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Peterson, DH, Cayan DR, Dettinger MD, Noble M, Riddle LG, Schemel LE, Smith RE, Uncles R, Walters R.  1996.  San Francisco Bay: observations, numerical simulation and statistical models. San Francisco Bay: the ecosystem. Further investigations into the natural history of San Francisco Bay and Delta with reference to the influence of man. ( Hollibaugh JT, Ed.).:9-34., San Francisco, Calif.: Pacific Division of the American Association for the Advancement of Science Abstract
Guzman-Morales, J, Gershunov A, Theiss J, Li HQ, Cayan D.  2016.  Santa Ana Winds of Southern California: Their climatology, extremes, and behavior spanning six and a half decades. Geophysical Research Letters. 43:2827-2834.   10.1002/2016gl067887   AbstractWebsite

Santa Ana Winds (SAWs) are an integral feature of the regional climate of Southern California/Northern Baja California region, but their climate-scale behavior is poorly understood. In the present work, we identify SAWs in mesoscale dynamical downscaling of a global reanalysis from 1948 to 2012. Model winds are validated with anemometer observations. SAWs exhibit an organized pattern with strongest easterly winds on westward facing downwind slopes and muted magnitudes at sea and over desert lowlands. We construct hourly local and regional SAW indices and analyze elements of their behavior on daily, annual, and multidecadal timescales. SAWs occurrences peak in winter, but some of the strongest winds have occurred in fall. Finally, we observe that SAW intensity is influenced by prominent large-scale low-frequency modes of climate variability rooted in the tropical and north Pacific ocean-atmosphere system.

Lundquist, JD, Cayan DR.  2002.  Seasonal and spatial patterns in diurnal cycles in streamflow in the western United States. Journal of Hydrometeorology. 3:591-603.   10.1175/1525-7541(2002)003<0591:saspid>;2   AbstractWebsite

The diurnal cycle in streamflow constitutes a significant part of the variability in many rivers in the western United States and can be used to understand some of the dominant processes affecting the water balance of a given river basin. Rivers in which water is added diurnally, as in snowmelt, and rivers in which water is removed diurnally, as in evapotranspiration and infiltration, exhibit substantial differences in the timing, relative magnitude, and shape of their diurnal flow variations. Snowmelt-dominated rivers achieve their highest sustained flow and largest diurnal fluctuations during the spring melt season. These fluctuations are characterized by sharp rises and gradual declines in discharge each day. In large snowmelt-dominated basins, at the end of the melt season, the hour of maximum discharge shifts to later in the day as the snow line retreats to higher elevations. Many evapotranspiration/infiltration-dominated rivers in the western states achieve their highest sustained flows during the winter rainy season but exhibit their strongest diurnal cycles during summer months, when discharge is low, and the diurnal fluctuations compose a large percentage of the total flow. In contrast to snowmelt-dominated rivers, the maximum discharge in evapotranspiration/infiltration-dominated rivers occurs consistently in the morning throughout the summer. In these rivers, diurnal changes are characterized by a gradual rise and sharp decline each day.

White, WB, Cayan DR, Niiler PP, Moisan J, Lagerloef G, Bonjean F, Legler D.  2005.  The seasonal cycle of diabatic heat storage in the Pacific Ocean. Progress in Oceanography. 64:1-29.   10.1016/j.pocean.2004.06.012   AbstractWebsite

This study quantifies uncertainties in closing the seasonal cycle of diabatic heat storage (DHS) over the Pacific Ocean from 20 degrees S to 60 degrees N through the synthesis of World Ocean Circulation Experiment (WOCE) reanalysis products from 1993 to 1999. These products are DHS from Scripps Institution of Oceanography (SIO); near-surface geostrophic and Ekman currents from Earth and Space Research (ESR); and air-sea heat fluxes from Comprehensive Ocean-Atmosphere Data Set (COADS), National Centers for Environmental Prediction (NCEP), and European Center for Mid-Range Weather Forecasts (ECMWF). With these products, we compute residual heat budget components by differencing long-term monthly means from the long-term annual mean. This allows the seasonal cycle of the DHS tendency to be modeled. Everywhere latent heat flux residuals dominate sensible heat flux residuals, shortwave heat flux residuals dominate longwave heat flux residuals, and residual Ekman heat advection dominates residual geostrophic heat advection, with residual dissipation significant only in the Kuroshio-Oyashio current extension. The root-mean-square (RMS) of the differences between observed and model residual DHS tendencies (averaged over 10 degrees latitude-by-20 degrees longitude boxes) is < 20 W m(-2) in the interior ocean and < 100 W m(-2) in the Kuroshio-Oyashio current extension. This reveals that the residual DHS tendency is driven everywhere by some mix of residual latent heat flux, shortwave heat flux, and Ekman heat advection. Suppressing bias errors in residual air-sea turbulent heat fluxes and Ekman heat advection through minimization of the RMS differences reduces the latter to < 10 W m(-2) over the interior ocean and < 25 W m(-2) in the Kuroshio-Oyashio current extension. This reveals air-sea temperature and specific humidity differences from in situ surface marine weather observations to be a principal source of bias error, overestimated over most of ocean but underestimated near the Intertropical Convergence Zone. (c) 2005 Elsevier Ltd. All rights reserved.

Yan, XH, Niiler PP, Nadiga SK, Stewart RH, Cayan DR.  1995.  Seasonal heat storage in the North Pacific: 1976–1989. Journal of Geophysical Research-Oceans. 100:6899-6926.   10.1029/94jc03230   AbstractWebsite

Volunteer observing ship expendable bathythermograph data obtained during the period 1976-1989, from 30 degrees N to 40 degrees N in the North Pacific, were used to compute the rate of change of heat storage to a fixed upper ocean temperature surface. The variability of the storage rate in the seasonal timescale, computed on a 5 degrees latitude by 5 degrees longitude spatial scale, is compared to the net surface heat flux computed from available surface marine weather data to the same spatial and temporal resolution. Averaged across the entire basin, the difference between the average monthly heat storage rate and the average monthly heat flux is 3.76 Wm(-1). The average basin-wide absolute heat storage rate agrees to within 7.5% of the average absolute heat flux for the whole basin. An empirical orthogonal decomposition of the spatial patterns of the difference between the heat storage rate and the net heat flux reveals no obvious trends in the heat flux computation or possible physical processes responsible for the difference. Instead, the eddies shed by the warm boundary current, the Kuroshio, is probably responsible for the major part of the difference patterns in locations where the difference values are maximum. The most important results of this study are that the heat storage rate computed to a fixed isotherm matches the net heat flux extremely well at the chosen locations and across the whole basin; and the heat storage rate computation is sensitive to the isotherm choice and to the space scale involved.

Kueppers, LM, Snyder MA, Sloan LC, Cayan D, Jin J, Kanamaru H, Kanamitsu M, Miller NL, Tyree M, Due H, Weare B.  2008.  Seasonal temperature responses to land-use change in the western United States. Global and Planetary Change. 60:250-264.   10.1016/j.gloplacha.2007.03.005   AbstractWebsite

In the western United States, more than 79 000 km 2 has been converted to irrigated agriculture and urban areas. These changes have the potential to alter surface temperature by modifying the energy budget at the land-atmosphere interface. This study reports the seasonally varying temperature responses of four regional climate models (RCMs) - RSM, RegCM3, MM5-CLM3, and DRCM - to conversion of potential natural vegetation to modern land-cover and land-use over a 1-year period. Three of the RCMs supplemented soil moisture, producing large decreases in the August mean (- 1.4 to -3.1 degrees C) and maximum (-2.9 to -6.1 degrees C) 2-m air temperatures where natural vegetation was converted to irrigated agriculture. Conversion to irrigated agriculture also resulted in large increases in relative humidity (9% to 36% absolute change). Modeled changes in the August minimum 2-m air temperature were not as pronounced or consistent across the models. Converting natural vegetation to urban land-cover produced less pronounced temperature effects in all models, with the magnitude of the effect dependent upon the preexisting vegetation type and urban parameterizations. Overall, the RCM results indicate that the temperature impacts of land-use change are most pronounced during the summer months, when surface heating is strongest and differences in surface soil moisture between irrigated land and natural vegetation are largest. (c) 2007 Elsevier B.V. All rights reserved.

Cayan, DR, Miller AJ, Barnett TP, Graham NE, Ritchie JN, Oberhuber JM.  1995.  Seasonal-interannual fluctuations in surface temperature over the Pacific: effects of monthly winds and heat fluxes. Natural climate variability on decade-to-century time scales. :133-150., Washington, D.C.: National Academy Press Abstract
Burns, JC, Cayan DR, Tong G, Bainto EV, Turner CL, Shike H, Kawasaki T, Nakamura Y, Yashiro M, Yanagawa H.  2005.  Seasonality and temporal clustering of Kawasaki syndrome. Epidemiology. 16:220-225.   10.1097/01.ede.0000152901.06689.d4   AbstractWebsite

Background: The distribution of a syndrome in space and time may suggest clues to its etiology. The cause of Kawasaki syndrome, a systemic vasculitis of infants and children, is unknown, but an infectious etiology is suspected. Methods: Seasonality and clustering of Kawasaki syndrome cases were studied in Japanese children with Kawasaki syndrome reported in nationwide surveys in Japan. Excluding the years that contained the 3 major nationwide epidemics, 84,829 cases during a 14-year period (1987-2000) were analyzed. To assess seasonality, we calculated mean monthly incidence during the study period for eastern and western Japan and for each of the 47 prefectures. To assess clustering, we compared the number of cases per day (daily incidence) with a simulated distribution (Monte Carlo analysis). Results: Marked spatial and temporal patterns were noted in both the seasonality and deviations from the average number of Kawasaki syndrome cases in Japan. Seasonality was bimodal with peaks in January and June/July and a nadir in October. This pattern was consistent throughout Japan and during the entire 14-year period. Some years produced very high or low numbers of cases, but the overall variability was consistent throughout the entire country. Temporal clustering of Kawasaki syndrome cases was detected with nationwide outbreaks. Conclusions: Kawasaki syndrome has a pronounced seasonality in Japan that is consistent throughout the length of the Japanese archipelago. Temporal clustering of cases combined with marked seasonality suggests an environmental trigger for this clinical syndrome.

Burns, JC, Herzog L, Fabri O, Tremoulet AH, Rodo X, Uehara R, Burgner D, Bainto E, Pierce D, Tyree M, Cayan D, Kawasaki Dis Global Climate C.  2013.  Seasonality of Kawasaki Disease: A global perspective. Plos One. 8   10.1371/journal.pone.0074529   AbstractWebsite

Background: Understanding global seasonal patterns of Kawasaki disease (KD) may provide insight into the etiology of this vasculitis that is now the most common cause of acquired heart disease in children in developed countries worldwide. Methods: Data from 1970-2012 from 25 countries distributed over the globe were analyzed for seasonality. The number of KD cases from each location was normalized to minimize the influence of greater numbers from certain locations. The presence of seasonal variation of KD at the individual locations was evaluated using three different tests: time series modeling, spectral analysis, and a Monte Carlo technique. Results: A defined seasonal structure emerged demonstrating broad coherence in fluctuations in KD cases across the Northern Hemisphere extra-tropical latitudes. In the extra-tropical latitudes of the Northern Hemisphere, KD case numbers were highest in January through March and approximately 40% higher than in the months of lowest case numbers from August through October. Datasets were much sparser in the tropics and the Southern Hemisphere extra-tropics and statistical significance of the seasonality tests was weak, but suggested a maximum in May through June, with approximately 30% higher number of cases than in the least active months of February, March and October. The seasonal pattern in the Northern Hemisphere extra-tropics was consistent across the first and second halves of the sample period. Conclusion: Using the first global KD time series, analysis of sites located in the Northern Hemisphere extra-tropics revealed statistically significant and consistent seasonal fluctuations in KD case numbers with high numbers in winter and low numbers in late summer and fall. Neither the tropics nor the Southern Hemisphere extra-tropics registered a statistically significant aggregate seasonal cycle. These data suggest a seasonal exposure to a KD agent that operates over large geographic regions and is concentrated during winter months in the Northern Hemisphere extratropics.

Franco, G, Cayan DR, Moser S, Hanemann M, Jones MA.  2011.  Second California Assessment: integrated climate change impacts assessment of natural and managed systems. Climatic Change. 109:1-19.   10.1007/s10584-011-0318-z   AbstractWebsite

Since 2006 the scientific community in California, in cooperation with resource managers, has been conducting periodic statewide studies about the potential impacts of climate change on natural and managed systems. This Special Issue is a compilation of revised papers that originate from the most recent assessment that concluded in 2009. As with the 2006 studies that influenced the passage of California's landmark Global Warming Solutions Act (AB32), these papers have informed policy formulation at the state level, helping bring climate adaptation as a complementary measure to mitigation. We provide here a brief introduction to the papers included in this Special Issue focusing on how they are coordinated and support each other. We describe the common set of downscaled climate and sea-level rise scenarios used in this assessment that came from six different global climate models (GCMs) run under two greenhouse gas emissions scenarios: B1 (low emissions) and A2 (a medium-high emissions). Recommendations for future state assessments, some of which are being implemented in an on-going new assessment that will be completed in 2012, are offered.

DeFlorio, MJ, Ghan SJ, Singh B, Miller AJ, Cayan DR, Russell LM, Somerville RCJ.  2014.  Semidirect dynamical and radiative effect of North African dust transport on lower tropospheric clouds over the subtropical North Atlantic in CESM 1.0. Journal of Geophysical Research: Atmospheres. 119:2013JD020997.   10.1002/2013JD020997   AbstractWebsite

This study uses a century length preindustrial climate simulation by the Community Earth System Model (CESM 1.0) to explore statistical relationships between dust, clouds, and atmospheric circulation and to suggest a semidirect dynamical mechanism linking subtropical North Atlantic lower tropospheric cloud cover with North African dust transport. The length of the run allows us to account for interannual variability of North African dust emissions and transport in the model. CESM's monthly climatology of both aerosol optical depth and surface dust concentration at Cape Verde and Barbados, respectively, agree well with available observations, as does the aerosol size distribution at Cape Verde. In addition, CESM shows strong seasonal cycles of dust burden and lower tropospheric cloud fraction, with maximum values occurring during boreal summer, when a strong correlation between these two variables exists over the subtropical North Atlantic. Calculations of Estimated Inversion Strength (EIS) and composites of EIS on high and low downstream North African dust months during boreal summer reveal that dust is likely increasing inversion strength over this region due to both solar absorption and reflection. We find no evidence for a microphysical link between dust and lower tropospheric clouds in this region. These results yield new insight over an extensive period of time into the complex relationship between North African dust and North Atlantic lower tropospheric clouds, which has previously been hindered by spatiotemporal constraints of observations. Our findings lay a framework for future analyses using different climate models and submonthly data over regions with different underlying dynamics.

Dettinger, MD, Cayan DR, Meyer M, Jeton AE.  2004.  Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River basins, Sierra Nevada, California, 1900-2099. Climatic Change. 62:283-317.   10.1023/B:CLIM.0000013683.13346.4f   AbstractWebsite

Hydrologic responses of river basins in the Sierra Nevada of California to historical and future climate variations and changes are assessed by simulating daily streamflow and water-balance responses to simulated climate variations over a continuous 200-yr period. The coupled atmosphere-ocean-ice-land Parallel Climate Model provides the simulated climate histories, and existing hydrologic models of the Merced, Carson, and American Rivers are used to simulate the basin responses. The historical simulations yield stationary climate and hydrologic variations through the first part of the 20th century until about 1975 when temperatures begin to warm noticeably and when snowmelt and streamflow peaks begin to occur progressively earlier within the seasonal cycle. A future climate simulated with business-as-usual increases in greenhouse-gas and aerosol radiative forcings continues those recent trends through the 21st century with an attendant + 2.5 degreesC warming and a hastening of snowmelt and streamflow within the seasonal cycle by almost a month. The various projected trends in the business-as-usual simulations become readily visible despite realistic simulated natural climatic and hydrologic variability by about 2025. In contrast to these changes that are mostly associated with streamflow timing, long-term average totals of streamflow and other hydrologic fluxes remain similar to the historical mean in all three simulations. A control simulation in which radiative forcings are held constant at 1995 levels for the 50 years following 1995 yields climate and streamflow timing conditions much like the 1980s and 1990s throughout its duration. The availability of continuous climate-change projection outputs and careful design of initial conditions and control experiments, like those utilized here, promise to improve the quality and usability of future climate-change impact assessments.

Lundquist, JD, Dettinger MD, Cayan DR.  2005.  Snow-fed streamflow timing at different basin scales: Case study of the Tuolumne River above Hetch Hetchy, Yosemite, California. Water Resources Research. 41   10.1029/2004wr003933   AbstractWebsite

Diurnal cycles in snow-fed streams provide a useful technique for measuring the time it takes water to travel from the top of the snowpack, where snowmelt typically peaks in the afternoon, to the river gauge, where the daily maximum flows may arrive many hours later. Hourly stage measurements in nested subbasins (6-775 km(2)) of the Tuolumne River in Yosemite National Park illustrate travel time delays at different basin scales during the spring 2002 and 2003 melt seasons. Travel times increase with longer percolation times through deeper snowpacks, increase with longer travel times over land and along longer stream channels, and increase with slower in-stream flow velocities. In basins smaller than 30 km(2), travel times through the snowpack dominate streamflow timing. In particular, daily peak flows shift to earlier in the day as snowpacks thin and mean discharges increase. In basins larger than 200 km(2), snowpack heterogeneity causes the hour of peak flow to be highly consistent, with little or no variation as the snowpack thins. Basins with areas in between 30 and 200 km(2) exhibit different sequences of diurnal streamflow timing in different years, sometimes acting like small basins and other times like large basins. From the start of the melt season until the day of peak snowmelt discharge, increasing travel distances in channels as the snow line retreats to higher elevations do not cause long enough travel delays to offset the observed decrease in mean travel times through the snowpack. A model that couples porous medium flow through thinning snowpacks with free surface flow in stream channels can reproduce the observed patterns, provided that the model incorporates snowpack heterogeneity.

White, WB, Cayan DR, Dettinger MD, Auad G.  2001.  Sources of global warming in upper ocean temperature during El Nino. Journal of Geophysical Research-Oceans. 106:4349-4367.   10.1029/1999jc000130   AbstractWebsite

Global average sea surface temperature (SST) from 40 degreesS to 60 degreesN fluctuates +/-0.3 degreesC on interannual period scales, with global warming (cooling) during El Nino (La Nina). About 90% of the global warming during El Nino occurs in the tropical global ocean from 20 degreesS to 20 degreesN, half because of large SST anomalies in the tropical Pacific associated with El Nino and the other half because of warm SST anomalies occurring over similar to 80% of the tropical global ocean. From examination of National Centers for Environmental Prediction [Kalnay et al., 1996] and Comprehensive Ocean-Atmosphere Data Set [Woodruff et al., 1993] reanalyses, tropical global warming during El Nino is associated with higher troposphere moisture content and cloud cover, with reduced trade wind intensity occurring during the onset phase of EI Nino. During this onset phase the tropical global average diabatic heat storage tendency in the layer above the main pycnocline is 1-3 Wm(-2) above normal. Its principal source is a reduction in the poleward Ekman heat flux out of the tropical ocean of 2-5 Wm(-2). Subsequently, peak tropical global warming during El Nino is dissipated by an increase in the flux of latent heat to the troposphere of 2-5 W m(-2), with reduced shortwave and longwave radiative fluxes in response to increased cloud cover tending to cancel each other. In the extratropical global ocean the reduction in poleward Ekman heat flux out of the tropics during the onset of El Nino tends to be balanced by reduction in the flux of latent heat to the troposphere. Thus global warming and cooling during Earth's internal mode of interannual climate variability arise from fluctuations in the global hydrological balance, not the global radiation balance. Since it occurs in the absence of extraterrestrial and anthropogenic forcing, global warming on decadal, interdecadal, and centennial period scales may also occur in association with Earth's internal modes of climate variability on those scales.

White, WB, Dettinger MD, Cayan DR.  2003.  Sources of global warming of the upper ocean on decadal period scales. Journal of Geophysical Research-Oceans. 108   10.1029/2002jc001396   AbstractWebsite

[1] Recent studies find global climate variability in the upper ocean and lower atmosphere during the twentieth century dominated by quasi-biennial, interannual, quasi-decadal and interdecadal signals. The quasi-decadal signal in upper ocean temperature undergoes global warming/cooling of - 0.1degreesC, similar to that occurring with the interannual signal (i. e., El Nino-Southern Oscillation), both signals dominated by global warming/cooling in the tropics. From the National Centers for Environmental Prediction troposphere reanalysis and Scripps Institution of Oceanography upper ocean temperature reanalysis we examine the quasi-decadal global tropical diabatic heat storage (DHS) budget from 1975 to 2000. We find the anomalous DHS warming tendency of 0.3-0.9 W m(-2) driven principally by a downward global tropical latent-plus-sensible heat flux anomaly into the ocean, overwhelming the tendency by weaker upward shortwave-minus-longwave heat flux anomaly to drive an anomalous DHS cooling tendency. During the peak quasidecadal warming the estimated dissipation of DHS anomaly of 0.2-0.5 W m(-2) into the deep ocean and a similar loss to the overlying atmosphere through air-sea heat flux anomaly are balanced by a decrease in the net poleward Ekman heat advection out of the tropics of 0.4-0.7 W m(-2). This scenario is nearly the opposite of that accounting for global tropical warming during the El Nino. These diagnostics confirm that even though the global quasi-decadal signal is phase-locked to the 11-year signal in the Sun's surface radiative forcing of -0.1 W m(-2), the anomalous global tropical DHS tendency cannot be driven by it directly.

Hidalgo, HG, Cayan DR, Dettinger MD.  2005.  Sources of variability of evapotranspiration in California. Journal of Hydrometeorology. 6:3-19.   10.1175/jhm-398.1   AbstractWebsite

The variability (1990-2002) of potential evapotranspiration estimates (ETo) and related meteorological variables from a set of stations from the California Irrigation Management System (CIMIS) is studied. Data from the National Climatic Data Center (NCDC) and from the Department of Energy from 1950 to 2001 were used to validate the results. The objective is to determine the characteristics of climatological ETo and to identify factors controlling its variability (including associated atmospheric circulations). Daily ETo anomalies are strongly correlated with net radiation (R-n) anomalies, relative humidity (RH), and cloud cover, and less with average daily temperature (T-avg). The highest intraseasonal variability of ETo daily anomalies occurs during the spring, mainly caused by anomalies below the high ETo seasonal values during cloudy days. A characteristic circulation pattern is associated with anomalies of ETo and its driving meteorological inputs, R-n, RH, and T-avg, at daily to seasonal time scales. This circulation pattern is dominated by 700-hPa geopotential height (Z(700)) anomalies over a region off the west coast of North America, approximately between 32degrees and 44degrees latitude, referred to as the California Pressure Anomaly (CPA). High cloudiness and lower than normal ETo are associated with the low-height (pressure) phase of the CPA pattern. Higher than normal ETo anomalies are associated with clear skies maintained through anomalously high Z(700) anomalies offshore of the North American coast. Spring CPA, cloudiness, maximum temperature (T-max), pan evaporation (E-pan), and ETo conditions have not trended significantly or consistently during the second half of the twentieth century in California. Because it is not known how cloud cover and humidity will respond to climate change, the response of ETo in California to increased greenhouse-gas concentrations is essentially unknown; however to retain the levels of ETo in the current climate, a decline of R-n by about 6% would be required to compensate for a warming of +3degreesC.

Yin, ZY, Estberg J, Hallisey EJ, Cayan DR.  2007.  Spatial patterns of lightning at different spatial scales in the western United States during August of 1990: A case study using Geographic Information Systems technology. Journal of Environmental Informatics. 9:4-17.   10.3808/jei.200700083   AbstractWebsite

Geographic information systems (GIS) have been widely used to study spatial variability in different atmospheric processes. In this study, we used a GIS approach to explore the potential to examine variation patterns of lightning strikes at different scales so that micro-, synoptic-, and planetary-scale processes can be linked in explaining and modeling the distribution patterns of lightning strikes. The data collected by the ground-based lightning detection system for an entire month in the western United States were used as an example. Lightning strike density surfaces were generated using different kernel bandwidths, or search radii. It has been recognized that density surfaces are useful in visual interpretation of spatial patterns at different scales, but there are insufficient data on how well such surfaces can be used in quantitative analysis of point distribution patterns. In our study, the resulting surfaces were compared quantitatively with gridded lightning strikes using meshes, or fishnets, of different cell sizes. The fishnet cell sizes ranged from 1 km for micro-scale processes to 50 km for synoptic- and planetary-scale processes. The results suggest that there is a threshold in the search radius or kernel bandwidth, above which a significant amount of errors would be introduced in quantitative analysis. It could be argued that it is possible to achieve a balance between the need for visual interpretation of distribution patterns and the need for quantitative analysis at different scales. We used the lightning data of August 1990 and digital elevation models of 1 km resolution to perform a case study on the relationship between lightning occurrence and topography. Our results indicate that at different spatial scales, the relationships between lightning density and topography may reflect different processes that influenced the spatial distribution pattern of the lightning occurrence.

Pagano, T, Pasteris P, Dettinger MD, Cayan DR, Redmond K.  2004.  Spring 2004: The heat is on Western US water supplies. EOS Trans. AGU. 85:385. Abstract
Cayan, DR, Peterson DH.  1993.  Spring climate and salinity in the San Francisco Bay Estuary. Water Resources Research. 29:293-303.   10.1029/92wr02152   AbstractWebsite

Salinity in the San Francisco Bay Estuary almost always experiences its yearly maximum during late summer, but climate variability produces marked interannual variations. The atmospheric circulation pattern impacts the estuary primarily through variations of runoff from rainfall and snowmelt from the Sierra Nevada and, secondarily, through variations in the near-surface salinity in the coastal ocean. While winter precipitation is the primary influence upon salinity in the estuary, spring climate variations also contribute importantly to salinity fluctuations. Spring atmospheric circulation influences both the magnitude and the timing of freshwater flows, through anomalies of precipitation and temperature. To help discriminate between the effects of these two influences, the record is divided into subsets according to whether spring conditions in the region are cool and wet, warm and wet, cool and dry, or warm and dry. Warm springs promote early snowmelt-driven flows, and cool springs result in delayed flows. In addition to effects of winter and spring climate variability operating on the watershed, there are more subtle effects that are transmitted into the estuary from the coastal ocean. These influences are most pronounced in cool and dry springs with high surface salinity (SS) in the coastal ocean versus cool and wet springs with low SS in the coastal ocean. A transect of SS records at stations from the mouth to the head of the bay suggests that the coastal ocean anomaly signal is attenuated from the mouth to the interior of the estuary. In contrast, a delayed, postsummer signal caused by winter and spring runoff variations from the upstream watershed are most pronounced at the head of the estuary and attenuate toward the mouth.

Lundquist, JD, Cayan DR, Dettinger MD.  2004.  Spring onset in the Sierra Nevada: When is snowmelt independent of elevation? Journal of Hydrometeorology. 5:327-342.   10.1175/1525-7541(2004)005<0327:soitsn>;2   AbstractWebsite

Short-term climate and weather systems can have a strong influence on mountain snowmelt, sometimes overwhelming the effects of elevation and aspect. Although most years exhibit a spring onset that starts first at lowest and moves to highest elevations, in spring 2002, flow in a variety of streams within the Tuolumne and Merced River basins of the southern Sierra Nevada all rose synchronously on 29 March. Flow in streams draining small high-altitude glacial subcatchments rose at the same time as that draining much larger basins gauged at lower altitudes, and streams from north- and south-facing cirques rose and fell together. Historical analysis demonstrates that 2002 was one among only 8 yr with such synchronous flow onsets during the past 87 yr, recognized by having simultaneous onsets of snowmelt at over 70% of snow pillow sites, having discharge in over 70% of monitored streams increase simultaneously, and having temperatures increase over 12degreesC within a 5-day period. Synchronous springs tend to begin with a low pressure trough over California during late winter, followed by the onset of a strong ridge and unusually warm temperatures. Synchronous springs are characterized by warmer than average winters and cooler than average March temperatures in California. In the most elevation-dependent, nonsynchronous years, periods of little or no storm activity, with warmer than average March temperatures, precede the onset of spring snowmelt, allowing elevation and aspect to influence snowmelt as spring arrives gradually.

Mahmud, A, Tyree M, Cayan D, Motallebi N, Kleeman MJ.  2008.  Statistical downscaling of climate change impacts on ozone concentrations in California. Journal of Geophysical Research-Atmospheres. 113   10.1029/2007jd009534   AbstractWebsite

The statistical relationship between the daily 1-hour maximum ozone (O(3)) concentrations and the daily maximum upper air temperature was explored for California's two most heavily polluted air basins: the South Coast Air Basin (SoCAB) and the San Joaquin Valley Air Basin (SJVAB). A coarse-scale analysis of the temperature at an elevation of 850-mbar pressure (T850) for the period 1980-2004 was obtained from the National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis data set for grid points near Upland (SoCAB) and Parlier (SJVAB). Daily 1-hour maximum ozone concentrations were obtained from the California Air Resources Board (CARB) for these locations over the same time period. The ozone concentrations measured at any given value of the Reanalysis T850 were approximately normally distributed. The 25%, 50%, and 75% quartile ozone concentrations increased linearly with T850, reflecting the effect of temperature on emissions and chemical reaction rates. A 2D Lagrangian (trajectory) form of the UCD/CIT photochemical air quality model was used in a perturbation study to explain the variability of the ozone concentrations at each value of T850. Wind speed, wind direction, temperature, relative humidity, mixing height, initial concentrations for VOC concentrations, background ozone concentrations, time of year, and overall emissions were perturbed in a realistic fashion during this study. A total of 62 model simulations were performed, and the results were analyzed to show that long-term changes to emissions inventories were the largest sources of ozone variability at a fixed value of T850. Projections of future T850 values in California were obtained from the Geophysical Fluid Dynamics Laboratory (GFDL) model under the Intergovernmental Panel on Climate Change (IPCC) A2 and B1 emissions scenarios for the years 2001 to 2100. The future temperature trends combined with the historical statistical relationships suggest that an additional 22-30 days year(-1) in California would experience O(3) >= 90 ppb under the A2 global emissions scenario, and an additional 6-13 days year(-1) would experience O(3) >= 90 ppb under the B1 global emissions scenario by the year 2050 (assuming the NO(x) and VOC emissions remained at 1990-2004 levels). These calculations help to quantify the climate "penalty" that must be overcome to improve air quality in California.

Pierce, DW, Cayan DR, Thrasher BL.  2014.  Statistical downscaling using Localized Constructed Analogs (LOCA). Journal of Hydrometeorology. 15:2558-2585.   10.1175/jhm-d-14-0082.1   AbstractWebsite

A new technique for statistically downscaling climate model simulations of daily temperature and precipitation is introduced and demonstrated over the western United States. The localized constructed analogs (LOCA) method produces downscaled estimates suitable for hydrological simulations using a multiscale spatial matching scheme to pick appropriate analog days from observations. First, a pool of candidate observed analog days is chosen by matching the model field to be downscaled to observed days over the region that is positively correlated with the point being downscaled, which leads to a natural independence of the downscaling results to the extent of the domain being downscaled. Then, the one candidate analog day that best matches in the local area around the grid cell being downscaled is the single analog day used there. Most grid cells are downscaled using only the single locally selected analog day, but locations whose neighboring cells identify a different analog day use a weighted combination of the center and adjacent analog days to reduce edge discontinuities. By contrast, existing constructed analog methods typically use a weighted average of the same 30 analog days for the entire domain. By greatly reducing this averaging, LOCA produces better estimates of extreme days, constructs a more realistic depiction of the spatial coherence of the downscaled field, and reduces the problem of producing too many light-precipitation days. The LOCA method is more computationally expensive than existing constructed analog techniques, but it is still practical for downscaling numerous climate model simulations with limited computational resources.

Bromirski, PD, Flick RE, Cayan DR.  2003.  Storminess variability along the California coast: 1858-2000. Journal of Climate. 16:982-993.   10.1175/1520-0442(2003)016<0982:svatcc>;2   AbstractWebsite

The longest available hourly tide gauge record along the West Coast (U. S.) at San Francisco yields meteorologically forced nontide residuals (NTR), providing an estimate of the variation in "storminess'' from 1858 to 2000. Mean monthly positive NTR (associated with low sea level pressure) show no substantial change along the central California coast since 1858 or over the last 50 years. However, in contrast, the highest 2% of extreme winter NTR levels exhibit a significant increasing trend since about 1950. Extreme winter NTR also show pronounced quasi-periodic decadal-scale variability that is relatively consistent over the last 140 years. Atmospheric sea level pressure anomalies (associated with years having high winter NTR) take the form of a distinct, large-scale atmospheric circulation pattern, with intense storminess associated with a broad, southeasterly displaced, deep Aleutian low that directs storm tracks toward the California coast.

Das, T, Hidalgo HG, Dettinger MD, Cayan DR, Pierce DW, Bonfils C, Barnett TP, Bala G, Mirin A.  2009.  Structure and detectability of trends in hydrological measures over the western United States. Journal of Hydrometeorology. 10:871-892.   10.1175/2009jhm1095.1   AbstractWebsite

This study examines the geographic structure of observed trends in key hydrologically relevant variables across the western United States at (1)/(8)degrees spatial resolution during the period 1950-99. Geographical regions, latitude bands, and elevation classes where these trends are statistically significantly different from trends associated with natural climate variations are identified. Variables analyzed include late-winter and spring temperature, winter-total snowy days as a fraction of winter-total wet days, 1 April snow water equivalent (SWE) as a fraction of October-March (ONDJFM) precipitation total [precip(ONDJFM)], and seasonal [JFM] accumulated runoff as a fraction of water-year accumulated runoff. Observed changes were compared to natural internal climate variability simulated by an 850-yr control run of the finite volume version of the Community Climate System Model, version 3 (CCSM3-FV), statistically downscaled to a (1)/(8)degrees grid using the method of constructed analogs. Both observed and downscaled model temperature and precipitation data were then used to drive the Variable Infiltration Capacity (VIC) hydrological model to obtain the hydrological variables analyzed in this study. Large trends (magnitudes found less than 5% of the time in the long control run) are common in the observations and occupy a substantial part (37%-42%) of the mountainous western United States. These trends are strongly related to the large-scale warming that appears over 89% of the domain. The strongest changes in the hydrologic variables, unlikely to be associated with natural variability alone, have occurred at medium elevations [750-2500 m for JFM runoff fractions and 500-3000 m for SWE/Precip(ONDJFM)] where warming has pushed temperatures from slightly below to slightly above freezing. Further analysis using the data on selected catchments indicates that hydroclimatic variables must have changed significantly (at 95% confidence level) over at least 45% of the total catchment area to achieve a detectable trend in measures accumulated to the catchment scale.

Giese, BS, Cayan DR.  1993.  Surface heat flux parameterizations and tropical Pacific sea surface temperature simulations. Journal of Geophysical Research-Oceans. 98:6979-6989.   10.1029/93jc00323   AbstractWebsite

Models of the tropical Pacific Ocean routinely use bulk formulae to estimate the surface fluxes of heat into and out of the ocean. The formulae generally require atmospheric variables such as cloud cover, surface wind speed, air-sea temperature difference, and relative humidity. Since these quantities are not provided by most tropical Pacific Ocean models, modelers have resorted to simplifying parameterizations in terms of a few primary variables. Several of these parameterizations are tested by using observed data and comparing fluxes calculated using the simplified parameterizations with those from the full bulk formula. The impact of the various parameterizations is also assessed in a tropical Pacific Ocean general circulation model by comparing model sea surface temperature (SST) with the monthly means and anomalies of observed tropical Pacific SST during a 6-year period that included both warm and cold episodes. The different flux parameterizations yield a broad mix of results. Overall, using a constant relative humidity of 0.80 does well, while using a constant downward radiation and a constant air temperature minus SST produces large discrepancies in comparison with the full flux parameterizations. In agreement with previous studies the model runs demonstrate that SST variability on the equator is dominated by wave processes whereas off the equator the surface flux of heat plays a crucial role in determining changes in SST.