Jos de Laat of KNMI has alerted us to the informative new paper
Kunkel, K, et al 2012: Monitoring and Understanding Trends in Extreme Storms: State of Knowledge. Bulletin of the American Meteorological Society 2012 ; doi: http://dx.doi.org/10.1175/BAMS-D-11-00262.1
The abstract reads [highlight added]
The state of knowledge regarding trends and an understanding of their causes is presented for a specific subset of extreme weather and climate types. For severe convective storms (tornadoes, hail storms, and severe thunderstorms), differences in time and space of practices of collecting reports of events make the use of the reporting database to detect trends extremely difficult. Overall, changes in the frequency of environments favorable for severe thunderstorms have not been statistically significant. For extreme precipitation, there is strong evidence for a nationally-averaged upward trend in the frequency and intensity of events. The causes of the observed trends have not been determined with certainty, although there is evidence that increasing atmospheric water vapor may be one factor. For hurricanes and typhoons, robust detection of trends in Atlantic and western North Pacific tropical cyclone (TC) activity is significantly constrained by data heterogeneity and deficient quantification of internal variability. Attribution of past TC changes is further challenged by a lack of consensus on the physical linkages between climate forcing and TC activity. As a result, attribution of trends to anthropogenic forcing remains controversial. For severe snowstorms and ice storms, the number of severe regional snowstorms that occurred since 1960 was more than twice that of the preceding 60 years. There are no significant multi-decadal trends in the areal percentage of the contiguous U.S. impacted by extreme seasonal snowfall amounts since 1900. There is no distinguishable trend in the frequency of ice storms for the U.S. as a whole since 1950.
The article is an important new contribution in the assessment of changes in climate metrics over time. I have, however, one comments about the analyses and their conclusions in regards to their suggestion of attributing an increase in extreme precipitation to an increase in atmospheric water vapor. Kunkel et al 2012 write
Karl and Trenberth (2003) have empirically demonstrated that for the same annual or seasonal precipitation totals, warmer climates generate more extreme precipitation events compared to cooler climates. This is consistent with water vapor being a critical limiting factor for the most extreme precipitation events. A number of analyses have documented significant positive trends in water vapor concentration and have linked these trends to human fingerprints in both changes of surface (Willet et al.2007) and atmospheric moisture (Santer et al. 2007).
The authors present analyses in their Table 2 to document an increase in atmospheric water vapor. They describe their analysis in the Table caption as
Table 2. Differences between two periods (1990-2009 minus 1971-845 1989) for daily, 1-in-5yr extreme events and maximum precipitable water values measured in the spatial vicinity of the extreme event location and within 24 hours of the event time.
However, in their analysis they use just two blocks of time (1990-2009) and (1971-1989) when different sliding analysis windows should have been used, in order to assess our robust there finding is with respect to sampling window.
They also should consider a peer-reviewed study which yields a different finding when assessing the overall North American trend in precipitable water;
Wang, J.-W., K. Wang, R.A. Pielke, J.C. Lin, and T. Matsui, 2008: Towards a robust test on North America warming trend and precipitable water content increase. Geophys. Res. Letts., 35, L18804, doi:10.1029/2008GL034564. http://pielkeclimatesci.files.wordpress.com/2009/10/r-337.pdf
where we report
An increase in the atmospheric moist content has been generally assumed when the lower-tropospheric temperature (Tcol) increases, with relative humidity holding steady. Rather than using simple linear regression, we propose a more rigorous trend detection method that considers time series memory. The autoregressive moving-average (ARMA) parameters for the time series of Tcol, precipitable water vapor (PWAV), and total precipitable water content (PWAT) from the North American Regional Reanalysis data were first computed. We then applied the Monte Carlo method to replicate the ARMA time series samples to estimate the variances of their Ordinary Least Square trends. Student’s t tests showed that Tcol from 1979 to 2006 increased significantly; however, PWAVand PWAT did not. This suggests that atmospheric temperature and water vapor trends do not follow the conjecture of constant relative humidity over North America. We thus urge further evaluations of Tcol, PWAV, and PWAT trends for the globe.
They also did not consider peer-reviewed papers on the role of land use change in altering extreme precipitation events, where irrigation of surrounding landscapes when dams are constructed, appears to enhance extreme precipitation at least in arid and semi-arid landscapes through the enhancement of convective available potential energy (CAPE); e.g. see
Degu, A. M., F. Hossain, D. Niyogi, R. Pielke Sr., J. M. Shepherd, N. Voisin, and T. Chronis, 2011: The influence of large dams on surrounding climate and precipitation patterns. Geophys. Res. Lett., 38, L04405, doi:10.1029/2010GL046482.
In this paper we wrote
Understanding the forcings exerted by large dams on local climate is key to establishing if artificial reservoirs inadvertently modify precipitation patterns in impounded river basins. Using a 30 year record of reanalysis data, the spatial gradients of atmospheric variables related to precipitation formation are identified around the reservoir shoreline for 92 large dams of North America. Our study reports that large dams influence local climate most in Mediterranean, and semi‐arid climates, while for humid climates the influence is least apparent. Clear spatial gradients of convective available potential energy, specific humidity and surface evaporation are also observed around the fringes between the reservoir shoreline and farther from these dams. Because of the increasing correlation observed between CAPE and extreme precipitation percentiles, our findings point to the possibility of storm intensification in impounded basins of the Mediterranean and arid climates of the United States.
Another example of a study that documents how landscape change in the United States can alter precipitation patterns, including intensity, is
Georgescu, M., D. B. Lobell, and C. B. Field (2009), The Potential Impact of US biofuels on Regional Climate, Geophys. Res. Lett., In Press, doi: 10.1029/2009GL040477
who reported that
Using the latest version of the WRF modeling system we conducted twenty-four, midsummer, continental-wide, sensitivity experiments by imposing realistic biophysical parameter limits appropriate for bio-energy crops in the Corn Belt of the United States….. Maximum, local changes in 2m temperature of the order of 1°C occur for the full breadth of albedo (ALB), minimum canopy resistance (RCMIN), and rooting depth (ROOT) specifications, while the regionally (105°W – 75°W and 35°N – 50°N) and monthly averaged response of 2m temperature was most pronounced for the ALB and RCMIN experiments, exceeding 0.2°C….The full range of albedo variability associated with biofuel crops may be sufficient to drive regional changes in summertime rainfall.
An increase in surface temperature would increase CAPE (and the resultant intensity of thunderstorms) if the water vapor content remained the same (or increased).
Urban landscapes also can contribute to enhancing the magnitude of extreme precipitation; e.g. see
Lei, M., D. Niyogi, C. Kishtawal, R. Pielke Sr., A. Beltrán-Przekurat, T. Nobis, and S. Vaidya, 2008: Effect of explicit urban land surface representation on the simulation of the 26 July 2005 heavy rain event over Mumbai, India. Atmos. Chem. Phys. Discussions, 8, 8773–8816
where among the conclusions is written
The results indicate that even for this synoptically active rainfall event, the vertical wind and precipitation are significantly influenced by urbanization, and the effect is more significant during the storm initiation…….The results suggest that urbanization can significantly contribute to extremes in monsoonal rain events that have been reported to be on the rise;
and see also, as another example,
Georgescu, M., G. Miguez-Macho, L. T. Steyaert, and C. P. Weaver (2009), Climatic effects of 30 years of landscape change over the Greater Phoenix, Arizona, region: 2. Dynamical and thermodynamical response, J. Geophys. Res., doi:10.1029/2008JD010762.
where in his guest post on February 9 2009 wrote
Our modeling results show a systematic difference in total accumulated precipitation between the most recent (2001) and least recent (1973) landscape reconstructions: a rainfall enhancement for 2001 relative to the 1973 landscape.
We recommend that in the next assessment led by Ken Kunkel and colleagues they include consideration of the role of landscape processes in affecting extreme weather over the United States (and elsewhere).