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Paula J. Brown and Arthur T. DeGaetano

Abstract

Most climatological datasets are beset with urban temperature influences that distort long-term trends. Using an hourly dataset of 41 urban and rural stations from the United States, discriminant functions were developed using diurnal temperature range indices based on temperature, dewpoint, and dewpoint depression that capture the differences between the two environments. Based on data for 1997–2006, diurnal temperature range and nighttime dewpoint depression range indices provide the best classification variables to statistically discriminate between urban and rural climates. Of the 41 stations, 93% were correctly classified by this technique in a cross-validation analysis. An additional discriminant analysis specific to coastal stations was needed because coastal climates were noted to be aberrant. Here, all stations tested were correctly classified by the procedure. Temporal trends in discriminant scores indicate periods of time during which urbanization was occurring or increasing. Instrumental and location changes were noted to affect both temperature and dewpoint series and therefore the classification. However, such discontinuities can potentially be adjusted and the homogenized data used with the classification technique. The use of this data-driven approach complements existing methods used to classify the urban character of stations, because it is objective, is applicable in the presatellite era, and can infer changes at higher temporal resolution.

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John T. Abatzoglou and Timothy J. Brown

Abstract

Summertime cloud-to-ground lightning strikes are responsible for the majority of wildfire ignitions across vast sections of the seasonally dry western United States. In this study, a strong connection between active phases of the Madden–Julian oscillation (MJO) and regional summertime lightning activity was found across the interior western United States. This intraseasonal mode of lightning activity emanates northward from the desert Southwest across the Great Basin and into the northern Rocky Mountains. The MJO is shown to provide favorable conditions for the northward propagation of widespread lightning activity through the amplification of the upper-level ridge over the western United States and the development of midtropospheric instability. Given the relative predictability of the MJO with long lead times, results allude to the potential for intraseasonal predictability of lightning activity and proactive fire management planning.

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Paula J. Brown and Arthur T. DeGaetano

Abstract

U.S. hourly surface observations are examined at 145 stations to identify annual and seasonal changes in temperature, dewpoint, relative humidity, and specific humidity since 1930. Because of numerous systematic instrument changes that have occurred, a homogeneity assessment was performed on temperatures and dewpoints. Dewpoints contained higher breakpoint detection rates associated with instrumentation changes than did temperatures. Temperature trends were tempered by adjusting the data, whereas dewpoints were unaffected. The effects were the same whether the adjustments were based on statistically detected or fixed-year breakpoints. Average long-term trends (1930–2010) indicate that temperature has warmed but that little change has occurred in dewpoint and specific humidity. Warming is strongest in spring. There is evidence of inhomogeneity in the relative humidity record that primarily affects data from prior to 1950. Therefore, long-term decreases in relative humidity, which are strongest in winter, need to be viewed with caution. Trends since 1947 indicate that the warming of temperatures has coincided with increases in dewpoints and a moistening of specific humidity. This moistening is especially pronounced during the summer in the Midwest. For the nation, trends in relative humidity show little change for the period 1947–2010, during which these data are more homogeneous. Moistening has occurred throughout the central United States while other regions have experienced drying. Urban-related warming and drying trends are present in the data, but their effect is minimal. Regional changes in land use and moisture availability are likely influencing trends in atmospheric moisture.

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Robin T. Clark and Simon J. Brown

Abstract

Atmospheric circulation patterns occurring on the warmest 10% of summer days for a region of Europe severely impacted by the 2003 heatwave have been identified using a perturbed parameter ensemble of regional high-resolution climate model simulations for the recent past. Changes in the frequency and duration of these circulation types, driven by the simulations following a moderate transient pathway of anthropogenic emissions, are then shown for the period 2070 to 2100. Increases in the future probability of hot days are then attributed separately to changes in the frequency and temperature intensity of the circulation types. Changes in temperature intensity are found to have an effect 2 to 3 times larger than in frequency.

The authors then consider how model uncertainty in changes of future temperature within circulation patterns compares to the uncertainty irrespective of circulation, in an attempt to exclude contributions to the overall uncertainty arising from changes in circulation. Within individual patterns, the range of meteorological physical processes may be narrower. However, no reduction in uncertainty was found when single patterns were considered. Contributions to the lack of narrowing from circulation-type duration, model vegetation root depth and changes in cloud cover, pressure gradient, and continental-scale warming are subsequently examined using relationships between changes in surface latent heat and temperature. Vegetation root depth is found to be the greatest contributor to the temperature uncertainty.

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Paula J. Brown and Arthur T. DeGaetano

Abstract

Hourly dewpoint temperature data for the 1951–2006 period at 10 stations in the contiguous United States were investigated to determine if inhomogeneities in their records could be detected. At least three instrument changes are known to have occurred during this time period. The relatively sparse network of stations with dewpoint temperature data in the United States necessitated a nonconventional method to create a reference series. Utilizing nighttime occurrences of fog, clear/calm conditions, and precipitation as meteorological situations during which dewpoint temperatures and minimum temperatures are similar, three potential reference series based on daily minimum temperature were developed to test for inhomogeneities. Four stations with independent network neighbors recording hourly dewpoint data provided a direct validation of the effect of inhomogeneities on dewpoint temperatures. It was determined that fog conditions and the combined results from all three meteorologically based tests performed best when detecting documented inhomogeneities. However, a larger number of undocumented inhomogeneities, a feature common in most traditional inhomogeneity tests, were also detected that may or may not be valid.

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J. T. Willis, K. A. Browning, and D. Atlas

Abstract

Simultaneous measurements of the radar cross section and fallspeed of 5 cm (and larger) ice spheres falling in free air have been obtained using a high-precision tracking radar operating at a wavelength of 5.47 cm. While they were dry, the spheres fell with supercritical Reynolds numbers and drag coefficients of only 0.24 to 0.30. These coefficients are much smaller than those normally attributed to hailstones under any conditions. The surface of one sphere, 5.1 cm in diameter, became wet during its fall. This was accompanied by a 5 db decrease in its normalized radar cross section and a twofold increase in its drag coefficient. The implications of these observations are discussed.

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Paula J. Brown, Raymond S. Bradley, and Frank T. Keimig

Abstract

The northeastern United States is one of the most variable climates in the world, and how climate extremes are changing is critical to populations, industries, and the environment in this region. A long-term (1870–2005) temperature and precipitation dataset was compiled for the northeastern United States to assess how the climate has changed. Adjustments were made to daily temperatures to account for changes in mean, variance, and skewness resulting from inhomogeneities, but precipitation data were not adjusted. Trends in 17 temperature and 10 precipitation indices at 40 stations were evaluated over three time periods—1893–2005, 1893–1950, and 1951–2005—and over 1870–2005 for a subset of longer-term stations. Temperature indices indicate strong warming with increases in the frequency of warm events (e.g., warm nights and warm summer days) and decreases in the frequency of cold events (e.g., ice days, frost days, and the cold spell duration indicator). The strongest warming is exhibited in the decrease in frost days and the increase in growing season length. Although maximum temperatures indices showed strong warming trends over the period 1893–1950, subsequent trends show little change and cooling. Few significant trends were present in the precipitation indices; however, they displayed a tendency toward wetter conditions. A stepwise multiple linear regression analysis indicated that some of the variability in the 27 indices from 1951 to 2002 was explained by the North Atlantic Oscillation, Pacific decadal oscillation, and Pacific–North American pattern. However, teleconnection patterns showed little influence on the 27 indices over a 103-yr period.

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K. A. Browning, J. Hallett, T. W. Harrold, and D. Johnson

Abstract

Attempts have been made to obtain samples of freshly fallen hailstones from severe storms in Oklahoma with the purpose of studying the nature and extent of spongy ice within natural hail. Interception by automobile of radar echoes with Ze > 105 mm6 m−3 has been found to provide a workable technique for collecting large hailstones as they fall to the ground. Observations suggest that the regions of highest reflectivity were associated more closely with the falls of large hail than with the accompanying heavy rain.

Immediate sectioning of the freshly fallen hailstones revealed the presence of thin shells of spongy ice in many of the larger stones. Calorimetric analyses gave liquid water contents of up to 12 ± 4% of the mass of the stones. Some of the hailstones were aspherical owing to preferential melting of the regions of spongy ice during fall. In the case of hailstones that were stored at sub-freezing temperatures, spongy ice shells could often still be identified from the presence of millimeter size cavities embedded within ice composed of large crystals.

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Robin T. Clark, Simon J. Brown, and James M. Murphy

Abstract

Changes in extreme daily temperature events are examined using a perturbed physics ensemble of global model simulations under present-day and doubled CO2 climates where ensemble members differ in their representation of various physical processes. Modeling uncertainties are quantified by varying poorly constrained model parameters that control atmospheric processes and feedbacks and analyzing the ensemble spread of simulated changes. In general, uncertainty is up to 50% of projected changes in extreme heat events of the type that occur only once per year.

Large changes are seen in distributions of daily maximum temperatures for June, July, and August with significant shifts to warmer conditions. Changes in extremely hot days are shown to be significantly larger than changes in mean values in some regions. The intensity, duration, and frequency of summer heat waves are expected to be substantially greater over all continents. The largest changes are found over Europe, North and South America, and East Asia. Reductions in soil moisture, number of wet days, and nocturnal cooling are identified as significant factors responsible for the changes.

Although uncertainty associated with the magnitude of expected changes is large in places, it does not bring into question the sign or nature of the projected changes. Even with the most conservative simulations, hot extreme events are still expected to substantially increase in intensity, duration, and frequency. This ensemble, however, does not represent the full range of uncertainty associated with future projections; for example, the effects of multiple parameter perturbations are neglected, as are the effects of structural changes to the basic nature of the parameterization schemes in the model.

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J. Rosinski, K. A. Browning, G. Langer, and C. T. Nagamoto

Abstract

The distribution of water-insoluble aerosol particles with diameters in the range 3 to 60 (m has been studied as a function of hailstone radius for a collection of large hailstones. Most of the stones had opaque (probably graupel) embryos; a few had transparent (probably frozen drop) embryos. The concentration of particles within the opaque hailstone embryos was substantially smaller than the concentration of particles within the outer parts of the same hailstones, suggesting that the bulk of the hailstone growth had occurred in a different part of the storm from the region of embryo formation. It seems that the opaque embryos (graupel) form in weak updrafts and then grow into large haillstones upon entering an updraft core which is fed by dust-laden air originating closer to the ground. The transparent embryos were found to contain many more particles than the opaque embryos, perhaps indicating a different place of origin and also indicating an active role by the large particles ingested by a storm in the development of the transparent embryos.

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