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Emily Bercos-Hickey, Christina M. Patricola, and William A. Gallus Jr

Abstract

The impact of climate change on severe storms and tornadoes remains uncertain, largely owing to inconsistencies in observational data and limitations of climate models. We performed ensembles of convection-permitting climate model simulations to examine how three tornadic storms would change if similar events were to occur in pre-industrial and future climates. The choice of events includes winter, nocturnal, and spring tornadic storms to provide insight into how the timing and seasonality of storms may affect their response to climate change. Updraft helicity (UH), convective available potential energy (CAPE), storm-relative helicity (SRH), and convective inhibition (CIN) were used to determine the favorability for the three tornadic storm events in the different climate states. We found that from the pre-industrial period to the present, the potential for tornadic storms decreased for the winter event and increased for the nocturnal and spring events. With future climate change, the potential for tornadic storms increased for the winter and nocturnal events in association with increased CAPE, and decreased for the spring event despite greater CAPE.

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Ryosuke Shibuya, Yukari Takayabu, and Hirotaka Kamahori

Abstract

This study examines disastrous historical precipitation cases that generate extreme precipitation simultaneously over a wide area in Japan (as in July 2018), defined as widespread extreme precipitation events. A statistically significant large-scale environment conducive for widespread extreme precipitation events over western Japan is investigated based on composite analysis. During a widespread precipitation event, a zonally elongated positive anomaly of the column-integrated water vapor extends from East China to western Japan. In the lower troposphere, a dipole of a geopotential height anomaly exists with positive and negative values at the east and west of the precipitation area, respectively. It is found that the negative geopotential anomaly is enhanced over East China at 2 days before the event and moves toward the precipitating area mainly due to the potential vorticity (PV) production term by diabatic heating, analogous to a diabatic Rossby wave. The temporal evolution of the dynamical forced vertical velocity is well in phase with that the PV production term, suggesting the importance of the coupling between the dynamical forced motion and diabatic heating. This result provides a physical understanding of the reason why both the background moisture and the baroclinicity are essential in the composited atmospheric fields and another view to the importance of the feedback parameter between the dynamical motion and diabatic heating.

Open access
Qiaoling Ren, Xingwen Jiang, Yang Zhang, Zhenning Li, and Song Yang

Abstract

It is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP. The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, midtropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as it was previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to a rainfall change (more in the south, less in the north) over East Asia.

Open access
Rodrigo J. Bombardi and William R. Boos

Abstract

This study examines the annual cycle of monsoon precipitation simulated by models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), then uses moist energy diagnostics to explain globally inhomogeneous projected future changes. Rainy season characteristics are quantified using a consistent method across the globe. Model bias is shown to include rainy season onsets tens of days later than observed in some monsoon regions (India, Australia, and North America) and overly large summer precipitation in others (North America, South America, and southern Africa). Projected next-century changes include rainy season lengthening in the two largest Northern Hemisphere monsoon regions (South Asia and central Sahel) and shortening in the two largest Southern Hemisphere regions (South America and southern Africa). Changes in the North American and Australian monsoons are less coherent across models. To understand these changes, relative moist static energy (MSE) is defined as the difference between local and tropical-mean surface air MSE. Future changes in relative MSE in each region correlate well with onset and demise date changes. Furthermore, Southern Hemisphere regions projected to undergo rainy season shortening are spanned by an increasing equator-to-pole MSE gradient, suggesting their rainfall will be increasingly inhibited by fluxes of dry extratropical air; Northern Hemisphere regions with projected lengthening of rainy seasons undergo little change in equator-to-pole MSE gradient. Thus, although model biases raise questions as to the reliability of some projections, these results suggest that globally inhomogeneous future changes in monsoon timing may be understood through simple measures of surface air MSE.

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Helga Kristin Olafsdottir, Holger Rootzén, and David Bolin

Abstract

Both the intensities of individual extreme rainfall events and the frequency of such events are important for infrastructure planning. We develop a new statistical extreme value model, the PGEV model, which makes it possible to use high-quality annual maximum series data instead of less well-checked daily data to estimate trends in intensity and frequency separately. The method is applied to annual maximum data from Vol. 10 of NOAA Atlas 14, dating from approximately 1900 to 2014, showing that in the majority of 333 rain gauge stations in the northeastern United States the frequency of extreme rainfall events increases as mean temperature increases, but that there is little evidence of trends in the distribution of the intensities of individual extreme rainfall events. The median of the frequency trends corresponds to extreme rainfall becoming 83% more frequent for each 1°C of temperature increase. Naturally, increasing trends in frequency also increase the yearly or decadal risks of very extreme rainfall events. Three other large areas in the contiguous United States, the Midwest, the Southeast, and Texas, are also studied, and show similar but weaker trends than those in the Northeast.

Open access
Bin Tang, Wenting Hu, and Anmin Duan

Abstract

A future projection of four extreme precipitation indices over the Indochina Peninsula and South China (INCSC) region with reference to the period 1958–2014 is conducted through the application of a multimodel ensemble approach and a rank-based weighting method. The weight of each model from phase 6 of the Coupled Model Intercomparison Project (CMIP6) is calculated depending on its historical simulation skill. Then, the weighted and unweighted ensembles are used for future projections. The results show that all four extreme precipitation indices are expected to increase over the INCSC region, both in the middle (2041–60) and at the end (2081–2100) of the twenty-first century, under three Shared Socioeconomic Pathway (SSP) scenarios. The increases in total extreme precipitation (R95p), extreme precipitation days (R95d), and the fraction of total rainfall from events exceeding the extreme precipitation threshold (R95pT) in the Indochina Peninsula are more significant than those in South China. The occurrence of extreme rainfall events may become more frequent in the future over the INCSC region, since the probability that R95pT increases is larger than 0.7 in the whole INCSC region. A comparison between the weighted and unweighted ensemble means shows that the uncertainty over South China is almost always reduced after applying the weighted scheme to future probabilistic projection, while the reductions in uncertainty over the Indochina Peninsula may depend on SSPs. The more extreme precipitation over the INCSC region in the future may be related to the larger water vapor supply and the more unstable local atmospheric stratification.

Open access
Nana Liu, L. Ruby Leung, and Zhe Feng

Abstract

The distribution of latent heating released by mesoscale convective systems (MCSs) plays a crucial role in global energy and water cycles. To investigate the characteristics of MCS latent heating, five years (2014–19) of Global Precipitation Measurement (GPM) Ku-band Precipitation Radar observations and latent heating retrievals are combined with a newly developed global high-resolution (~10 km, hourly) MCS tracking dataset. The results suggest that midlatitude MCSs are shallower and have a lower maximum precipitation rate than tropical MCSs. However, MCSs occurring in the midlatitudes have larger precipitation areas and higher stratiform rain volume fraction, in agreement with previous studies. With substantial spatial and seasonal variability, MCS latent heating profiles are top-heavier in the middle and high latitudes than those in the tropics. Larger magnitudes of latent heating in the stratiform regions are found over the ocean than over land, which is the case for both the tropics and midlatitudes. The larger magnitude is related to a larger precipitating area/volume rather than a higher storm height or more intense convective core typically associated with land systems. A majority of midlatitude MCSs have a relatively high (>70%) stratiform fraction while this is not the case for tropical MCSs, suggesting that midlatitude MCSs tend to produce more stratiform rain while tropical MCSs are more convective. Importantly, the results of this study indicate that storm intensity, latent heating, and rainfall are different metrics of MCSs that can provide multiple constraints to inform development of convection parameterizations in global models.

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Evan Jones, Allison A. Wing, and Rhys Parfitt

Abstract

This study compares the spread in climatological tropical cyclone (TC) precipitation across eight different reanalysis datasets: NCEP-CFSR, ERA-20C, ERA-40, ERA5, ERA-Interim, JRA-55, MERRA-2, and NOAA-20C. TC precipitation is assigned using manual tracking via a fixed 500-km radius from each TC center. The reanalyses capture similar general spatial patterns of TC precipitation and TC precipitation fraction, defined as the fraction of annual precipitation assigned to TCs, and the spread in TC precipitation is larger than the spread in total precipitation across reanalyses. The spread in TC precipitation relative to the inter-reanalysis mean TC precipitation, or relative spread, is larger in the east Pacific than in the west Pacific. Partitioned by reanalysis intensity, the largest relative spread across reanalyses in TC precipitation is from high-intensity TCs. In comparison with satellite observations, reanalyses show lower climatological mean annual TC precipitation over most areas. A comparison of area-averaged precipitation rate in TCs composited over reanalysis intensity shows the spread across reanalyses is larger for higher intensity TCs. Testing the sensitivity of TC precipitation assignment to tracking method shows that climatological mean annual TC precipitation is systematically larger when assigned via manual tracking versus objective tracking. However, this tendency is minimized when TC precipitation is normalized by TC density. Overall, TC precipitation in reanalyses is affected by not only horizontal output resolution or any TC preprocessing, but also data assimilation and parameterization schemes. The results indicate that improvements in the representation of TCs and their precipitation in reanalyses are needed to improve overall precipitation.

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Xiaolan L. Wang, Mercè Casas-Prat, Yang Feng, Alexander Crosby, and Val R. Swail

Abstract

This study presents and analyzes Environment Canada’s Davis Strait Baffin Bay (EC-DSBB) Wind and Wave Reanalysis for the period 1979–2016 to characterize the historical changes in the surface wind speed and ocean surface waves. The trend analysis is carried out only for the months of May–December, when there is a significant ice-free sea area. The results show that 10-m wind speed (W s) has increased significantly in most areas of the domain in September–December, with some significant decreases over the open water area in June and July. The W s increases are most extensive in September, with significant increases in both the mean and extremes. It is also shown that the mean wind direction (W d) has a distinctive seasonal variation, being mainly northward and northwestward in June–August, and predominantly southward and southeastward in May and September–December. The most notable changes in W d are seen in June. The results also show that significant wave height (H s) and wave power (W p) have significantly increased in September–December and decreased in June. For example, the September regional mean H s has increased at a rate of 0.4% yr−1. In September–December, the local W s increases seem to be the main driver for the H s and W p increases, but the southeastward direction is favored by increasing fetch as sea ice retreats. In September and December, the positive trend in both W s and H s has intensified in the 2001–16. In June, however, the mean W d and the changes therein also play an important role in the H s changes, which are more affected by remotely generated waves.

Open access
Georgina Falster, Bronwen Konecky, Midhun Madhavan, Samantha Stevenson, and Sloan Coats

Abstract

Characterizing variability in the global water cycle is fundamental to predicting impacts of future climate change; understanding the role of the Pacific Walker circulation (PWC) in the regional expression of global water cycle changes is critical to understanding this variability. Water isotopes are ideal tracers of the role of the PWC in global water cycling because they retain information about circulation-dependent processes including moisture source, transport, and delivery. We collated publicly available measurements of precipitation δ 18O (δ 18OP) and used novel data processing techniques to synthesize long (34 yr), globally distributed composite records from temporally discontinuous δ 18OP measurements. We investigated relationships between global-scale δ 18OP variability and PWC strength, as well as other possible drivers of global δ 18OP variability—including El Niño–Southern Oscillation (ENSO) and global mean temperature—and used isotope-enabled climate model simulations to assess potential biases arising from uneven geographical distribution of the observations or our data processing methodology. Covariability underlying the δ 18OP composites is more strongly correlated with the PWC (r = 0.74) than any other index of climate variability tested. We propose that the PWC imprint in global δ 18OP arises from multiple complementary processes, including PWC-related changes in moisture source and transport length, and a PWC- or ENSO-driven “amount effect” in tropical regions. The clear PWC imprint in global δ 18OP implies a strong PWC influence on the regional expression of global water cycle variability on interannual to decadal time scales, and hence that uncertainty in the future state of the PWC translates to uncertainties in future changes in the global water cycle.

Open access