71 research outputs found
Robust Relationship Between Mid-latitudes CAPE and Moist Static Energy in Present and Future Simulations
Convective available potential energy (CAPE), a metric associated with severe
weather, is expected to increase with warming. Under the most widely-accepted
theory, developed for strongly convective regimes, mean CAPE should rise
following the Clausius-Clapeyron (C-C) relationship at 6-7%/K. We show here
that although the magnitude of CAPE change in high-resolution model output is
only slightly underestimated with simple theories, it is insufficient to
describe the distributional changes, which has a down-sloping structure and is
crucial for impact assessment. A more appropriate framework for understanding
CAPE changes uses the tight correlation between CAPE and moist static energy
(MSE) surplus. Atmospheric profiles develop appreciable CAPE only when MSE
surplus becomes positive; beyond this point, CAPE increases as 25% of the
rise in MSE surplus. Because this relationship is robust across climate states,
changes in future CAPE distributions can be well-captured by a simple scaling
of present-day data using only three parameters.Comment: Submitted for publication in Geophysical Research Letters (26
November 2022); 13 pages, 4 figure
Estimating changes in temperature extremes from millennial scale climate simulations using generalized extreme value (GEV) distributions
Changes in extreme weather may produce some of the largest societal impacts
of anthropogenic climate change. However, it is intrinsically difficult to
estimate changes in extreme events from the short observational record. In this
work we use millennial runs from the CCSM3 in equilibrated pre-industrial and
possible future conditions to examine both how extremes change in this model
and how well these changes can be estimated as a function of run length. We
estimate changes to distributions of future temperature extremes (annual minima
and annual maxima) in the contiguous United States by fitting generalized
extreme value (GEV) distributions. Using 1000-year pre-industrial and future
time series, we show that the magnitude of warm extremes largely shifts in
accordance with mean shifts in summertime temperatures. In contrast, cold
extremes warm more than mean shifts in wintertime temperatures, but changes in
GEV location parameters are largely explainable by mean shifts combined with
reduced wintertime temperature variability. In addition, changes in the spread
and shape of the GEV distributions of cold extremes at inland locations can
lead to discernible changes in tail behavior. We then examine uncertainties
that result from using shorter model runs. In principle, the GEV distribution
provides theoretical justification to predict infrequent events using time
series shorter than the recurrence frequency of those events. To investigate
how well this approach works in practice, we estimate 20-, 50-, and 100-year
extreme events using segments of varying lengths. We find that even using GEV
distributions, time series that are of comparable or shorter length than the
return period of interest can lead to very poor estimates. These results
suggest caution when attempting to use short observational time series or model
runs to infer infrequent extremes.Comment: 33 pages, 22 figures, 1 tabl
Global Variations of HDO and HDO/H2O Ratios in the Upper Troposphere and Lower Stratosphere Derived from ACE-FTS Satellite Measurements
High-quality satellite observations of water and deuterated water in the upper troposphere and lower stratosphere (UTLS) from the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) are used to map global climatological behavior. Spatial and temporal variability in these data suggest that convection plays a significant role in setting water vapor isotopic composition in these regions. In many instances, enhancements in HDO/H2O (i.e., δD) are closely tied to patterns of climatological deep convection and uncorrelated with water vapor, although convection appears to have different isotopic effects in different locations. The ACE-FTS data reveal seasonal variations in the tropics and allow mapping of climatological regional structure. These data reveal strong regional isotopic enhancement associated with the North American summer monsoon but not the Asian monsoon or the western Pacific warm pool. We suggest that the isotopic effects of deep convection near the tropopause are moderated by the ambient relative humidity, which controls the amount of convective ice that evaporates. Local convective signals can in turn affect global behavior: the North America monsoon influence introduces a Northern Hemisphere-Southern Hemisphere asymmetry in water isotopic composition in the lower stratosphere that extends into the tropics and influences the apparent seasonal cycle in averaged tropical UTLS data. Seasonal variation in tropical lower stratospheric water isotopic composition extends up to ∼20 km in ACE retrievals, but in contrast to previous reports, there is no clear evidence of propagation beyond the lowermost stratosphere. The reliability of these observations is supported by the broad consistency of ACE-FTS averaged tropical profiles with previous remote and in situ δD measurements. © 2012 by the American Geophysical Union
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