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DeFlorio, MJ, Waliser DE, Guan B, Lavers DA, Ralph FM, Vitart F.  2018.  Global assessment of atmospheric river prediction skill. Journal of Hydrometeorology. 19:409-426.   10.1175/jhm-d-17-0135.1   AbstractWebsite

Atmospheric rivers (ARs) are global phenomena that transport water vapor horizontally and are associated with hydrological extremes. In this study, the Atmospheric River Skill (ATRISK) algorithm is introduced, which quantifies AR prediction skill in an object-based framework using Subseasonal to Seasonal (S2S) Project global hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The dependence of AR forecast skill is globally characterized by season, lead time, and distance between observed and forecasted ARs. Mean values of daily AR prediction skill saturate around 7-10 days, and seasonal variations are highest over the Northern Hemispheric ocean basins, where AR prediction skill increases by 15%-20% at a 7-day lead during boreal winter relative to boreal summer. AR hit and false alarm rates are explicitly considered using relative operating characteristic (ROC) curves. This analysis reveals that AR forecast utility increases at 10-day lead over the North Pacific/western U.S. region during positive El Nino-Southern Oscillation (ENSO) conditions and at 7-and 10-day leads over the North Atlantic/U.K. region during negative Arctic Oscillation (AO) conditions and decreases at a 10-day lead over the North Pacific/western U.S. region during negative Pacific-North America (PNA) teleconnection conditions. Exceptionally large increases in AR forecast utility are found over the North Pacific/western United States at a 10-day lead during El Nino + positive PNA conditions and over the North Atlantic/United Kingdom at a 7-day lead during La Nina + negative PNA conditions. These results represent the first global assessment of AR prediction skill and highlight climate variability conditions that modulate regional AR forecast skill.

Enzminger, TL, Small EE, Borsa AA.  2018.  Accuracy of snow water equivalent estimated from GPS vertical displacements: A synthetic loading case study for western US mountains. Water Resources Research. 54:581-599.   10.1002/2017wr021521   AbstractWebsite

GPS monitoring of solid Earth deformation due to surface loading is an independent approach for estimating seasonal changes in terrestrial water storage (TWS). In western United States (WUSA) mountain ranges, snow water equivalent (SWE) is the dominant component of TWS and an essential water resource. While several studies have estimated SWE from GPS-measured vertical displacements, the error associated with this method remains poorly constrained. We examine the accuracy of SWE estimated from synthetic displacements at 1,395 continuous GPS station locations in the WUSA. Displacement at each station is calculated from the predicted elastic response to variations in SWE from SNODAS and soil moisture from the NLDAS-2 Noah model. We invert synthetic displacements for TWS, showing that both seasonal accumulation and melt as well as year-to-year fluctuations in peak SWE can be estimated from data recorded by the existing GPS network. Because we impose a smoothness constraint in the inversion, recovered TWS exhibits mass leakage from mountain ranges to surrounding areas. This leakage bias is removed via linear rescaling in which the magnitude of the gain factor depends on station distribution and TWS anomaly patterns. The synthetic GPS-derived estimates reproduce approximately half of the spatial variability (unbiased root mean square error similar to 50%) of TWS loading within mountain ranges, a considerable improvement over GRACE. The inclusion of additional simulated GPS stations improves representation of spatial variations. GPS data can be used to estimate mountain-range-scale SWE, but effects of soil moisture and other TWS components must first be subtracted from the GPS-derived load estimates.

Sherbakov, T, Malig B, Guirguis K, Gershunov A, Basu R.  2018.  Ambient temperature and added heat wave effects on hospitalizations in California from 1999 to 2009. Environmental Research. 160:83-90.   10.1016/j.envres.2017.08.052   AbstractWebsite

Investigators have examined how heat waves or incremental changes in temperature affect health outcomes, but few have examined both simultaneously. We utilized distributed lag nonlinear models (DLNM) to explore temperature associations and evaluate possible added heat wave effects on hospitalizations in 16 climate zones throughout California from May through October 1999-2009. We define heat waves as a period when daily mean temperatures were above the zone- and month-specific 95th percentile for at least two consecutive days. DLNMs were used to estimate climate zone-specific non-linear temperature and heat wave effects, which were then combined using random effects meta-analysis to produce an overall estimate for each. With higher temperatures, admissions for acute renal failure, appendicitis, dehydration, ischemic stroke, mental health, noninfectious enteritis, and primary diabetes were significantly increased, with added effects from heat waves observed for acute renal failure and dehydration. Higher temperatures also predicted statistically significant decreases in hypertension admissions, respiratory admissions, and respiratory diseases with secondary diagnoses of diabetes, though heat waves independently predicted an added increase in risk for both respiratory types. Our findings provide evidence that both heat wave and temperature exposures can exert effects independently.

Lamjiri, MA, Dettinger MD, Ralph FM, Guan B.  2017.  Hourly storm characteristics along the US West Coast: Role of atmospheric rivers in extreme precipitation. Geophysical Research Letters. 44:7020-7028.   10.1002/2017gl074193   AbstractWebsite

Gridded hourly precipitation observations over the conterminous U.S., from 1948 to 2002, are analyzed to determine climatological characteristics of storm precipitation totals. Despite generally lower hourly intensities, precipitation totals along the U.S. West Coast (USWC) are comparable to those in southeast U.S. (SEUS). Storm durations, more so than hourly intensities, strongly modulate precipitation- total variability over the USWC, where the correlation coefficients between storm durations and storm totals range from 0.7 to 0.9. Atmospheric rivers (ARs) contribute 30-50% of annual precipitation on the USWC and make such large contributions to extreme storms that 60-100% of the most extreme storms, i.e., storms with precipitation- total return intervals longer than 2 years, are associated with ARs. These extreme storm totals are more strongly tied to storm durations than to storm hourly or average intensities, emphasizing the importance of AR persistence to extreme storms on the USWC.

Vanos, JK, Herdt AJ, Lochbaum MR.  2017.  Effects of physical activity and shade on the heat balance and thermal perceptions of children in a playground microclimate. Building and Environment. 126:119-131.   10.1016/j.buildenv.2017.09.026   AbstractWebsite

Outdoor thermal comfort (TC) is an important parameter in assessing the value and health utility of a recreational space. Given the public health significance of child heat illness, the ability to model children's heat balance and TC during activity has received little attention. The current pilot study tests the performance of an outdoor human heat balance model on children playing in warm/hot outdoor environments in sun and shade. Fourteen children aged 9-13 participated in the 8-day study in Texas in spring 2016, performing physical activity while wearing heartrate monitors and completing thermal perception surveys (e.g., actual thermal sensation (ATS)). Surveys were compared to predicted thermal sensation (PTS) based on principles of human-environment heat exchange using personal data and a suite of on-site microclimate information. Results demonstrate the model to significantly predict ATS votes (Spearman's rho = 0.504). Subjective preferred change was also significantly correlated to modeled PTS (rho = -0.607). Radiation, air temperature, windspeed, and level of tiredness were significant predictors of ATS. Finally, the mean human energy balance was significantly lower in the shade (-168 W m(-2)), thus lowering heat stress potential, with the model predicting ATS with little-to-no error (0.2 and 0.0 scale error units in sun and shade, respectively). This study demonstrates an ability to estimate a child's heat balance while accounting for changes in major heat contributors (e.g., radiation, metabolism), and is the first study to evaluate TC of children during activity in outdoor built environments. New insights of heat perception may aid in recognition of often under-recognized heat stress. (C) 2017 Elsevier Ltd. All rights reserved.

Sellars, SL, Kawzenuk B, Nguyen P, Ralph FM, Sorooshian S.  2017.  Genesis, pathways, and terminations of intense global water vapor transport in association with large-scale climate patterns. Geophysical Research Letters. 44:12465-12475.   10.1002/2017gl075495   AbstractWebsite

The CONNected objECT (CONNECT) algorithm is applied to global Integrated Water Vapor Transport data from the NASA's Modern-Era Retrospective Analysis for Research and Applications - Version 2 reanalysis product for the period of 1980 to 2016. The algorithm generates life-cycle records in time and space evolving strong vapor transport events. We show five regions, located in the midlatitudes, where events typically exist (off the coast of the southeast United States, eastern China, eastern South America, off the southern tip of South Africa, and in the southeastern Pacific Ocean). Global statistics show distinct genesis and termination regions and global seasonal peak frequency during Northern Hemisphere late fall/winter and Southern Hemisphere winter. In addition, the event frequency and geographical location are shown to be modulated by the Arctic Oscillation, Pacific North American Pattern, and the quasi-biennial oscillation. Moreover, a positive linear trend in the annual number of objects is reported, increasing by 3.58 objects year-over-year. Plain Language Summary A computational science approach to tracking global atmospheric water vapor plumes is applied to a NASA data set from 1980 to 2016. Results show regions of the globe where intense water vapor transport often exists, including their genesis and termination locations. Winter time months tend to have more water vapor plumes in both the Southern and Northern Hemispheres. In addition, climate phenomena also have an impact on the frequency and location of these water vapor plumes.

Wang, Y, Zhang GJ, He YJ.  2017.  Simulation of precipitation extremes using a stochastic convective parameterization in the NCAR CAM5 under different resolutions. Journal of Geophysical Research-Atmospheres. 122:12875-12891.   10.1002/2017jd026901   AbstractWebsite

With the incorporation of the Plant-Craig stochastic deep convection scheme into the Zhang-McFarlane deterministic parameterization in the Community Atmospheric Model version 5 (CAM5), its impact on extreme precipitation at different resolutions (2 degrees, 1 degrees, and 0.5 degrees) is investigated. CAM5 with the stochastic deep convection scheme (experiment (EXP)) simulates the precipitation extreme indices better than the standard version (control). At 2 degrees and 1 degrees resolutions, EXP increases high percentile (>99th) daily precipitation over the United States, Europe, and China, resulting in a better agreement with observations. However, at 0.5 degrees resolution, due to enhanced grid-scale precipitation with increasing resolution, EXP overestimates extreme precipitation over southeastern U.S. and eastern Europe. The reduced biases in EXP at each resolution benefit from a broader probability distribution function of convective precipitation intensity simulated. Among EXP simulations at different resolutions, if the spatial averaging area over which input quantities used in convective closure are spatially averaged in the stochastic convection scheme is comparable, the modeled convective precipitation intensity decreases with increasing resolution, when gridded to the same resolution, while the total precipitation is not sensitive to model resolution, exhibiting some degree of scale-awareness. Sensitivity tests show that for the same resolution, increasing the size of spatial averaging area decreases convective precipitation but increases the grid-scale precipitation.

Merrifield, A, Lehner F, Xie SP, Deser C.  2017.  Removing Circulation Effects to Assess Central US Land-Atmosphere Interactions in the CESM Large Ensemble. Geophysical Research Letters. 44:9938-9946.   10.1002/2017gl074831   AbstractWebsite

Interannual variability of summer surface air temperature (SAT) in the central United States (U.S.) is influenced by atmospheric circulation and land surface feedbacks. Here a method of dynamical adjustment is used to remove the effects of circulation on summer SAT variability over North America in the Community Earth System Model Large Ensemble. The residual SAT variability is shown to reflect thermodynamic feedbacks associated with land surface conditions. In particular, the central U.S. is a hot spot of land-atmosphere interaction, with residual SAT accounting for more than half of the total SAT variability. Within the hot spot, residual SAT anomalies show higher month-to-month persistence through the warm season and a redder spectrum than dynamically induced SAT anomalies. Residual SAT variability in this region is also shown to be related to preseason soil moisture conditions, surface flux variability, and local atmospheric pressure anomalies.

Ralph, FM, Iacobellis SF, Neiman PJ, Cordeira JM, Spackman JR, Waliser DE, Wick GA, White AB, Fairall C.  2017.  Dropsonde observations of total integrated water vapor transport within North Pacific atmospheric rivers. Journal of Hydrometeorology. 18:2577-2596.   10.1175/jhm-d-17-0036.1   AbstractWebsite

Aircraft dropsonde observations provide the most comprehensive measurements to date of horizontal water vapor transport in atmospheric rivers (ARs). The CalWater experiment recently more than tripled the number of ARs probed with the required measurements. This study uses vertical profiles of water vapor, wind, and pressure obtained from 304 dropsondes across 21 ARs. On average, total water vapor transport ( TIVT) in an AR was 4.7 x 10(8) +/- 2 x 10(8) kg s(-1). This magnitude is 2.6 times larger than the average discharge of liquid water from the Amazon River. The mean AR width was 890 +/- 270 km. Subtropical ARs contained larger integrated water vapor ( IWV) but weaker winds than midlatitude ARs, although average TIVTs were nearly the same. Mean TIVTs calculated by defining the lateral "edges'' of ARs using an IVT threshold versus an IWV threshold produced results that differed by less than 10% across all cases, but did vary between the midlatitudes and subtropical regions.

Ralph, FM, Iacobellis SF, Neiman PJ, Cordeira JM, Spackman JR, Waliser DE, Wick GA, White AB, Fairall C.  2017.  Dropsonde observations of total integrated water vapor transport within North Pacific atmospheric rivers. Journal of Hydrometeorology. 18:2577-2596.   10.1175/jhm-d-17-0036.1   AbstractWebsite

Aircraft dropsonde observations provide the most comprehensive measurements to date of horizontal water vapor transport in atmospheric rivers (ARs). The CalWater experiment recently more than tripled the number of ARs probed with the required measurements. This study uses vertical profiles of water vapor, wind, and pressure obtained from 304 dropsondes across 21 ARs. On average, total water vapor transport ( TIVT) in an AR was 4.7 x 10(8) +/- 2 x 10(8) kg s(-1). This magnitude is 2.6 times larger than the average discharge of liquid water from the Amazon River. The mean AR width was 890 +/- 270 km. Subtropical ARs contained larger integrated water vapor ( IWV) but weaker winds than midlatitude ARs, although average TIVTs were nearly the same. Mean TIVTs calculated by defining the lateral "edges'' of ARs using an IVT threshold versus an IWV threshold produced results that differed by less than 10% across all cases, but did vary between the midlatitudes and subtropical regions.

Chao, Y, Farrara JD, Bjorkstedt E, Chai F, Chavez F, Rudnick DL, Enright W, Fisher JL, Peterson WT, Welch GF, Davis CO, Dugdale RC, Wilkerson FP, Zhang HC, Zhang YL, Ateljevich E.  2017.  The origins of the anomalous warming in the California coastal ocean and San Francisco Bay during 2014-2016. Journal of Geophysical Research-Oceans. 122:7537-7557.   10.1002/2017jc013120   AbstractWebsite

During 2014 exceptionally warm water temperatures developed across a wide area off the California coast and within San Francisco Bay (SFB) and persisted into 2016. Observations and numerical model output are used to document this warming and determine its origins. The coastal warming was mostly confined to the upper 100 m of the ocean and was manifested strongly in the two leading modes of upper ocean (0-100 m) temperature variability in the extratropical eastern Pacific. Observations suggest that the coastal warming in 2014 propagated into nearshore regions from the west while later indicating a warming influence that propagated from south to north into the region associated with the 2015-2016 El Nino event. An analysis of the upper ocean (0-100 m) heat budget in a Regional Ocean Modeling System (ROMS) simulation confirmed this scenario. The results from a set of sensitivity runs with the model in which the lateral boundary conditions varied supported the conclusions drawn from the heat budget analysis. Concerning the warming in the SFB, an examination of the observations and the heat budget in an unstructured-grid numerical model simulation suggested that the warming during the second half of 2014 and early 2016 originated in the adjacent California coastal ocean and propagated through the Golden Gate into the Bay. The finding that the coastal and Bay warming are due to the relatively slow propagation of signals from remote sources raises the possibility that such warming events may be predictable many months or even several seasons in advance. Plain Language Summary The origins of the exceptionally warm water temperatures that developed off the California coast and in San Francisco Bay were studied using observations and computer model experiments. The coastal warming was mostly confined to the upper ocean. The coastal warming in 2014 was found to have moved into coastal waters from further offshore in the northeastern Pacific. Warming persisted into 2015-2016 as a warming influence from the south associated with the 2015-16 El Nino event in the tropical Pacific Ocean. The model experiments suggested confirmed that propagation of the warming signals from the west and north into the California coastal ocean and suggested that the warming in San Francisco Bay was found to have originated primarily in the adjacent California coastal ocean. The finding that the coastal and Bay warming are due to the relatively slow propagation of signals from remote sources raises the possibility that such warming events may be predictable many months or even several seasons in advance.

Murphy, BN, Woody MC, Jimenez JL, Carlton AMG, Hayes PL, Liu S, Ng NL, Russell LM, Setyan A, Xu L, Young J, Zaveri RA, Zhang Q, Pye HOT.  2017.  Semivolatile POA and parameterized total combustion SOA in CMAQv5.2: impacts on source strength and partitioning. Atmospheric Chemistry and Physics. 17:11107-11133.   10.5194/acp-17-11107-2017   AbstractWebsite

Mounting evidence from field and laboratory observations coupled with atmospheric model analyses shows that primary combustion emissions of organic compounds dynamically partition between the vapor and particulate phases, especially as near-source emissions dilute and cool to ambient conditions. The most recent version of the Community Multiscale Air Quality model version 5.2 (CMAQv5.2) accounts for the semivolatile partitioning and gas-phase aging of these primary organic aerosol (POA) compounds consistent with experimentally derived parameterizations. We also include a new surrogate species, potential secondary organic aerosol from combustion emissions (pcSOA), which provides a representation of the secondary organic aerosol (SOA) from anthropogenic combustion sources that could be missing from current chemical transport model predictions. The reasons for this missing mass likely include the following: (1) unspeciated semivolatile and intermediate volatility organic compound (SVOC and IVOC, respectively) emissions missing from current inventories, (2) multigenerational aging of organic vapor products from known SOA precur-sors (e.g., toluene, alkanes), (3) underestimation of SOA yields due to vapor wall losses in smog chamber experiments, and (4) reversible organic compounds-water interactions and/or aqueous-phase processing of known organic vapor emissions. CMAQ predicts the spatially averaged contribution of pcSOA to OA surface concentrations in the continental United States to be 38.6 and 23.6% in the 2011 winter and summer, respectively. Whereas many past modeling studies focused on a particular measurement campaign, season, location, or model configuration, we endeavor to evaluate the model and important uncertain parameters with a comprehensive set of United States-based model runs using multiple horizontal scales (4 and 12 km), gas-phase chemical mechanisms, and seasons and years. The model with representation of semivolatile POA improves predictions of hourly OA observations over the traditional nonvolatile model at sites during field campaigns in southern California (CalNex, May-June 2010), northern California (CARES, June 2010), the southeast US (SOAS, June 2013; SEARCH, January and July, 2011). Model improvements manifest better correlations (e.g., the correlation coefficient at Pasadena at night increases from 0.38 to 0.62) and reductions in underprediction during the photochemically active afternoon period (e.g., bias at Pasadena from -5.62 to -2.42 mu gm(-3)). Daily averaged predictions of observations at routine-monitoring networks from simulations over the continental US (CONUS) in 2011 show modest improvement during winter, with mean biases reducing from 1.14 to 0.73 mu gm(-3), but less change in the summer when the decreases from POA evaporation were similar to the magnitude of added SOA mass. Because the model-performance improvement realized by including the relatively simple pcSOA approach is similar to that of more-complicated parameterizations of OA formation and aging, we recommend caution when applying these more-complicated approaches as they currently rely on numerous uncertain parameters. The pcSOA parameters optimized for performance at the southern and northern California sites lead to higher OA formation than is observed in the CONUS evaluation. This may be due to any of the following: variations in real pcSOA in different regions or time periods, too-high concentrations of other OA sources in the model that are important over the larger domain, or other model issues such as loss processes. This discrepancy is likely regionally and temporally dependent and driven by interferences from factors like varying emissions and chemical regimes.

Jacques, AA, Horel JD, Crosman ET, Vernon FL.  2017.  Tracking Mesoscale Pressure Perturbations Using the USArray Transportable Array. Monthly Weather Review. 145:3119-3142.   10.1175/mwr-d-16-0450.1   AbstractWebsite

Mesoscale convective phenomena induce pressure perturbations that can alter the strength and magnitude of surface winds, precipitation, and other sensible weather, which, in some cases, can inflict injuries and damage to property. This work extends prior research to identify and characterize mesoscale pressure features using a unique resource of 1-Hz pressure observations available from the USArray Transportable Array (TA) seismic field campaign. A two-dimensional variational technique is used to obtain 5-km surface pressure analysis grids every 5 min from 1 March to 31 August 2011 from the TA observations and gridded surface pressure from the Real-Time Mesoscale Analysis over a swath of the central United States. Bandpass-filtering and feature-tracking algorithms are employed to isolate, identify, and assess prominent mesoscale pressure perturbations and their properties. Two case studies, the first involving mesoscale convective systems and the second using a solitary gravity wave, are analyzed using additional surface observation and gridded data resources. Summary statistics for tracked features during the period reviewed indicate a majority of perturbations last less than 3 h, produce maximum perturbation magnitudes between 2 and 5 hPa, and move at speeds ranging from 15 to 35ms(-1). The results of this study combined with improvements nationwide in real-time access to pressure observations at subhourly reporting intervals highlight the potential for improved detection and nowcasting of high-impact mesoscale weather features.

Kamae, Y, Shiogama H, Imada Y, Mori M, Arakawa O, Mizuta R, Yoshida K, Takahashi C, Arai M, Ishii M, Watanabe M, Kimoto M, Xie SP, Ueda H.  2017.  Forced response and internal variability of summer climate over western North America. Climate Dynamics. 49:403-417.   10.1007/s00382-016-3350-x   AbstractWebsite

Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951-2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO-AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS.

Wei, SS, Shearer PM.  2017.  A sporadic low-velocity layer atop the 410 km discontinuity beneath the Pacific Ocean. Journal of Geophysical Research-Solid Earth. 122:5144-5159.   10.1002/2017jb014100   AbstractWebsite

Waveforms of SS precursors recorded by global stations are analyzed to investigate lateral heterogeneities of upper mantle discontinuities on a global scale. A sporadic low-velocity layer immediately above the 410 km discontinuity (LVL-410) is observed worldwide, including East Asia, western North America, eastern South America, the Pacific Ocean, and possibly the Indian Ocean. Our best data coverage is for the Pacific Ocean, where the LVL-410 covers 33-50% of the resolved region. Lateral variations of our LVL-410 observations show no geographical correlation with 410 km discontinuity topography or tomographic models of seismic velocity, suggesting that the LVL-410 is not caused by regional thermal anomalies. We interpret the LVL-410 as partial melting due to dehydration of ascending mantle across the 410 km discontinuity, which is predicted by the transition zone water filter hypothesis. Given the low vertical resolution of SS precursors, it is possible that the regions without a clear LVL-410 detection also have a thin layer. Therefore, the strong lateral heterogeneity of the LVL-410 in our observations suggests partial melting with varying intensities across the Pacific and further provides indirect evidence of a hydrous mantle transition zone with laterally varying water content.

Hu, HC, Dominguez F, Wang Z, Lavers DA, Zhang G, Ralph FM.  2017.  Linking atmospheric river hydrological impacts on the US West Coast to Rossby wave breaking. Journal of Climate. 30:3381-3399.   10.1175/jcli-d-16-0386.1   AbstractWebsite

Atmospheric rivers (ARs) have significant hydrometeorological impacts on the U.S. West Coast. This study presents the connection between the characteristics of large-scale Rossby wave breaking (RWB) over the eastern North Pacific and the regional-scale hydrological impacts associated with landfalling ARs on the U.S. West Coast (36 degrees-49 degrees N). ARs associated with RWB account for two-thirds of the landfalling AR events and >70% of total AR-precipitation in the winter season. The two regimes of RWB-anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB)-are associated with different directions of the vertically integrated water vapor transport (IVT). AWB-ARs impinge in a more westerly direction on the coast whereas CWB-ARs impinge in a more southwesterly direction. Most of the landfalling ARs along the northwestern coast of the United States (states of Washington and Oregon) are AWB-ARs. Because of their westerly impinging angles when compared to CWB-ARs, AWBARs arrive more orthogonally to the western Cascades and more efficiently transform water vapor into precipitation through orographic lift than CWB-ARs. Consequently, AWB-ARs are associated with the most extreme streamflows in the region. Along the southwest coast of the United States (California), the southwesterly impinging angles of CWBARs are more orthogonal to the local topography. Furthermore, the southwest coast CWB-ARs have more intense IVT. Consequently, CWB-ARs are associated with the most intense precipitation. As a result, most of the extreme streamflows in southwest coastal basins are associated with CWB-ARs. In summary, depending on the associated RWB type, ARs impinge on the local topography at a different angle and have a different spatial signature of precipitation and streamflow.

Sanchez-Lorenzo, A, Enriquez-Alonso A, Calbo J, Gonzalez JA, Wild M, Folini D, Norris JR, Vicente-Serrano SM.  2017.  Fewer clouds in the Mediterranean: consistency of observations and climate simulations. Scientific Reports. 7   10.1038/srep41475   AbstractWebsite

Clouds play a major role in the climate system, but large uncertainties remain about their decadal variations. Here we report a widespread decrease in cloud cover since the 1970 s over the Mediterranean region, in particular during the 1970 s-1980 s, especially in the central and eastern areas and during springtime. Confidence in these findings is high due to the good agreement between the interannual variations of cloud cover provided by surface observations and several satellite-derived and reanalysis products, although some discrepancies exist in their trends. Climate model simulations of the historical experiment from the Coupled Model Intercomparison Project Phase 5 (CMIP5) also exhibit a decrease in cloud cover over the Mediterranean since the 1970 s, in agreement with surface observations, although the rate of decrease is slightly lower. The observed northward expansion of the Hadley cell is discussed as a possible cause of detected trends.

Lonsdale, CR, Hegarty JD, Cady-Pereira KE, Alvarado MJ, Henze DK, Turner MD, Capps SL, Nowak JB, Neuman A, Middlebrook AM, Bahreini R, Murphy JG, Markovic MZ, VandenBoer TC, Russell LM, Scarino AJ.  2017.  Modeling the diurnal variability of agricultural ammonia in Bakersfield, California, during the CalNex campaign. Atmospheric Chemistry and Physics. 17:2721-2739.   10.5194/acp-17-2721-2017   AbstractWebsite

NH3 retrievals from the NASA Tropospheric Emission Spectrometer (TES), as well as surface and aircraft observations of NH3(g) and submicron NH4(p), are used to evaluate modeled concentrations of NH3(g) and NH4(p) from the Community Multiscale Air Quality (CMAQ) model in the San Joaquin Valley (SJV) during the California Research at the Nexus of Air Quality and Climate Change (CalNex) campaign. We find that simulations of NH3 driven with the California Air Resources Board (CARB) emission inventory are qualitatively and spatially consistent with TES satellite observations, with a correlation coefficient (r(2)) of 0.64. However, the surface observations at Bakersfield indicate a diurnal cycle in the model bias, with CMAQ overestimating surface NH3 at night and underestimating it during the day. The surface, satellite, and aircraft observations all suggest that daytime NH3 emissions in the CARB inventory are underestimated by at least a factor of 2, while the nighttime overestimate of NH3(g) is likely due to a combination of overestimated NH3 emissions and underestimated deposition. Running CMAQ v5.0.2 with the bi-directional NH3 scheme reduces NH3 concentrations at night and increases them during the day. This reduces the model bias when compared to the surface and satellite observations, but the increased concentrations aloft significantly increase the bias relative to the aircraft observations. We attempt to further reduce model bias by using the surface observations at Bakers-field to derive an empirical diurnal cycle of NH3 emissions in the SJV, in which nighttime and midday emissions differ by about a factor of 4.5. Running CMAQv5.0.2 with a bi-directional NH3 scheme together with this emissions diurnal profile further reduces model bias relative to the surface observations. Comparison of these simulations with the vertical profile retrieved by TES shows little bias except for the lowest retrieved level, but the model bias relative to flight data aloft increases slightly. Our results indicate that both diurnally varying emissions and a bi-directional NH3 scheme should be applied when modeling NH3(g) and NH4(p) in this region. The remaining model errors suggest that the bi-directional NH3 scheme in CMAQ v5.0.2 needs further improvements to shift the peak NH3 land-atmosphere flux to earlier in the day. We recommend that future work include updates to the current CARB NH3 inventory to account for NH3 from fertilizer application, livestock, and other farming practices separately; adding revised information on crop management practices specific to the SJV region to the bi-directional NH3 scheme; and top-down studies focused on determining the diurnally varying biases in the canopy compensation point that determines the net land-atmosphere NH3 fluxes.

Brusca, RC, Alvarez-Borrego S, Hastings PA, Findley LT.  2017.  Colorado River flow and biological productivity in the Northern Gulf of California, Mexico. Earth-Science Reviews. 164:1-30.   10.1016/j.earscirev.2016.10.012   AbstractWebsite

A review of published research indicates that the Northern Gulf of California is, historically and currently, one of the most biologically productive marine regions on Earth. This high productivity is driven by a unique mix of factors, including: coastal upwelling, wind-driven mixing, extreme tidal mixing and turbulence, thermohaline circulation that moves intermediate waters into the mixed layer, coastal-trapped waves, regular sediment resuspension, and, to a lesser extent, agricultural runoff, released nutrients from erosion of ancient Colorado River Delta sediments, and perhaps input from decomposing tidal-flat plant debris. It has been suggested that decreased Colorado River flow, due to anthropogenic water impoundments and diversions, has had a negative impact on the health of the Northern Gulf of California ecosystem, particularly by reducing primary productivity and/or stock production of finfish and shellfish. However, there is no evidence that surface flow from the Colorado River is now, nor has ever been an important driver of primary productivity in the Northern Gulf, and nutrient/chlorophyll studies show no relationship to Colorado River flow (or, if anything, reduced nutrient/chlorophyll levels occur during high river-flow periods). And, there is very limited and equivocal evidence to support the claim that reduced river flow has significantly impacted secondary productivity in the Northern Gulf. The marine ecosystem of the Northern Gulf remains rich in nutrients, high in biodiversity and productivity, and appears to continue to be healthy, except for the impacts of historical and current fisheries. Human extraction of shrimp, Gulf corvina, totoaba (largely illegally), and other marine resources, remain very high in this region. There also is no evidence that reduced Colorado River flow has negatively impacted the health of the critically endangered vaquita porpoise, and assertions that it has done so deflect attention from the actual cause of decline-bycatch in legal and illegal gillnet fisheries. A review of Colorado River Delta research confirms that, historically and perhaps as long as the river has reached the Gulf of California, there have been long periods of no flow, or greatly reduced flow to the sea. Thus, the ecosystem is historically adapted to broadly fluctuating river flows and elevated salinities. Although commonly used by recent researchers, measurements of Colorado River water crossing the border into Mexico do not provide a reliable proxy for how much water (if any) actually reaches the Upper Gulf because of the complex nature of internal basins and diversions in the region. (C) 2016 Elsevier B.V. All rights reserved.

Benmarhnia, T, Bailey Z, Kaiser D, Auger N, King N, Kaufman JS.  2016.  A Difference-in-Differences Approach to Assess the Effect of a Heat Action Plan on Heat-Related Mortality, and Differences in Effectiveness According to Sex, Age, and Socioeconomic Status (Montreal, Quebec). Environmental Health Perspectives. 124:1694-1699.   10.1289/EHP203   AbstractWebsite

BACKGROUND: The impact of heat waves on mortality and health inequalities is well documented. Very few studies have assessed the effectiveness of heat action plans (HAPs) on health, and none has used quasi-experimental methods to estimate causal effects of such programs.OBJECTIVES: We developed a quasi-experimental method to estimate the causal effects associated with HAPs that allows the identification of heterogeneity across subpopulations, and to apply this method specifically to the case of the Montreal (Quebec, Canada) HAP.METHODS: A difference-in-differences approach was undertaken using Montreal death registry data for the summers of 2000-2007 to assess the effectiveness of the Montreal HAP, implemented in 2004, on mortality. To study equity in the effect of HAP implementation, we assessed whether the program effects were heterogeneous across sex (male vs. female), age (>= 65 years vs. < 65 years), and neighborhood education levels (first vs. third tertile). We conducted sensitivity analyses to assess the validity of the estimated causal effect of the HAP program.RESULTS: We found evidence that the HAP contributed to reducing mortality on hot days, and that the mortality reduction attributable to the program was greater for elderly people and people living in low-education neighborhoods.CONCLUSION: These findings show promise for programs aimed at reducing the impact of extreme temperatures and health inequities. We propose a new quasi-experimental approach that can be easily applied to evaluate the impact of any program or intervention triggered when daily thresholds are reached.

Sanchez, SC, Charles CD, Carriquiry JD, Villaescusa JA.  2016.  Two centuries of coherent decadal climate variability across the Pacific North American region. Geophysical Research Letters. 43:9208-9216.   10.1002/2016gl069037   AbstractWebsite

The decadal variability of the Pacific Ocean and North American hydroclimate are subjects of immediate concern for society, yet the length of the instrumental record limits full mechanistic understanding of this variability. Here we introduce a 178year, seasonally resolved coral oxygen isotopic record from Clarion Island (18 degrees N, 115 degrees W), a sampling a subtropical region that is strongly influenced by the decadal-scale fluctuations of the North Pacific Gyre Oscillation and a region that serves as a critical locus for the communication of climate anomalies with the tropics. This Mexican Pacific coral record is highly correlated to coral records from the central tropical Pacific and tree ring records from western North America. Significant changes in the amplitude of oceanic decadal variability in the early nineteenth century are mirrored in the drought reconstructions in western North America. The spatial manifestation of this relationship was relatively invariant, despite notable changes in the climatic mean state.

Mitchell, EK, Fialko Y, Brown KM.  2016.  Velocity-weakening behavior of Westerly granite at temperature up to 600 degrees C. Journal of Geophysical Research-Solid Earth. 121:6932-6946.   10.1002/2016jb013081   AbstractWebsite

The deep limit to seismicity in continental crust is believed to be controlled by a transition from velocity-weakening to velocity-strengthening friction based on experimental measurements of the rate dependence of friction at different temperatures. Available experimental data on granite suggest a transition to stable creep at about 350 degrees C (approximate to 15km depth). Here we present results from unconfined experiments on Westerly granite at both dry and hydrated conditions that show increasingly unstable slip (velocity-weakening behavior) at temperature up to 600 degrees C. A comparison of previously published experimental results with those presented in this study suggests that the rate and state friction parameters strongly depend on normal stress and pore pressure at high (>400 degrees C) temperature, which may help explain regional variations in the depth distribution of earthquakes in continental crust. Temperature dependence of the rate and state friction parameters may also contribute to strong dynamic weakening observed in high-speed friction experiments on crystalline rocks such as granite and gabbro.

Lundquist, JD, Roche JW, Forrester H, Moore C, Keenan E, Perry G, Cristea N, Henn B, Lapo K, McGurk B, Cayan DR, Dettinger MD.  2016.  Yosemite Hydroclimate Network: Distributed stream and atmospheric data for the Tuolumne River watershed and surroundings. Water Resources Research. 52:7478-7489.   10.1002/2016wr019261   AbstractWebsite

Regions of complex topography and remote wilderness terrain have spatially varying patterns of temperature and streamflow, but due to inherent difficulties of access, are often very poorly sampled. Here we present a data set of distributed stream stage, streamflow, stream temperature, barometric pressure, and air temperature from the Tuolumne River Watershed in Yosemite National Park, Sierra Nevada, California, USA, for water years 2002-2015, as well as a quality-controlled hourly meteorological forcing time series for use in hydrologic modeling. We also provide snow data and daily inflow to the Hetch Hetchy Reservoir for 1970-2015. This paper describes data collected using low-visibility and low-impact installations for wilderness locations and can be used alone or as a critical supplement to ancillary data sets collected by cooperating agencies, referenced herein. This data set provides a unique opportunity to understand spatial patterns and scaling of hydroclimatic processes in complex terrain and can be used to evaluate downscaling techniques or distributed modeling. The paper also provides an example methodology and lessons learned in conducting hydroclimatic monitoring in remote wilderness.

Fewings, MR, Washburn L, Dorman CE, Gotschalk C, Lombardo K.  2016.  Synoptic forcing of wind relaxations at Pt. Conception, California. Journal of Geophysical Research-Oceans. 121:5711-5730.   10.1002/2016jc011699   AbstractWebsite

Over the California Current upwelling system in summer, the prevailing upwelling-favorable winds episodically weaken (relax) or reverse direction for a few days. Near Pt. Conception, California, the wind usually does not reverse, but wind relaxation allows poleward oceanic coastal flow with ecological consequences. To determine the offshore extent and synoptic forcing of these wind relaxations, we formed composite averages of wind stress from the QuikSCAT satellite and atmospheric pressure from the North American Regional Reanalysis (NARR) using 67 wind relaxations during summer 2000-2009. Wind relaxations at Pt. Conception are the third stage of an event sequence that repeatedly affects the west coast of North America in summer. First, 5-7 days before the wind weakens near Pt. Conception, the wind weakens or reverses off Oregon and northern California. Second, the upwelling-favorable wind intensifies along central California. Third, the wind relaxes at Pt. Conception, and the area of weakened winds extends poleward to northern California over 3-5 days. The NARR underestimates the wind stress within similar to 200 km of coastal capes by a factor of 2. Wind relaxations at Pt. Conception are caused by offshore extension of the desert heat low. This synoptic forcing is related to event cycles that cause wind reversal as in Halliwell and Allen (1987) and Mass and Bond (1996), but includes weaker events. The wind relaxations extend similar to 600 km offshore, similarly to the California-scale hydraulic expansion fan shaping the prevailing winds, and similar to 1000 km alongshore, limited by an opposing pressure gradient force at Cape Mendocino.

Ralph, FM, Prather KA, Cayan D, Spackman JR, DeMott P, Dettinger M, Fairall C, Leung R, Rosenfeld D, Rutledge S, Waliser D, White AB, Cordeira J, Martin A, Helly J, Intrieri J.  2016.  CalWater field studies designed to quantify the roles of atmospheric rivers and aerosols in modulating US West Coast precipitation in a changing climate. Bulletin of the American Meteorological Society. 97:1209-1228.   10.1175/bams-d-14-00043.1   AbstractWebsite

The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemistry, cloud microphysics, and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on the remaining gaps, a vision was developed to extend these studies offshore over the eastern North Pacific and to enhance land-based measurements from 2014 to 2018 (CalWater-2). The dataset and selected results from CalWater-1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.