively on how tropical cyclones (TCs) intensify by extracting heat energy from the ocean. That work may improve TC predictions; however, it is especially difficult to predict the rapid intensification (RI) of such cyclones, which represents a formidable threat to coastal populations.

Balaguru et al. (2020a) used a suite of observations and numerical experiments to show how salinity in the upper ocean significantly impacts RI in the western tropical Atlantic Ocean. This result opens the way to improved operational predictions using satellite observations of salinity (Reul et al. 2020). Increased magnitude of hurricane activity in the central and eastern tropical Atlantic in recent decades further motivates the need to understand factors contributing to RI (Balaguru et al. 2018).

Connections of TCs to modes of variability have also been of interest to this team. Balaguru et al. (2020b) showed how the ENSO Longitude Index (ELI) better explains the interannual variations in the upper-ocean heat content compared to traditional ENSO indices. The ELI has the potential to better predict TC activity in the eastern North Pacific basin at lead times of five to six months.

In collaboration with the E3SM team, as reflected in Balaguru et al. (2020c), WACCEM contributed to analysis and evaluation of TCs in E3SM v1 simulations at both low (around 100-km) and high (around 25-km) resolutions. The work suggests that E3SM is a useful tool for studying the role of the ocean in TCs, along with past and future changes.

References

ITCZ: Recent and Future Changes

Dong, L., L.R. Leung, J. Lu, and F. Song. 2021. “Double-ITCZ as an Emergent Constraint for Future Precipitation Over Mediterranean Climate Regions in the North Hemisphere.” Geophys. Res. Lett., doi:10.1029/2020GL091569.

Hagos, S.M., L.R. Leung, O.A. Garuba, C. Demott, B. Harrop, J. Lu, and M. Ahn. 2021. "The relationship between precipitation and precipitable water in CMIP6 simulations and implications for tropical climatology and change, J. Clim., 1-41, doi:10.1175/JCLI-D-20-0211.1.

Lau, W.K.M., K.-M. Kim, J.-D. Chern, W.K. Tao, and L.R. Leung. 2019. “Structural Changes and Variability of the ITCZ Induced by Radiation-Cloud-Convection-Circulation Interactions: Inferences from the Goddard Multi-scale Modeling Framework (GMMF) Experiments.” Clim. Dyn., doi:10.1007/s00382-019-05000-y.

Zhou, W., L.R. Leung, J. Lu, D. Yang, and F. Song. 2020a. “Contrasting Recent and Future ITCZ Changes from Distinct Tropical Warming Patterns.” Geophys. Res. Lett., 47, doi:10.1029/2020GL089846.

Teleconnected Changes: Monsoon Precipitation and More

Harrop, B.E., M.S. Pritchard, H. Parishani, A. Gettelman, S. Hagos, P.H. Lauritzen, L.R. Leung, J. Lu, K.G. Pressel, and K. Sakaguchi. 2021. “Conservation of Dry Air, Water, and Energy in CAM and its Impact on Tropical Rainfall.” J. Clim., in review.

Lu, J., D. Xue, L.R. Leung, F. Liu, F. Song, B. Harrop, and W. Zhou. 2021. “The Leading Modes of Asian Summer Monsoon Variability as Pulses of Atmospheric Energy Flow.” Geophys. Res. Lett., doi:10.1029/2020GL091629.

Song, F., L.R. Leung, J. Lu, L. Dong, W. Zhou, B. Harrop, and Y. Qian. 2021. “Emergence of Seasonal Delay of Tropical Rainfall During 1979-2018.” Nature Clim. Change, revised.

Song, F., J. Lu, L.R. Leung, and F. Liu. 2020. “Contrasting Phase Changes of Precipitation Annual Cycle over Land and Ocean Under Global Warming.” Geophys. Res. Lett., 47, doi:10.1029/2020GL090327.

Song, F., L.R. Leung, J. Lu, and L. Dong. 2018a. “Future Changes in Seasonality of the North Pacific and North Atlantic Subtropical Highs.” Geophys. Res. Lett., 45, doi: 10.1029/2018GL079940.

Song, F., L.R. Leung, J. Lu, and L. Dong. 2018b. “Seasonally-Dependent Responses of Subtropical Highs and Tropical Rainfall to Global Warming”. Nature Clim. Change, 8, 787-792, doi:10.1038/s41558-018-0244-4.

Zhou, W., L.R. Leung, F. Song, and J. Lu. 2020b. “Future Great Plains Low-Level Jet Changes Governed by Seasonally-Dependent Pattern Shifts of North Atlantic Subtropical High.” Geophys. Res. Lett., doi:10.1029/2020GL090356.

Mesoscale Convective Systems and Their Large-Scale Environments

Feng, Z., L.R. Leung, N. Liu, J. Wang, R.A. Houze Jr., J. Li, J.C. Hardin, and J. Guo. 2021. “A Global High-Resolution Mesoscale Convective System Database Using Satellite-derived Cloud Tops, Surface Precipitation, and Tracking.” J. Geophys. Res., in review.

Feng, Z., F. Song, K. Sakaguchi, and L.R. Leung. 2020. “Evaluation of Mesoscale Convective Systems in Climate Simulations: Methodological Development and Results from MPAS-CAM over the U.S.” J. Clim., early online. doi:10.1175/JCLI-D-20-0136.1.

Feng, Z., R.A. Houze, Jr., L.R. Leung, F. Song, J. Hardin, J. Wang, W. Gustafson, Jr., and C. Homeyer. 2019. “Spatiotemporal Characteristics and Large-scale Environment of Mesoscale Convective Systems East of the Rocky Mountains.” J. Clim., 32, 7303-7328, doi:10.1175/JCLI-D-19-0137.1.

Feng, Z., L. R. Leung, S. Hagos, R. A. Houze, Jr., C. D. Burleyson, and K. Balaguru. 2016. “More Frequent Intense and Long-Lived Storms Dominate the Trend in Central U.S. Rainfall.” Nat. Commun., 7, 13429, doi: 10.1038/ncomms13429.

Hu, H., L.R. Leung, and Z. Feng. 2020a. “Observed Warm-Season Characteristics of MCS and non-MCS Rainfall and Their Recent Changes in the Central United States.” Geophys. Res. Lett., 47, doi:10.1029/2019GL086783.

Liu, N., L.R. Leung, and Z. Feng. 2021. “Global Mesoscale Convective System Latent Heating Characteristics from GPM and IMERG Retrievals and an MCS Tracking Dataset.” J. Clim., in review.

Song, F., Z. Feng, L.R. Leung, S. Wang, B. Pokharel, X. Chen, K. Sakaguchi, and C. Wang. 2021. “The Role of Propagating Environments in the Initiation of Summer Mesoscale Convective Systems over the U.S. Great Plains.” J. Geophys. Res., in review.

Song, F., Z. Feng, L.R. Leung, R.A. Houze, Jr., J. Wang, J. Hardin, and C. Homeyer. 2019. “Contrasting the Spring and Summer Large-Scale Environments Associated with Mesoscale Convective Systems Over the U.S. Great Plains.” J. Clim., 32, 6749-6767, doi:10.1175/JCLI-D-18-0839.1.

Mesoscale Convective Systems: Hydrologic Footprints and Land-Atmosphere Interactions

Hu, H., Z. Feng, and L.R. Leung. 2021a. “Linking Flood Frequency with Mesoscale Convective Systems in the Central US.” Geophys. Res. Lett., submitted.

Hu, H., L.R. Leung, and Z. Feng. 2021b. “Earlier-Season Mesoscale Convective Systems Dominate Soil Moisture-Precipitation Feedback for Summer Rainfall in Central US.” Proc. Nat. Acad. Sci., submitted.

Hu, H., L.R. Leung, and Z. Feng. 2020a. “Observed Warm-Season Characteristics of MCS and non-MCS Rainfall and Their Recent Changes in the Central United States.” Geophys. Res. Lett., 47, doi:10.1029/2019GL086783.

Hu, H., L.R. Leung, and Z. Feng. 2020b. “Understanding the Distinct Impacts of MCS and Non-MCS Rainfall on the Surface Water Balance in the Central US Using a Numerical Water-Tagging Technique.” J. Hydrometeor., 21, 2343-2357, doi:10.1175/JHM-D-20-0081.1.

Predictability of the Madden-Julian Oscillation and Connection with Atmospheric Rivers

Chen, X., L.R. Leung, Z. Feng, and F. Song. 2021. “Roles of Different Convective Systems in the Atmospheric Overturning of South Asian Summer Monsoon.” J. Clim., in review.

Chen, X., O.M. Pauluis, L.R. Leung, and F. Zhang. 2020. “Significant Contribution of Mesoscale Overturning to Tropical Mass and Energy Transport Revealed by the ERA5 Reanalysis.” Geophys. Res. Lett., 47, doi:10.1029/2019GL085333.

Hagos, S.M., L.R. Leung, O. Garuba, and C. Patricola. 2021. “Influence of Background Divergent Moisture Flux on the Frequency of Atmospheric Rivers.” J. Clim., in review.

Hagos, S., C. Zhang, O. Garuba, L.R. Leung, C.D. Burleyson, and K. Balaguru. 2020. “Impacts of Insolation and Soil Moisture on Seasonality of Interactions Between the Madden-Julian Oscillation and Maritime Continent.” J. Geophys. Res., 125, doi:10.1029/2020JD032382.

Hagos, S., C. Zhang, L.R. Leung, C.D. Burleyson, and K. Balaguru. 2019. “Zonal Migration of Monsoon Moisture Flux Convergence and the Strength of Madden-Julian Oscillation Events.” Geophys. Res. Lett., 46, doi:10.1029/2019GL083468.

Lu, J., D. Xue, Y. Gao, G. Chen, R. L. Leung, and P. Staten, 2018. “Enhanced Hydrological Extremes in the Western United States under Global Warming Through the Lens of Water Vapor Wave Activity.” npj Climate and Atmospheric Sciences, 1:7, doi:10.1038/s41612-018-0017-9.

Tropical Cyclones and the Role of the Ocean

Balaguru, K., G.R. Foltz, and L.R. Leung. 2018. “Increasing Magnitude of Hurricane Rapid Intensification in the Central and Eastern Tropical Atlantic.” Geophys. Res. Lett., 45, doi: 10.1029/2018GL077597.

Balaguru, K., G.R. Foltz, L.R. Leung, J. Kaplan, W. Xu, N. Reul, and B. Chapron. 2020a. “Pronounced Impact of Salinity on Rapidly Intensifying Tropical Cyclones.” Bull. Amer. Meteor. Soc., 101 (9), E1497-E1511, doi:10.1175/BAMS-D-19-0303.1.

Balaguru, K., C.M. Patricola, S.M. Hagos, L.R. Leung, and L. Dong. 2020b. “Enhanced Predictability of Eastern North Pacific Tropical Cyclone Activity Using the ENSO Longitude Index.” Geophys. Res. Lett., 47, doi:10.1029/2020GL088849.

Balaguru, K., L.R. Leung, L. van Roekel, J.-C. Golaz, P. Ullrich, P.M. Caldwell, S.M. Hagos, B.E. Harrop, and A. Mametjanov. 2020c. “Characterizing Tropical Cyclones in the Energy Exascale Earth System Model Version 1.” J. Adv. Mod. Earth Syst., 12, doi:10.1029/2019MS002024.

Reul, N., B. Chapron, S.A. Grodsky, S. Buimbard, V. Kudryavtsev, G.R. Foltz, and K. Balaguru. 2020. “Satellite Observations of the Sea Surface Salinity Response to Tropical Cyclones.” Geophys. Res. Lett., 48, doi:10.1029/2020GL091478.