Principal Investigator


At a Glance

A modeling study indicates that the urbanization of Houston acted to enhance the flooding from Hurricane Harvey (2017) in the city, both because less of Harvey’s rainfall was able to infiltrate the soil due to an increase in impervious surface coverage and because the increased surface “roughness” of the urban landscape acted to locally enhance Harvey’s rainfall. An observed 28-year increase in rapid intensification rates of North Atlantic hurricanes is unusual and may already include a signal from human radiative forcing; however, uncertainties in the hurricane data record over the rest of the world preclude a confident assessment of recent changes in rapid intensification. A modeling study indicates that explosive volcanic activity, such as the 1963 Mount Agung eruption, could impact global tropical cyclone (TC) activity in the years that follow, but that the response will be fundamentally different for different volcanoes. The goal of this work is to improve the understanding of the mechanisms behind and limits to the predictability of TC activity over the past few and next centuries. The work connects to broad questions in the climate science community, such as uncertainty over what TC changes are likely to occur over the coming century, and the extent to which intrinsic climate variability and natural forcing may be dominant over the impact of greenhouse forcing.

 


Research Highlight

The goal of this work by the Vecchi group is to improve the understanding of the character of, mechanisms behind and limits on the predictability of, variations and changes in the statistics of TC activity over the past few and coming centuries. Key tools in these studies are climate and atmospheric model studies, along with analyses of the observed record, to better understand the extent to which observed multi-decadal to centennial changes in TC activity have been driven by large-scale factors (such as ocean temperature changes, greenhouse gases, volcanic eruptions, and El Niño) versus random atmospheric fluctuations. Furthermore, the group has explored aspects of the extent to which natural and human-induced climate drivers (such as volcanic eruptions and greenhouse-induced warming) and non-climatic factors (such as urbanization) have influenced the character and impact of TC activity.

They have also explored controls on the extreme rainfall and flooding associated with TCs, such as the devastating flooding in Houston during Hurricane Harvey in 2017. It was found that urbanization in Houston contributed both to locally enhance/concentrate the extreme rainfall over the city during Hurricane Harvey, and to increase the amount of flooding from the rainfall (Zhang et al., 2018). A very surprising result of this is that the urban surface characteristics of Houston, largely its enhanced “surface roughness” coming from the many tall buildings, lead to a concentration of the rainfall induced by Harvey (Figure 3.1). In addition, the study showed that urbanization changed surface characteristics so that the rainfall was less able to infiltrate the soil, and led to enhanced flooding compared to the natural state. It was found that urbanization led to an approximately 20-fold increase in the likelihood of such flooding – with both increase in rainfall and flood conversion playing a role.

Figure 3.1. Including Houston’s urban surface characteristics enhances rainfall over Houston from Hurricane Harvey. Rainfall in the Houston area (a) observed during Hurricane Harvey (25-30 August 2017), and from two ensembles of atmospheric model experiments in which (b) urban conditions were prescribed over Houston and (c) in which the land surface conditions over Houston are artificially set to be grassland. Notice the increased rainfall over Houston in the middle panel relative to the bottom panel. (Zhang et al., 2018).

The group continues to focus on the rapid intensification of TCs, in which TC intensity increases rapidly in less than a day such as with hurricanes Harvey, Irma, and Maria in 2017. Rapidly intensifying TCs present a particular challenge to society since they tend to be poorly forecast on the weather scale and a rapid change in TC intensity can leave society with little time to prepare for TC impacts. A manuscript was seen through publication (Bhatia et al., 2018) that shows that projected warming over the 21st century is expected to increase the potential intensity (theoretical upper bound) of hurricanes and therefore increase the global proportion of storms making rapid intensification, with changes relative to the late- 20th century expected as soon as the next couple of decades. Another manuscript, in press (Bhatia et al., 2019), finds that the fraction of Atlantic hurricanes undergoing rapid intensification has increased over recent decades faster than expected from internal climate variations, but that observational uncertainties preclude us from making reliable statements yet about the changes in the rate of rapid intensification across the rest of the globe.

In addition, the team has researched the impact of volcanic forcing on global-scale TC and hydroclimate variations – with a particular focus on the similarities and differences between forcing from different 20th century volcanoes (Yang et al., submitted). These questions were explored through targeted climate model experiments in order to test the hypothesis that the hydroclimate and TC sensitivity to volcanoes depends fundamentally on the hemisphere in which the volcanic plume is most pronounced. The studies found fundamentally different TC and hydroclimate responses to Pinatubo (1991), the most intense volcanic eruption in the 20th century with a stratospheric plume symmetric about the equator; Santa María (1902), whose plume had a northern hemisphere maximum; and Agung (1963), which had its plume primarily in the southern hemisphere. For asymmetric volcanoes, rainfall and TC activity shift away from the hemisphere of strongest stratospheric aerosol forcing. And even though Pinatubo was the strongest volcano in terms of global radiative forcing and surface temperature change, both Santa María and Agung drove larger regional rainfall and TC differences due to displacements of climatological wet/ dry regions. These experiments help provide a baseline to help interpret climate changes over the past millennium, in which volcanos were one of the main drivers. They also help assess the extent to which the impact of cooling from volcanic forcing and the impact of surface warming from greenhouse gas buildup can serve as useful analogues when looking at regional climate changes and extremes.

 


References

Bhatia, K., G.A. Vecchi, T. Knutson, H. Murakami, J. Kossin, K. Dixon and C. Whitlock, 2019. Recent increases in tropical cyclone intensification rates. Nature Communications, 10(1): 635. doi.org/10.1038/ s41467-019-08471-z.

Yang, W., G.A. Vecchi, S. Fueglistaler, L.W. Horowitz, D.J. Luet, A.G. Muñoz, D. Paynter, and S. Underwood, 2019. Climatic Impacts from Asymmetric Large Volcanic Eruptions in a TC-Permitting Climate Model. Geophys. Res. Lett., submitted.