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Guirguis, K, Gershunov A, Cayan DR, Pierce DW.  2018.  Heat wave probability in the changing climate of the Southwest US. Climate Dynamics. 50:3853-3864.   10.1007/s00382-017-3850-3   AbstractWebsite

Analyses of observed non-Gaussian daily minimum and maximum temperature probability distribution functions (PDFs) in the Southwest US highlight the importance of variance and warm tail length in determining future heat wave probability. Even if no PDF shape change occurs with climate change, locations with shorter warm tails and/or smaller variance will see a greater increase in heat wave probability, defined as exceedances above the historical 95th percentile threshold, than will long tailed/larger variance distributions. Projections from ten downscaled CMIP5 models show important geospatial differences in the amount of warming expected for a location. However, changes in heat wave probability do not directly follow changes in background warming. Projected changes in heat wave probability are largely explained by a rigid shift of the daily temperature distribution. In some locations where there is more warming, future heat wave probability is buffered somewhat by longer warm tails. In other parts of the Southwest where there is less warming, heat wave probability is relatively enhanced because of shorter tailed PDFs. Effects of PDF shape changes are generally small by comparison to those from a rigid shift, and fall within the range of uncertainty among models in the amount of warming expected by the end of the century.

Gershunov, A, Cayan DR.  2003.  Heavy daily precipitation frequency over the contiguous United States: Sources of climatic variability and seasonal predictability. Journal of Climate. 16:2752-2765.   10.1175/1520-0442(2003)016<2752:hdpfot>2.0.co;2   AbstractWebsite

By matching large-scale patterns in climate fields with patterns in observed station precipitation, this work explores seasonal predictability of precipitation in the contiguous United States for all seasons. Although it is shown that total seasonal precipitation and frequencies of less-than-extreme daily precipitation events can be predicted with much higher skill, the focus of this study is on frequencies of daily precipitation above the seasonal 90th percentile (P90), a variable whose skillful prediction is more challenging. Frequency of heavy daily precipitation is shown to respond to ENSO as well as to non-ENSO interannual and interdecadal variability in the North Pacific. Specification skill achieved by a statistical model based on contemporaneous SST forcing with and without an explicit dynamical atmosphere is compared and contrasted. Statistical models relating the SST forcing patterns directly to observed station precipitation are shown to perform consistently better in all seasons than hybrid (dynamical-statistical) models where the SST forcing is first translated to atmospheric circulation via three separate general circulation models and the dynamically computed circulation anomalies are statistically related to observed precipitation. Skill is summarized for all seasons, but in detail for January-February-March, when it is shown that predictable patterns are spatially robust regardless of the approach used. Predictably, much of the skill is due to ENSO. While the U. S. average skill is modest, regional skill levels can be quite high. It is also found that non-ENSO-related skill is significant, especially for the extreme Southwest and that this is due mostly to non-ENSO interannual and decadal variability in the North Pacific SST forcing. Although useful specification skill is achieved by both approaches, hybrid predictability is not pursued further in this effort. Rather, prognostic analysis is carried out with the purely statistical approach to analyze P90 predictability based on antecedent SST forcing. Skill at various lead times is investigated and it is shown that significant regional skill can be achieved at lead times of several months even in the absence of strong ENSO forcing.

Li, HQ, Kanamitsu M, Hong SY, Yoshimura K, Cayan DR, Misra V.  2014.  A high-resolution ocean-atmosphere coupled downscaling of the present climate over California. Climate Dynamics. 42:701-714.   10.1007/s00382-013-1670-7   AbstractWebsite

A fully coupled regional ocean-atmosphere model system that consists of the regional spectral model and the regional ocean modeling system for atmosphere and ocean components, respectively, is applied to downscale the present climate (1985-1994) over California from a global simulation of the Community Climate System Model 3.0 (CCSM3). The horizontal resolution of the regional coupled modeling system is 10 km, while that of the CCSM3 is at a spectral truncation of T85 (approximately 1.4A degrees). The effects of the coupling along the California coast in the boreal summer and winter are highlighted. Evaluation of the sea surface temperature (SST) and 2-m air temperature climatology shows that alleviation of the warm bias along the California coast in the global model output is clear in the regional coupled model run. The 10-m wind is also improved by reducing the northwesterly winds along the coast. The higher resolution coupling effect on the temperature and specific humidity is the largest near the surface, while the significant impact on the wind magnitude appears at a height of approximately 850-hPa heights. The frequency of the Catalina Eddy and its duration are increased by more than 60 % in the coupled downscaling, which is attributed to enhanced offshore sea-breeze. Our study indicates that coupling is vital to regional climate downscaling of mesoscale phenomena over coastal areas.

Barnett, TP, Pierce DW, Hidalgo HG, Bonfils C, Santer BD, Das T, Bala G, Wood AW, Nozawa T, Mirin AA, Cayan DR, Dettinger MD.  2008.  Human-induced changes in the hydrology of the western United States. Science. 319:1080-1083.   10.1126/science.1152538   AbstractWebsite

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climate change detection and attribution study, using a high- resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show that up to 60% of the climate- related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human- induced. These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.

Pandey, GR, Cayan DR, Dettinger MD, Georgakakos KP.  2000.  A hybrid orographic plus statistical model for downscaling daily precipitation in northern California. Journal of Hydrometeorology. 1:491-506.   10.1175/1525-7541(2000)001<0491:ahopsm>2.0.co;2   AbstractWebsite

A hybrid (physical-statistical) scheme is developed to resolve the finescale distribution of daily precipitation over complex terrain. The scheme generates precipitation by combining information from the upper-air conditions and From sparsely distributed station measurements: thus, it proceeds in two steps. First, an initial estimate of the precipitation is made using a simplified orographic precipitation model. It is a steady-state, multilayer, and two-dimensional model following the concepts of Rhea, The model is driven by the 2.5 degrees x 2.5 degrees gridded National Oceanic and Atmospheric Administration-National Centers for Environmental Prediction upper-air profiles, and its parameters are tuned using the observed precipitation structure of the region, Precipitation is generated assuming a forced lifting of the air parcels as they cross the mountain barrier following a straight trajectory. Second, the precipitation is adjusted using errors between derived precipitation and observations from nearby sites. The study area covers the northern half of California, including coastal mountains, central valley, and the Sierra Nevada. The model is run for a 5-km rendition of terrain for days of January-March over the period of 1988-95. A jackknife analysis demonstrates the validity of the approach. The spatial and temporal distributions of the simulated precipitation field agree well with the observed precipitation, Further, a mapping of model performance indices (correlation coefficients, model bias, root-mean-square error, and threat scores) from an array of stations from the region indicates that the model performs satisfactorily in resolving daily precipitation at 5-km resolution.

Georgakakos, KP, Bae DH, Cayan DR.  1995.  Hydroclimatology of continental watersheds: 1. Temporal Analyses. Water Resources Research. 31:655-675.   10.1029/94wr02375   AbstractWebsite

The linkage between meteorology/climate and hydrology of temperate latitude catchments on daily to decade time scales is studied. Detailed hydrology is provided by a hydrologic catchment model, adapted from the operational streamflow forecast model of the National Weather Service River Forecast System. The model is tuned to respond to observed daily precipitation and potential evaporation input. Results from the Bird Creek basin with outlet near Sperry, Oklahoma, and from the Boone River basin with outlet at Webster City, Iowa, indicate that the model quite accurately simulates the observed daily discharge over 40 years at each of the two 2000-km(2) basins. Daily cross-correlations between observed and simulated basin outflows were better than 0.8 for both basins over a 40-year historical period. Soil moisture variability over a period of four decades is studied, and an assessment of temporal and spatial (as related to the separation distance of the two basins) scales present in the estimated soil moisture record is made. Negative soil. water anomalies have larger magnitudes than positive anomalies, and comparison of the simulated soil water records of the two basins indicates spatial scales of variability that in several cases are as long as the interbasin distance. The temporal scales of soil water content are considerably longer than those of the forcing atmospheric variables for all seasons and both basins. Timescales of upper and total soil water content anomalies are typically 1 and 3 months, respectively. Linkage between the hydrologic components and both local and regional-to-hemispheric atmospheric variability is studied, both for atmosphere forcing hydrology and hydrology forcing atmosphere. For both basins, crosscorrelation analysis shows that local precipitation strongly forces soil water in the upper soil layers with a 10-day lag. There is no evidence of soil water feedback to local precipitation. However, significant cross-correlation values are obtained for upper soil water leading daily maximum temperature with 5-10 day lags, especially during periods of extremely high or low soil water content. Complementary results of a spatial hydroclimatic analysis are presented in a companion paper (Cayan and Georgakakos, this issue).

Cayan, DR, Georgakakos KP.  1995.  Hydroclimatology of continental watersheds: 2. Spatial analyses. Water Resources Research. 31:677-697.   10.1029/94wr02376   AbstractWebsite

We diagnose the spatial patterns and further examine temporal behavior of anomalous monthly-seasonal precipitation, temperature, and atmospheric circulation in relationship to hydrologic (soil water and potential evapotranspiration) fluctuations at two watersheds in the central United States. The bulk hydrologic balance at each of the two watersheds, Boone River, Iowa (BN), and Bird Creek, Oklahoma (BC), was determined from the rainfall-runoff-routing watershed model described in part 1. There are many similarities among the hydroclimatic linkages at the two basins. In both, relationships with precipitation and temperature indicate that the forcing occurs on regional scales, much larger than the individual watersheds. Precipitation exhibits anomaly variability over 500-km scales, and sometimes larger. Anomalous temperature, which is strongly correlated with potential evapotranspiration, often extends from the Great Plains to the Appalachian Mountains. Seasonally, the temperature and precipitation anomalies tend to have greatest spatial coherence in fall and least in summer. The temperature and precipitation tend to have out-of-phase anomalies (e.g., warm associated with dry). Thus low soil water conditions are reinforced by low precipitation and high potential evapotranspiration, and vice versa for high soil water. Soil water anomalies in each basin accumulate over a history of significant large-scale climate forcing that usually appears one or two seasons in advance. These forcing fields are produced by atmospheric circulation anomaly patterns that often take on hemispheric scales. BN and BC have strong similarities in their monthly circulation patterns producing heavy/light monthly precipitation episodes, the primary means of forcing of the watersheds. The patterns exhibit regional high or low geopotential anomalies just upstream over the western United States or near the center of the country. The regional circulation features are often part of a train, with teleconnections upstream over the North Pacific and downstream over the North Atlantic/Eurasia sector. Synoptic scale events exhibit very similar patterns to the monthly circulations, only more intense.