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The partitioning of precipitation into frozen and liquid components influences snow-derived water resources and flood hazards in mountain environments. We used a 915-MHz Doppler radar wind profiler upstream of the northern Sierra Nevada to estimate the hourly elevation where snow melts to rain, or the snow level, during winter (December–February) precipitation events spanning water years (WY) 2008–2017. During this ten-year period, a Mann-Kendall test indicated a significant (p < 0.001) positive trend in snow level with a Thiel-Sen slope of 72 m year−1. We estimated total precipitation falling as snow (snow fraction) between WY1951 and 2017 using nine daily mid-elevation (1200–2000 m) climate stations and two hourly stations spanning WY2008–2017. The climate-station-based snow fraction estimates agreed well with snow-level radar values (R2 = 0.95, p < 0.01), indicating that snow fractions represent a reasonable method to estimate changes in frozen precipitation. Snow fraction significantly (p < 0.001) declined during WY2008–2017 at a rate of 0.035 (3.5%) year−1. Single-point correlations between detrended snow fraction and sea-surface temperatures (SST) suggested that positive SST anomalies along the California coast favor liquid phase precipitation during winter. Reanalysis-derived integrated moisture transported upstream of the northern Sierra Nevada was negatively correlated with snow fraction (R2 = 0.90, p < 0.01), with atmospheric rivers representing the likely circulation mechanism producing low-snow-fraction storms.

Assessing regional hydrologic responses to past climate changes can offer a guide for how water resources might respond to ongoing and future climate change. Here we employed a coupled water balance and lake evaporation model to examine Walker Lake behaviors during the Medieval Climate Anomaly (MCA), a time of documented hydroclimatic extremes. Together, a 14C-based shoreline elevation chronology, submerged subfossil tree stumps in the West Walker River, and regional paleoproxy evidence indicate a ~50 year pluvial episode that bridged two 140+ year droughts. We developed estimates of MCA climates to examine the transient lake behavior and evaluate watershed responses to climate change. Our findings suggest the importance of decadal climate persistence to elicit large lake-level fluctuations. We also simulated the current 2012–2015 California-Nevada drought and found that the current drought exceeds MCA droughts in mean severity but not duration.

Strong afternoon downslope Sundowner winds in southern California's Santa Ynez Mountains favor wildfire growth. To determine whether Sundowners are different from Santa Ana winds (SAW), we use surface observations from 1979–2014 to develop a climatology of extreme Sundowner days. The climatology was compared against an existing SAW index from 1979–2012. Sundowner occurrence peaks in late spring whereas SAWs peak during winter. SAWs demonstrate amplified 500 hPa geopotential heights over western North America and strong anomalously positive inland mean sea level pressures. In contrast, Sundowner-only conditions occur during zonal 500 hPa flow and moderate negative inland sea level pressure anomalies.

Incense sticks and mustard oil are the two most popular combustion fuels during rituals and social ceremonies in Asian countries. Given their widespread use in both closed and open burning activities, it is important to quantify the spectral radiative properties of aerosols emitted from the combustion of both fuels. This information is needed by climate models to assess the impact of these aerosols on radiative forcing. In this study, we used a 3-wavelength integrated photoacoustic-nephelometer – operating simultaneously at 405, 532 and 781 nm – to measure the optical coefficients of aerosols emitted from the laboratory combustion of mustard oil lamp and two types of incense sticks. From the measured optical coefficients at three wavelengths, time-varying single scattering albedo (SSA), absorption Angstrom exponent (AAE), and scattering Angstrom exponent (SAE) were calculated. For incense smoke particles, the time-averaged mean AAE values were found to be as high as 8.32 (between 405 and 532 nm) and 6.48 (between 532 and 781 nm). This spectrally-varying characteristic of AAE indicates that brown carbon – a class of organic carbon which strongly absorbs solar radiation in the blue and near ultraviolet – is the primary component of incense smoke aerosols. For aerosols emitted from the burning of mustard oil lamp, the time-averaged mean AAE values were ∼1.3 (between 405 and 781 nm) indicating that black carbon (BC) is the primary constituent. Scanning electron microscopy combined with image processing revealed the morphology of incense smoke aerosols to be non-coalescing and weakly-bound aggregates with a mean two-dimensional (2-d) fractal dimension (Df)=1.9±0.07, while the mustard oil smoke aerosols had typical fractal-like BC aggregate morphology with a mean 2-d Df=1.85±0.09.

An urban heat island (UHI) effect is identified in Reno, Nevada by analyzing regional temperature trends calculated from seven long- term observation stations for the summer and winter seasons between 1950 and 2014. The UHI is maximized during summer (JuneeAugust) and characterized by asymmetric in- creases in minimum (~1.1 C/decade, p < 0.01) versus maximum temperature (~0.1 C/decade, p < 0.01) trends in excess of trends from regional climate stations. Comparisons of historical Reno temperatures with an ensemble of 66 bias- corrected and downscaled global climate model (GCM) outputs spanning 1950e2014 demonstrates cold biases of 1.5e4.5 C during summer with minimum temperature having the largest bias. We show that a secondary bias correction step utilizing the statistical downscaling method of quantileequantile mapping (QQM) can reduce biases in future climate projections assuming no changes to the UHI. The QQM results in an additional total warming of ensemble mean temperatures by ~3 C for downscaled GCM output and ~4 C for re-gridded 1 grid resolution GCM output for 2030 e2049 under the RCP8.5 emissions scenario. These temperature differences produce additional increases in summer potential evapotranspiration of 10% compared to non-QQM bias-corrected GCM output. It was shown that the QQM method represents a useful and computationally efficient method for bias correction of temperature projections for cities where UHI effects exist. Planning and impacts studies of urban water resources can benefit from these improved climate projections, particularly in regions where downscaled GCM output is unavailable.

This paper implements an individual station-based downscaled approach based on a quantil–quantile bias correction mapping to further downscale regional gridded simulated output into individual station locations. We describe and propose a framework to optimize the usefulness of this downscaling approach over small watersheds in mountain regions, where downscale gridded data (∼10km) is still too coarse for use in sub-regional hydrologic simulation studies. Two regional downscaled data sets were downscaled to individual station locations: the Desert Research Institute-Regional Climate Model based on the Weather Research and Forecasting model (DRI-RCM; 12 km resolution); and the US Bureau of Reclamation Bias-Corrected Spatial Disaggregation (BCSD; 12 km resolution). We applied the station-based downscaled data in the integrated hydrologic model, GSFLOW, as adapted to the Incline Creek watersheds in the northeastern Lake Tahoe basin. Hydrologic model results showed that the proposed downscaled approach provides reasonable downscaled climate data that can be used to evaluate sub-regional scale hydrologic processes.

On 27 September 2010 the Los Angeles Civic Center reached its all-time record maximum temperature of 45 °C before 1330 LDT with several other regional stations observing all-time record breaking heat early in that afternoon. This record event is associated with a general circulation pattern predisposed to hemispheric wave breaking. Three days before the event, wave breaking organizes complex terrain and coastal-induced processes that lead to isentropic surface folding into the Los Angeles Basin. The first wave break occurs over the western two-thirds of North America leading to trough elongation across the southwestern U.S. Collocated with this trough is an isentropic potential vorticity filament that is the locus of a thermally indirect circulation central to warming and associated thickness increases and ridging westward across the Great Basin. In response to this circulation, two subsynoptic wave breaks are triggered along the Pacific Coast and the IPV filament is coupled to the breaking waves and the interaction produces a subsynoptic low pressure center and a deep vortex aloft over the southeastern California desert. This coupling leads to advection of an elevated mixed layer over Point Conception the night before the record-breaking heat that creates a coastally trapped low pressure area southwest of Los Angeles. The two low pressure centers create a low-level pressure gradient and east-southeasterly jet directed offshore over the Los Angeles Basin by sunrise on the 27th. This allows the advection of low-level warm air from the inland terrain towards the coastally-trapped disturbance and descending circulation resulting in record heating.

Diablo winds are dry and gusty north-northeasterly downslope windstorms that affect the San Francisco Bay Area in Northern California. On the evening of 8 October 2017, Diablo winds contributed to the ignitions and rapid spread of the “Northern California firestorm”, including the Tubbs Fire, which burned 2800 homes in Santa Rosa, resulting in 22 fatalities and $1.2 B USD in damages. We analyzed 18 years of data from a network of surface meteorological stations and showed that Diablo winds tend to occur overnight through early morning in fall, winter and spring. In addition to the area north of the San Francisco Bay Area, conditions similar to Diablo winds commonly occur in the western Sierra Nevada. Both of these areas are characterized by high wind speeds and low relative humidity, but they neither tend to be warmer than climatology nor have a higher gust factor, or ratio of wind gusts to mean wind speeds, than climatology.

An overstocked montane, mixedconifer forest on the west shore of the Lake Tahoe Basin was thinned using a Fecon masticator, leaving woodchips and tree shreddings on-site as mulch. No significant differences in soil compaction were found in 13 of 15 comparisons of soil-profile resistance values at several distances from the machine track and varying depths. We then watered the mastication sites with a rainfall simulator, and measured runoff, infiltration and erosion. The treatments included woodchip-covered and bare-soil plots corresponding to mulched track, as well as native grass, bare soils and relatively undisturbed soil plots. Sediment yields were greatest from the bare soils, followed by the undisturbed, grass and woodchip plots. Mastication appears to be an effective thinning treatment for overstocked forests with few discernible negative impacts on soil compaction or lake-polluting runoff.

The overall objective of this study was to improve the representation of regional ocean circulation in the North Pacific by using high resolution atmospheric forcing that accurately represents mesoscale processes in ocean-atmosphere regional (North Pacific) model configuration. The goal was to assess the importance of accurate representation of mesoscale processes in the atmosphere and the ocean on large scale circulation. This is an important question, as mesoscale processes in the atmosphere which are resolved by the high resolution mesoscale atmospheric models such as Weather Research and Forecasting (WRF), are absent in commonly used atmospheric forcing such as CORE forcing, employed in e.g. the Community Climate System Model (CCSM).