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Dazhi Xi
,
Shuai Wang
, and
Ning Lin

Abstract

In this study, the relationship between tropical cyclone (TC) intensity and rain rate over the ocean is investigated using a full-physics numerical model (WRF) and a physics-based TC rainfall model (TCR). TC intensity is found to be nearly linearly correlated with the average rain rate in the inner core [∼0.97 (mm h−1 m−2)/(m s−1)], while the correlation is weak at outer radii. This difference is induced because TC intensity is significantly correlated with both the vertical velocity and specific humidity in the inner core but is not significantly correlated with the vertical velocity in the outer radii. Further investigation shows that the intensity–rain-rate relationship at the outer radii is influenced by the TC evolution stage. The rain rate for the outer radii is positively correlated with TC intensity for nondecaying TCs, while this correlation is reduced for decaying TCs due to systematic downdrafts in the outer radii. In the context of climate change, the sensitivity of the TC rain rate to sea surface temperature (SST) is found to be +9% per 1 K increase of SST, roughly the product of the sensitivity of TC intensity to SST (+3%) and the Clausius-Clapeyron scaling (+7%). Coupled with synthetic storms, evolution of the TC rain rate over the twenty-first century under the SSP5-8.5 scenario is projected by the TCR (calibrated with the WRF simulations). The annual increase rates of averaged TC rain rate are 0.17% and 0.20% for the inner core and outer radii, respectively, larger than the annual increase rate of TC intensity (0.046%) but comparable to that of cube of intensity (0.18%).

Open access
Sarah Chapman
,
James Bacon
,
Cathryn E. Birch
,
Edward Pope
,
John H. Marsham
,
Hellen Msemo
,
Edson Nkonde
,
Kenneth Sinachikupo
, and
Charles Vanya

Abstract

Climate change is expected to increase the frequency and intensity of rainfall extremes. Understanding future changes in rainfall is necessary for adaptation planning. Eastern Africa is vulnerable to rainfall extremes because of low adaptive capacity and high future population growth. Convection-permitting climate models have been found to better represent moderate (yearly) rainfall extremes than parameterized convection models, but there is limited analysis of rare extremes that occur less frequently than once per year. These events often have the largest socioeconomic impacts. We use extreme value theory and regional frequency analysis to quantify rare rainfall extremes over East Africa in a convection-permitting climate model (CP4A). We compare the results with its parameterized counterpart (P25), the Coordinated Regional Climate Downscaling Experiment for the African region (CORDEX-Africa) ensemble, and observations to understand how the convection parameterization impacts the results. We find that CP4A better matches observations than the parameterized models. With climate change, we find the parameterized convection models have unrealistically high changes in the shape parameter of the extreme value distribution, which controls the tail behavior (i.e., the most extreme events), leading to large increases in return levels of events with a return period of >20 years. This suggests that parameterized convection models may not be suitable for looking at relative changes in rare rainfall events with climate change and that convection-permitting models should be preferred for this type of work. With the more realistic CP4A, RCP8.5 end-of-century climate change leads to 1-in-100-yr events becoming 1-in-23-yr events, which will necessitate serious adaptation efforts to avoid devastating socioeconomic impacts.

Significance Statement

We use a new, high-resolution climate model to examine how rare extreme rainfall events in East Africa might change in the future with climate change and compare the results with those from standard-resolution climate models. We find that the standard-resolution models have unrealistically large increases in rainfall for events that occur less frequently than every 20 years. The high-resolution model is more realistic and is required to illustrate possible future changes in rare rainfall extremes. Extreme events will become more common with climate change, and in the more realistic model we show that a 1-in-100-yr event may become a 1-in-23-yr event by the end of the century if greenhouse gas emissions are not significantly reduced.

Restricted access
Vicky Gorman
,
Tiffany Risch
,
Viviane Silva
,
Morgan B. Yarker
, and
Wendy Abshire
Free access
Till Kuhlbrodt
,
Aurore Voldoire
,
Matthew D. Palmer
,
Olivier Geoffroy
, and
Rachel E. Killick

Abstract

Ocean heat content (OHC) is one of the most relevant metrics tracking the current global heating. Therefore, simulated OHC time series are a cornerstone for assessing the scientific performance of Earth System models and global climate models. Here we present a detailed analysis of OHC change in simulations of the historical climate (1850-2014) performed with two pairs of CMIP6 (Coupled Model Intercomparison Project phase 6) models: UKESM1 and HadGEM3-GC3.1-LL; and CNRM-ESM2-1 and CNRM-CM6-1. The small number of models enables us to analyse OHC change globally and for individual ocean basins, making use of a novel ensemble of observational products. For the top 700 m of the global ocean, the two CNRM models reproduce the observed OHC change since the 1960s closely. The two UK models (UKESM1.0-LL and HadGEM3-GC3.1-LL) compensate a lack of warming in the 0-700 m layer in the 1970s and 1980s with warming below 2000 m. The observed warming between 700 and 2000 m is substantially underestimated by all models. An increased relevance for ocean heat uptake in the Atlantic after 1991 – suggested by observations – is picked up by the UK models but less so by the CNRM models, probably related to an AMOC strengthening in the UK models. The regional ocean heat uptake characteristics differ even though all four models share the same ocean component (NEMO ORCA1). Differences in the simulated global, full depth OHC time series can be attributed to differences in the models’ total effective radiative forcing.

Restricted access
Yibing Su
,
James A Smith
, and
Gabriele Villarini

Abstract

The Lower Mississippi River has experienced a cluster of extreme floods during the past two decades. The Bonnet Carré spillway, which is located on the Mississippi River immediately upstream of New Orleans, has been operated 15 times since its completion in 1931, with 7 occurrences after 2008. In this study, we examine rainfall and atmospheric water balance components associated with Lower Mississippi River flooding in 2008, 2011, 2015-2016, 2017, 2018, and 2019. We focus on multiple time scales - 1, 3, 7, and 14 days - reflecting contributions from individual storm systems and the aggregate contributions from a sequence of storm systems. Atmospheric water balance variables - integrated water vapor flux (IVT) and precipitable water - are central to our assessment of the storm environment for Lower Mississippi flood events. We find anomalously large IVT corridors accompany the critical periods of heavy rainfall and are organized in southwest-to-northeast orientation over the Mississippi domain. Atmospheric Rivers play an important role as agents of extremes in water vapor flux and rainfall. We conduct climatological analyses of IVT and precipitable water extremes across the four time scales using 40 years of North American Regional Reanalysis (NARR) fields from 1979 to 2018. We find significant increasing trends in both variables at all time scales. Increases in IVT especially cover large regions of the Mississippi domain. The findings point to increased vulnerability faced by the Mississippi flood control system in the current and future climate.

Restricted access
Xiaohui Xie
,
Yan Wang
,
Zhiyu Liu
,
Xiaohui Liu
,
Dake Chen
,
Dongsheng Zhang
, and
Jiannan Wang

Abstract

The bottom boundary layer (BBL) contributes significantly to the global energy dissipation of low-frequency flows in the abyssal ocean, but how this dissipation occurs remains poorly understood. Using in-situ data collected near the BBL at an abyssal seamount in the western Pacific Ocean, we demonstrate that strong bottom-trapped flows over sloping topography can lose their energy to near-inertial waves (NIWs) generated via the adjustment of the bottom Ekman layer. The NIWs with near-resonant frequencies corresponding to internal waves with propagation direction parallel to the topographic slope are observed. These waves are strongest in the BBL and have a correlation with the off-seamount sub-inertial flows largely attributed to the Ekman transport driven by the bottom-trapped anticyclonic circulation over the seamount. The bottom-intensified NIWs are observed to have dominant upward propagating energy and hypothesized to be generated via Ekman flow-topography interactions in the BBL. Energy loss from the near-bottom flows to radiating NIWs (~8 × 10−4 W/m2) is estimated to be substantially larger than that due to bottom drag dissipation (~2 × 10−4 W/m2), suggesting the important role of internal wave generation via the Ekman transport adjustment in damping the sub-inertial flows over the sloping seafloor.

Restricted access
Kwang-Hyung Kim
,
Chris D. Hewitt
,
Hideki Kanamaru
,
Jorge Alvar-Beltrán
,
Ana Heureux
,
Sook-Young Park
,
Min-Hye Jung
, and
Robert Stefanski

Abstract

Agricultural stakeholders can effectively manage the risks and opportunities arising from climate change and variability by enhancing climate services in agriculture. Key to understanding and addressing the climate challenge is the provision and the use of climate information to aid decision-makers and policymakers. Climate services are now integral to the United Nations Framework Convention on Climate Change, the Intergovernmental Panel on Climate Change’s Assessment Reports, governments’ national adaptation plans, funding bodies, and a growing number of sectors and industries worldwide. The article provides our personal perspective, experience, and views on the important and timely issue of managing better the risks and opportunities to the agriculture sector and community that are arising from changes in climate. We describe a framework to help drive action to tackle the climate challenge comprising enhanced knowledge and information products, efficient information delivery and use, and assured policy and institutional support, in an iterative loop.

Full access
James A. Carton
and
Gennady A. Chepurin

Abstract

This paper describes the new Regional Arctic Ocean/sea ice Reanalysis (RARE) with a domain that spans a subpolar/polar cap poleward of 45°N. Sequential data assimilation constrains temperature and salinity using World Ocean Database profiles as well as in-situ and satellite SST, and PIOMAS sea ice thickness estimates. The 41-year (1980-2020) RARE1.15.2 reanalysis with resolution varying between 2-5 km horizontally and 1-10m vertically in the upper 100m is examined.

To explore the impact of resolution RARE1.15.2 is compared to a coarser resolution SODA3.15.2, which uses the same modeling and data assimilation system. Improving resolution in the reanalysis system improves agreement with observations. It produces stronger more compact currents, enhances eddy kinetic energy, and strengthens along-isopycnal heat and salt transports, but reduces vertical exchanges and thus strengthens upper ocean haline stratification. RARE1.15.2 and SODA3.15.2 are also compared to the Hadley Center EN4.2.2 statistical objective analysis. In regions of reasonable data coverage such as the Nordic Seas the three products produce similar time-mean distributions of temperature and salinity. But in regions of poor coverage and in regions where the coverage changes in time EN4.2.2 suffers more from those inhomogeneities.

Finally, the impact on the Arctic of interannual temperature fluctuations in the subpolar gyres on the Arctic Ocean is compared. The influence of the subpolar North Pacific is limited to a region surrounding Bering Strait. The influence of the subpolar North Atlantic, in contrast, spreads throughout the Nordic Seas and Barents Sea in all three products within two years.

Restricted access
Justin Hudson
and
Eric Maloney

Abstract

The ‘barrier effect’ of the Maritime Continent (MC) is a known hurdle in understanding the propagation of the Madden-Julian Oscillation (MJO). To understand the differing dynamics of MJO events that propagate versus stall over the MC, a new tracking algorithm utilizing 30-96 day filtered NOAA Interpolated OLR anomalies is presented. Using this algorithm, MJO events can be identified, tracked, and described in terms of their propagation characteristics. Latent heat flux from OAFLUX, and CYGNSS surface winds and fluxes, are compared for MJO events that do and do not propagate through the MC. Events that successfully propagate through the MC demonstrate regional surface flux anomalies that are stronger, more-spatially coherent, and have a larger fetch. The spatial scale of convective anomalies for events that successfully propagate through the MC region is also larger than for terminating events. Large-scale enhancement of latent heat fluxes near and to the east of the Dateline, equally driven by dynamic and thermodynamic effects, also accompanies MJO events that successfully propagate through the MC. These findings are placed in the context of recent theoretical models of the MJO in which latent heat fluxes are important for propagation and destabilization.

Restricted access
Jun Li
,
W. Paul Menzel
,
Timothy J. Schmit
, and
Johannes Schmetz

Abstract

A hyperspectral infrared (IR) sounder from geostationary orbit provides nearly continuous measurements of atmospheric thermodynamic and dynamic information within a weather cube, specifically the atmospheric temperature, moisture, and wind information at different pressure levels that are critical for improving high-impact weather (HIW) nowcasting and numerical weather prediction (NWP). Geostationary hyperspectral IR sounders (GeoHIS) have been on board China’s Fengyun-4 series since 2016 and will be on board Europe’s Meteosat Third Generation (MTG) series in the 2024 time frame; the United States and other countries are also planning to include GeoHIS instruments on their next generation of geostationary weather satellites. Although availability of on-orbit GeoHIS data are limited currently, studies have been conducted and progress has been made on developing the applications of high-temporal-resolution GeoHIS observations. These include but are not limited to deriving three-dimensional wind fields for nowcasting and NWP applications, trending atmospheric instability for warning in preconvective environments, conducting impact studies with data from the experimental Geostationary Interferometric Infrared Sounder (GIIRS) on board Fengyun-4A, preparing observing system simulation experiments (OSSEs), and monitoring diurnal variation of atmospheric composition. This paper provides an overview of the current applications of GeoHIS, discusses the data processing challenges, and provides perspectives on future development. The purpose is to provide direction on utilization of the current and assist preparation for the upcoming GeoHIS observations for nowcasting, NWP and other applications.

Free access