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Alfaro, EJ, Gershunov A, Cayan D.  2006.  Prediction of summer maximum and minimum temperature over the central and western United States: The roles of soil moisture and sea surface temperature. Journal of Climate. 19:1407-1421.   10.1175/jcli3665.1   AbstractWebsite

A statistical model based on canonical correlation analysis (CCA) was used to explore climatic associations and predictability of June-August (JJA) maximum and minimum surface air temperatures (Tmax and Tmin) as well as the frequency of Tmax daily extremes (Tmax90) in the central and western United States (west of 90 degrees W). Explanatory variables are monthly and seasonal Pacific Ocean SST (PSST) and the Climate Division Palmer Drought Severity Index (PDSI) during 1950-2001. Although there is a positive correlation between Tmax and Tmin, the two variables exhibit somewhat different patterns and dynamics. Both exhibit their lowest levels of variability in summer, but that of Tmax is greater than Tmin. The predictability of Tmax is mainly associated with local effects related to previous soil moisture conditions at short range (one month to one season), with PSST providing a secondary influence. Predictability of Tmin is more strongly influenced by large-scale (PSST) patterns, with PDSI acting as a short-range predictive influence. For both predictand variables (Tmax and Tmin), the PDSI influence falls off markedly at time leads beyond a few months, but a PSST influence remains for at least two seasons. The maximum predictive skill for JJA Tmin, Tmax, and Tmax90 is from May PSST and PDSI. Importantly. skills evaluated for various seasons and time leads undergo a seasonal cycle that has maximum levels in summer. At the seasonal time frame, summer Tmax prediction skills are greatest in the Midwest, northern and central California, Arizona, and Utah. Similar results were found for Tmax90. In contrast, Tmin skill is spread over most of the western region, except for clusters of low skill in the northern Midwest and southern Montana, Idaho, and northern Arizona.

Cavanaugh, NR, Gershunov A, Panorska AK, Kozubowski TJ.  2015.  The probability distribution of intense daily precipitation. Geophysical Research Letters. 42:1560-1567.   10.1002/2015gl063238   AbstractWebsite

The probability tail structure of over 22,000 weather stations globally is examined in order to identify the physically and mathematically consistent distribution type for modeling the probability of intense daily precipitation and extremes. Results indicate that when aggregating data annually, most locations are to be considered heavy tailed with statistical significance. When aggregating data by season, it becomes evident that the thickness of the probability tail is related to the variability in precipitation causing events and thus that the fundamental cause of precipitation volatility is weather diversity. These results have both theoretical and practical implications for the modeling of high-frequency climate variability worldwide.

Guirguis, K, Gershunov A, Schwartz R, Bennett S.  2011.  Recent warm and cold daily winter temperature extremes in the Northern Hemisphere. Geophysical Research Letters. 38   10.1029/2011gl048762   AbstractWebsite

The winters of 2009-2010 and 2010-2011 brought frigid temperatures to parts of Europe, Russia, and the U. S. We analyzed regional and Northern Hemispheric (NH) daily temperature extremes for these two consecutive winters in the historical context of the past 63 years. While some parts clearly experienced very cold temperatures, the NH was not anomalously cold. Extreme warm events were much more prevalent in both magnitude and spatial extent. Importantly, the persistent negative state of the North Atlantic Oscillation (NAO) explained the bulk of the observed cold anomalies, however the warm extremes were anomalous even accounting for the NAO and also considering the states of the Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO). These winters' widespread and intense warm extremes together with a continuing hemispheric decline in cold snap activity was a pattern fully consistent with a continuation of the warming trend observed in recent decades. Citation: Guirguis, K., A. Gershunov, R. Schwartz, and S. Bennett (2011), Recent warm and cold daily winter temperature extremes in the Northern Hemisphere, Geophys. Res. Lett., 38, L17701, doi:10.1029/2011GL048762.

Alfaro, EJ, Pierce DW, Steinemann AC, Gershunov A.  2005.  Relationships between the irrigation-pumping electrical loads and the local climate in Climate Division 9, Idaho. Journal of Applied Meteorology. 44:1972-1978.   10.1175/jam2315.1   AbstractWebsite

The electrical load from irrigation pumps is an important part of the overall electricity demand in many agricultural areas of the U.S. west. The date the pumps turn on and the total electrical load they present over the summer varies from year to year, partly because of climate fluctuations. Predicting this variability would be useful to electricity producers that supply the region. This work presents a contingency analysis and linear regression scheme for forecasting summertime irrigation pump loads in southeastern Idaho. The basis of the predictability is the persistence of spring soil moisture conditions into summer, and the effect it has on summer temperatures. There is a strong contemporaneous relationship between soil moisture and temperature in the summer and total summer pump electrical loads so that a reasonable prediction of summer pump electrical loads based on spring soil moisture conditions can be obtained in the region. If one assumes that decision makers will take appropriate actions based on the forecast output, the net economic benefit of forecast information is approximately $2.5 million per year, making this prediction problem an important seasonal summer forecasting issue with significant economic implications.

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.

Kozubowski, TJ, Panorska AK, Qeadan F, Gershunov A, Rominger D.  2009.  Testing Exponentiality Versus Pareto Distribution via Likelihood Ratio. Communications in Statistics-Simulation and Computation. 38:118-139.   10.1080/03610910802439121   AbstractWebsite

We consider the problem of maximum likelihood estimation of the parameters of the Pareto Type II (Lomax) distribution. We show that in certain parametrization and after modification of the parameter space to include exponential distribution as a special case, the MLEs of parameters always exist. Moreover, the MLEs have a non standard asymptotic distribution in the exponential case due to the lack of regularity. Further, we develop a likelihood ratio test for exponentiality versus Pareto II distribution. We emphasize that this problem is non standard, and the limiting null distribution of the deviance statistic in not chi-square. We derive relevant asymptotic theory as well as a convenient computational formula for the critical values for the test. An empirical power study and power comparisons with other tests are also provided. A problem from climatology involving precipitation data from hundreds of meteorological stations across North America provides a motivation for and an illustration of the new test.

Gershunov, A, Michaelsen J.  1996.  Vertical variability of water vapor in the midlatitude upper troposphere. Contributions to Atmospheric Physics [Beitraege zur Physik der Atmosphaere.], Wiesbaden, Germany. 69:205-214. AbstractWebsite

Radiative, dynamical and phase-change considerations of tropospheric moisture, all point to moisture in the upper troposphere as a major determinant of the global climate. Hemispheric-scale vertical variability of upper-tropospheric moisture is observed through a multivariate statistical analysis of three years of monthly mean SAGE-II data for 1986-88. Midlatitude zonally averaged vertical variability of moisture in the upper troposphere is separated into coherent modes using principal components analysis. Bulk vertical variations in the upper troposphere are separated from a dynamical mode of variability representing vertical moisture gradient and horizontal advection. The procedure is repeated for the northern and southern midlatitudes. We discuss the vertical structure and temporal variability of the meaningful modes and observe a north-south hemispheric asymmetry in the characteristics of the vertical moisture variability. It is hypothesized that midlatitude wave cyclones are responsible for the poleward and vertical transport of water vapor to and in the midlatitude upper troposphere.