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Kenneth S. Gage, Christopher R. Williams, Paul E. Johnston, Warner L. Ecklund, Robert Cifelli, Ali Tokay, and David A. Carter

Abstract

The National Oceanic and Atmospheric Administration’s Aeronomy Laboratory has modified a standard 915-MHz profiler for use as a precipitation profiler in support of Tropical Rainfall Measuring Mission ground validation field campaigns. This profiler was modified to look vertically with a fixed dish antenna. It was operated during the Texas and Florida Underflights Experiment (TEFLUN) A in south Texas in April–May 1998 and during TEFLUN B in central Florida in August–September 1998. Collocated with the profiler was a Distromet, Inc., RD-69 Joss–Waldvogel disdrometer in Texas and Florida and a two-dimensional video disdrometer in Florida. The disdrometers are used to calibrate the profiler at the lowest range gates. At higher altitudes, the calibrated profiler reflectivities are compared with observations made by scanning radars such as the Weather Surveillance Radar-1988 Doppler in Dickinson, Texas, and Melbourne, Florida, and the S-band Doppler dual-polarization radar in Florida. The authors conclude that it is possible to use profilers as transfer standards to calibrate and to validate the reflectivities measured by the scanning radars.

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Xiaoyan Jiang, Sara A. Rauscher, Todd D. Ringler, David M. Lawrence, A. Park Williams, Craig D. Allen, Allison L. Steiner, D. Michael Cai, and Nate G. McDowell

Abstract

Rapid and broad-scale forest mortality associated with recent droughts, rising temperature, and insect outbreaks has been observed over western North America (NA). Climate models project additional future warming and increasing drought and water stress for this region. To assess future potential changes in vegetation distributions in western NA, the Community Earth System Model (CESM) coupled with its Dynamic Global Vegetation Model (DGVM) was used under the future A2 emissions scenario. To better span uncertainties in future climate, eight sea surface temperature (SST) projections provided by phase 3 of the Coupled Model Intercomparison Project (CMIP3) were employed as boundary conditions. There is a broad consensus among the simulations, despite differences in the simulated climate trajectories across the ensemble, that about half of the needleleaf evergreen tree coverage (from 24% to 11%) will disappear, coincident with a 14% (from 11% to 25%) increase in shrubs and grasses by the end of the twenty-first century in western NA, with most of the change occurring over the latter half of the twenty-first century. The net impact is a ~6 GtC or about 50% decrease in projected ecosystem carbon storage in this region. The findings suggest a potential for a widespread shift from tree-dominated landscapes to shrub and grass-dominated landscapes in western NA because of future warming and consequent increases in water deficits. These results highlight the need for improved process-based understanding of vegetation dynamics, particularly including mortality and the subsequent incorporation of these mechanisms into earth system models to better quantify the vulnerability of western NA forests under climate change.

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Deborah A. McGrath, Jonathan P. Evans, C. Ken Smith, David G. Haskell, Neil W. Pelkey, Robert R. Gottfried, Charles D. Brockett, Matthew D. Lane, and E. Douglass Williams

Abstract

Over the past two decades, forests in the southeastern United States have undergone dramatic changes as the result of urban sprawl and conversion to intensively managed pine plantations. The Cumberland Plateau, an important ecoregion in the southeastern United States, contains some of the largest remaining tracts of privately owned, native hardwood forest in North America. These ecologically important forests have been undergoing increasingly rapid rates of hardwood-to-pine conversion, much of which has gone undetected by large-scale statewide inventories. Forest conversion in Tennessee's southern Cumberland Plateau provides a case study highlighting the need for interdisciplinary and spatially explicit assessments of the impact and drivers of land-use change at smaller scales. Aerial and satellite imagery were used to create computer-generated maps of land use and forest cover for a 243 000 ha study area within a seven-county region of the southern Cumberland Plateau in Tennessee to track and document patterns of forest change and conversion between 1981 and 2000. The ecological impact of forest harvesting and hardwood-to-pine conversion was evaluated by (i) monitoring aquatic macroinvertebrate diversity, (ii) tracking breeding-bird populations, and (iii) comparing calcium (Ca) stores and cycling in a chronosequence of hardwood to first- and second-rotation loblolly pine (Pinus taeda) plantations. It was found that 14% of native forest cover had been lost since 1981, 74% of which resulted from hardwood-to-pine conversion. It was also found that the rate of conversion to pine doubled from 1997 to 2000. Water quality in streams, as measured by the abundance of critical macroinvertebrates, was significantly lower in recently logged sites than in undisturbed native forest. Surveys of breeding-bird populations showed that pine plantations of several age classes had lower species richness and evenness than did native oak–hickory forests. Despite similar soil concentrations of Ca in native hardwood, mature first-rotation, and early second-rotation pine, changes were found in aboveground Ca storage that suggest substantial system Ca losses that may limit productivity of second-rotation pine or regrowth of oak–hickory forest. As part of the ongoing research on the socioeconomic drivers of land-use change on the Cumberland Plateau, it was found that Tennessee's major forest conservation incentive program only delays forest conversion for a few years while subsidizing landowners who would not have converted their land in the absence of the program. These results demonstrate the need for more detailed and multidisciplinary research conducted at smaller scales so as to enhance the understanding of the impact and drivers of land-use change at larger scales.

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David W. Stahle, Edward R. Cook, Dorian J. Burnette, Max C. A. Torbenson, Ian M. Howard, Daniel Griffin, Jose Villanueva Diaz, Benjamin I. Cook, A. Park Williams, Emma Watson, David J. Sauchyn, Neil Pederson, Connie A. Woodhouse, Gregory T. Pederson, David Meko, Bethany Coulthard, and Christopher J. Crawford

Abstract

Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

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A. Park Williams, Richard Seager, Max Berkelhammer, Alison K. Macalady, Michael A. Crimmins, Thomas W. Swetnam, Anna T. Trugman, Nikolaus Buenning, Natalia Hryniw, Nate G. McDowell, David Noone, Claudia I. Mora, and Thom Rahn

Abstract

In 2011, exceptionally low atmospheric moisture content combined with moderately high temperatures to produce a record-high vapor pressure deficit (VPD) in the southwestern United States (SW). These conditions combined with record-low cold-season precipitation to cause widespread drought and extreme wildfires. Although interannual VPD variability is generally dominated by temperature, high VPD in 2011 was also driven by a lack of atmospheric moisture. The May–July 2011 dewpoint in the SW was 4.5 standard deviations below the long-term mean. Lack of atmospheric moisture was promoted by already very dry soils and amplified by a strong ocean-to-continent sea level pressure gradient and upper-level convergence that drove dry northerly winds and subsidence upwind of and over the SW. Subsidence drove divergence of rapid and dry surface winds over the SW, suppressing southerly moisture imports and removing moisture from already dry soils. Model projections developed for the fifth phase of the Coupled Model Intercomparison Project (CMIP5) suggest that by the 2050s warming trends will cause mean warm-season VPD to be comparable to the record-high VPD observed in 2011. CMIP5 projections also suggest increased interannual variability of VPD, independent of trends in background mean levels, as a result of increased variability of dewpoint, temperature, vapor pressure, and saturation vapor pressure. Increased variability in VPD translates to increased probability of 2011-type VPD anomalies, which would be superimposed on ever-greater background VPD levels. Although temperature will continue to be the primary driver of interannual VPD variability, 2011 served as an important reminder that atmospheric moisture content can also drive impactful VPD anomalies.

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Christopher R. Williams, V. N. Bringi, Lawrence D. Carey, V. Chandrasekar, Patrick N. Gatlin, Ziad S. Haddad, Robert Meneghini, S. Joseph Munchak, Stephen W. Nesbitt, Walter A. Petersen, Simone Tanelli, Ali Tokay, Anna Wilson, and David B. Wolff
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Christopher R. Williams, V. N. Bringi, Lawrence D. Carey, V. Chandrasekar, Patrick N. Gatlin, Ziad S. Haddad, Robert Meneghini, S. Joseph Munchak, Stephen W. Nesbitt, Walter A. Petersen, Simone Tanelli, Ali Tokay, Anna Wilson, and David B. Wolff
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John R. Mecikalski, Wayne F. Feltz, John J. Murray, David B. Johnson, Kristopher M. Bedka, Sarah T. Bedka, Anthony J. Wimmers, Michael Pavolonis, Todd A. Berendes, Julie Haggerty, Pat Minnis, Ben Bernstein, and Earle Williams

Advanced Satellite Aviation Weather Products (ASAP) was jointly initiated by the NASA Applied Sciences Program and the NASA Aviation Safety and Security Program in 2002. The initiative provides a valuable bridge for transitioning new and existing satellite information and products into Federal Aviation Administration (FAA) Aviation Weather Research Program (AWRP) efforts to increase the safety and efficiency of project addresses hazards such as convective weather, turbulence (clear air and cloud induced), icing, and volcanic ash, and is particularly applicable in extending the monitoring of weather over data-sparse areas, such as the oceans and other observationally remote locations.

ASAP research is conducted by scientists from NASA, the FAA AWRP's Product Development Teams (PDT), NOAA, and the academic research community. In this paper we provide a summary of activities since the inception of ASAP that emphasize the use of current-generation satellite technologies toward observing and mitigating specified aviation hazards. A brief overview of future ASAP goals is also provided in light of the next generation of satellite sensors (e.g., hyperspectral; high spatial resolution) to become operational in the 2007–18 time frame.

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Christopher R. Williams, V. N. Bringi, Lawrence D. Carey, V. Chandrasekar, Patrick N. Gatlin, Ziad S. Haddad, Robert Meneghini, S. Joseph Munchak, Stephen W. Nesbitt, Walter A. Petersen, Simone Tanelli, Ali Tokay, Anna Wilson, and David B. Wolff

Abstract

Rainfall retrieval algorithms often assume a gamma-shaped raindrop size distribution (DSD) with three mathematical parameters N w, D m, and μ. If only two independent measurements are available, as with the dual-frequency precipitation radar on the Global Precipitation Measurement (GPM) mission core satellite, then retrieval algorithms are underconstrained and require assumptions about DSD parameters. To reduce the number of free parameters, algorithms can assume that μ is either a constant or a function of D m. Previous studies have suggested μ–Λ constraints [where Λ = (4 + μ)/D m], but controversies exist over whether μ–Λ constraints result from physical processes or mathematical artifacts due to high correlations between gamma DSD parameters. This study avoids mathematical artifacts by developing joint probability distribution functions (joint PDFs) of statistically independent DSD attributes derived from the raindrop mass spectrum. These joint PDFs are then mapped into gamma-shaped DSD parameter joint PDFs that can be used in probabilistic rainfall retrieval algorithms as proposed for the GPM satellite program. Surface disdrometer data show a high correlation coefficient between the mass spectrum mean diameter D m and mass spectrum standard deviation σ m. To remove correlations between DSD attributes, a normalized mass spectrum standard deviation is constructed to be statistically independent of D m, with representing the most likely value and std representing its dispersion. Joint PDFs of D m and μ are created from D m and . A simple algorithm shows that rain-rate estimates had smaller biases when assuming the DSD breadth of than when assuming a constant μ.

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Steven C. Hardiman, Ian A. Boutle, Andrew C. Bushell, Neal Butchart, Mike J. P. Cullen, Paul R. Field, Kalli Furtado, James C. Manners, Sean F. Milton, Cyril Morcrette, Fiona M. O’Connor, Ben J. Shipway, Chris Smith, David N. Walters, Martin R. Willett, Keith D. Williams, Nigel Wood, N. Luke Abraham, James Keeble, Amanda C. Maycock, John Thuburn, and Matthew T. Woodhouse

Abstract

A warm bias in tropical tropopause temperature is found in the Met Office Unified Model (MetUM), in common with most models from phase 5 of CMIP (CMIP5). Key dynamical, microphysical, and radiative processes influencing the tropical tropopause temperature and lower-stratospheric water vapor concentrations in climate models are investigated using the MetUM. A series of sensitivity experiments are run to separate the effects of vertical advection, ice optical and microphysical properties, convection, cirrus clouds, and atmospheric composition on simulated tropopause temperature and lower-stratospheric water vapor concentrations in the tropics. The numerical accuracy of the vertical advection, determined in the MetUM by the choice of interpolation and conservation schemes used, is found to be particularly important. Microphysical and radiative processes are found to influence stratospheric water vapor both through modifying the tropical tropopause temperature and through modifying upper-tropospheric water vapor concentrations, allowing more water vapor to be advected into the stratosphere. The representation of any of the processes discussed can act to significantly reduce biases in tropical tropopause temperature and stratospheric water vapor in a physical way, thereby improving climate simulations.

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