Showing papers in "Bulletin of the American Meteorological Society in 2003"

The Changing Character of Precipitation

Abstract: From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital....

The Common Land Model

Abstract: The Common Land Model (CLM) was developed for community use by a grassroots collaboration of scientists who have an interest in making a general land model available for public use and further development. The major model characteristics include enough unevenly spaced layers to adequately represent soil temperature and soil moisture, and a multilayer parameterization of snow processes; an explicit treatment of the mass of liquid water and ice water and their phase change within the snow and soil system; a runoff parameterization following the TOPMODEL concept; a canopy photo synthesis-conductance model that describes the simultaneous transfer of CO2 and water vapor into and out of vegetation; and a tiled treatment of the subgrid fraction of energy and water balance. CLM has been extensively evaluated in offline mode and coupling runs with the NCAR Community Climate Model (CCM3). The results of two offline runs, presented as examples, are compared with observations and with the simulation of three other la...

Breaking the Cloud Parameterization Deadlock

Abstract: A key factor limiting the reliability of simulations of anthropogenic climate change is the inability to accurately represent the various effects of clouds on climate. Despite the best efforts of the community, the problem has resisted solution for several decades. The reasons for this are briefly reviewed and it is argued that it will be many more decades before the problem can be solved through the approaches to cloud parameterization that have been used up to now. An alternative approach, called superparameterization, is then outlined, in which high-resolution cloud system-resolving models (CSRMs) are used in place of the conventional cloud parameterizations. Tests performed with the Community Atmosphere Model show that superparameterizations can give more realistic simulations of the current climate, including greatly improved simulations of the Madden–Julian oscillation and other tropical wave disturbances. Superparameterizations increase the cost of climate simulation by a factor of several hundred ...

Climate and wildfire in the western united states

Abstract: A 21-yr gridded monthly fire-starts and acres-burned dataset from U.S. Forest Service, Bureau of Land Management, National Park Service, and Bureau of Indian Affairs fire reports recreates the seasonality and interannual variability of wildfire in the western United States. Despite pervasive human influence in western fire regimes, it is striking how strongly these data reveal a fire season responding to variations in climate. Correlating anomalous wildfire frequency and extent with the Palmer Drought Severity Index illustrates the importance of prior and accumulated precipitation anomalies for future wildfire season severity. This link to antecedent seasons' moisture conditions varies widely with differences in predominant fuel type. Furthermore, these data demonstrate that the relationship between wildfire season severity and observed moisture anomalies from antecedent seasons is strong enough to forecast fire season severity at lead times of one season to a year in advance.

Dynamics and chemistry of marine stratocumulus - DYCOMS II

Abstract: The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigat...

Potential Predictability of the Madden-Julian Oscillation.

Abstract: The objective of this study is to estimate the limit of dynamical predictability of the Madden–Julian oscillation (MJO). Ensembles of “twin” predictability experiments were carried out with the NASA Goddard Laboratory for the Atmospheres (GLA) atmospheric general circulation model (AGCM) using specified annual cycle SSTs. Initial conditions were taken from a 10-yr control simulation during periods of strong MJO activity identified via extended empirical orthogonal function (EOF) analysis of 30–90-day bandpassed tropical rainfall. From this analysis, 15 cases were chosen when the MJO convective center was located over the Indian Ocean, Maritime Continent, western Pacific Ocean, and central Pacific Ocean, respectively, making 60 MJO cases in total. In addition, 15 cases were selected that exhibited very little to no MJO activity. Two different sets of small random perturbations were added to these 75 initial states. Simulations were then performed for 90 days from each of these 150 perturbed initial conditi...

Regional climate research - Needs and opportunities

Abstract: The Workshop on Regional Climate Research: Needs and Opportunities was held 2–4 April 2001 at the National Center for Atmospheric Research, Boulder, Colorado. The workshop was cosponsored by the National Science Foundation and the Department of Energy with the goals to 1) assess current approaches used in downscaling; 2) inform program managers of the status of regional climate research; and 3) define a future path for regional climate research. The workshop was organized into five sessions with presentations and discussion to address issues related to the regional climate problem, global climate modeling, statistical and dynamical downscaling, data and model diagnostics and validation, and downscaling applications. Sixty-eight invited participants attended the workshop from the U.S. and international research community. This report summarizes the presentations and discussion of the workshop and highlights the recommendations derived from the meeting. Important general recommendations include the...

A New High-Resolution Blended Real-Time Global Sea Surface Temperature Analysis

Abstract: A new blended high-resolution real-time global sea surface temperature analysis (RTG_SST), developed specifically for use in operational numerical weather forecasting models, was implemented in NCEP's operational job stream on 30 January 2001, immediately following investigations of miss-forecast precipitation events in the mid-Atlantic states. Each daily analysis uses the most recent 24-h receipts of in situ and satellite-derived surface temperature data and provides a global SST field on a 0.5° × 0.5° (latitude-longitude) grid. The RTG_SST provides the sea surface temperature fields for the regional Meso Eta Model, replacing the previously used National Environmental Satellite, Data, and Information Service (NESDIS) 50-km satellite-only SST analysis. Forecast events leading to the implementation of the RTG_SST are described; comparison is made of the properties used in this new analysis with those of the Reynolds-Smith (RS) analysis and the NESDIS 50-km analysis; data ingestion, analysis, and verificati...

Improvement of Microphysical Parameterization through Observational Verification Experiment

Abstract: Despite continual increases in numerical model resolution and significant improvements in the forecasting of many meteorological parameters, progress in quantitative precipitation forecasting (QPF) has been slow. This is attributable in part to deficiencies in the bulk microphysical parameterization (BMP) schemes used in mesoscale models to simulate cloud and precipitation processes. These deficiencies have become more apparent as model resolution has increased. To address these problems requires comprehensive data that can be used to isolate errors in QPF due to BMP schemes from those due to other sources. These same data can then be used to evaluate and improve the microphysical processes and hydrometeor fields simulated by BMP schemes. In response to the need for such data, a group of researchers is collaborating on a study titled the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). IMPROVE has included two field campaigns carried out in th...

Multimodel Ensembling in Seasonal Climate Forecasting at IRI

Abstract: The International Research Institute (IRI) for Climate Prediction seasonal forecast system is based largely on the predictions of ensembles of several atmospheric general circulation models (AGCMs) forced by two versions of an SST prediction—one consisting of persisted SST anomalies from the current observations and one of evolving SST anomalies as predicted by a set of dynamical and statistical SST prediction models. Recently, an objective multimodel ensembling procedure has replaced a more laborious and subjective weighting of the predictions of the several AGCMs. Here the skills of the multimodel predictions produced retrospectively over the first 4 years of IRI forecasts are examined and compared with the skills of the more subjectively derived forecasts actually issued. The multimodel ensemble predictions are generally found to be an acceptable replacement, although the precipitation forecasts do benefit from inclusion of empirical forecast tools. Planned pattern-level model output statistics (MOS) c...

How Predictable is El Niño

Abstract: Nobody anticipated that El Nino would be weak and prolonged in 1992, but brief and intense in 1997/98. Why are various El Nino episodes so different, and so difficult to predict? The answer involves the important role played by random atmospheric disturbances (such as westerly wind bursts) in sustaining the weakly damped Southern Oscillation, whose complementary warm and cold phases are, respectively, El Nino and La Nina. As in the case of a damped pendulum sustained by modest blows at random times, so the predictability of El Nino is limited, not by the amplification of errors in initial conditions as in the case of weather, but mainly by atmospheric disturbances interacting with the Southern Oscillation. Given the statistics of the wind fluctuations, the probability distribution function of future sea surface temperature fluctuations in the eastern equatorial Pacific can be determined by means of an ensemble of calculations with a coupled ocean–atmosphere model. Each member of the ensemble star...

Paleoclimatic Analogs to Twentieth-Century Moisture Regimes Across the United States

Abstract: Instrumental Palmer Drought Severity Indexes (PDSI) averaged over the western United States and Great Plains document three major decadal moisture regimes during the twentieth century: the early twentieth-century pluvial, the Dust Bowl drought, and the 1950s drought. Tree-ring reconstructed PDSI for the contiguous Unites States replicates these three twentieth-century moisture regimes, and have been used to search for possible analogs over the past 500 yr. The early twentieth-century wet regime from 1905 to 1917 appears to have been the wettest episode across the West since A.D.1500, but similar pluvials occurred in the nineteenth, seventeenth, and sixteenth centuries. The Dust Bowl drought (1929–40) was most severe over the northern Plains to the northern Rockies. No close analogs are found for the full severity and geographical focus of the Dust Bowl drought over the past 500 yr. The 1950s drought (1946–56) was concentrated over the Southwest and was replicated by some 12 droughts of similar sp...

Evaluation of the IRI's “Net Assessment” Seasonal Climate Forecasts: 1997–2001

Abstract: The International Research Institute for Climate Prediction (IRI) net assessment seasonal temperature and precipitation forecasts are evaluated for the 4-yr period from October–December 1997 to October–December 2001. These probabilistic forecasts represent the human distillation of seasonal climate predictions from various sources. The ranked probability skill score (RPSS) serves as the verification measure. The evaluation is offered as time-averaged spatial maps of the RPSS as well as area-averaged time series. A key element of this evaluation is the examination of the extent to which the consolidation of several predictions, accomplished here subjectively by the forecasters, contributes to or detracts from the forecast skill possible from any individual prediction tool. Overall, the skills of the net assessment forecasts for both temperature and precipitation are positive throughout the 1997–2001 period. The skill may have been enhanced during the peak of the 1997/98 El Nino, particularly for t...

The wgne assessment of short-term quantitative precipitation forecasts

Abstract: Twenty-four-hour and 48-h quantitative precipitation forecasts (QPFs) from 11 operational numerical weather prediction models have been verified for a 4-yr period against rain gauge observations over the United States, Germany, and Australia to assess their skill in predicting the occurrence and amount of daily precipitation. Model QPFs had greater skill in winter than in summer, and greater skill in midlatitudes than in Tropics, where they performed only marginally better than “ persistence.” The best agreement among models, as well as the best ability to discriminate raining areas, occurred for a low rain threshold of 1–2 mm d−1. In contrast, the skill for forecasts of rain greater than 20 mm d−1 was generally quite low, reflecting the difficulty in predicting precisely when and where heavy rain will fall. The location errors for rain systems, determined using pattern matching with the observations, were typically about 100 km for 24-h forecasts, with smaller errors occurring for the heaviest r...

A New Perspective of Stratosphere–Troposphere Exchange

Abstract: Stratosphere–troposphere exchange (STE) is important for the chemical composition of both the stratosphere and troposphere. Modifications of STE in a changing climate may affect stratospheric ozone depletion and the oxidizing capacity of the troposphere significantly. However, STE is still poorly understood and inadequately quantified, due to the involvement of physical and dynamical processes on local to global scales and to conceptual problems. In this study, a present-day global climatology of STE is developed that is based, from a data standpoint, on 15 yr of global meteorological reanalyses, and, from a conceptual standpoint, on a Lagrangian perspective that considers the pathways of exchange air parcels and their residence times in the troposphere and lowermost stratosphere. To this end, two complementary Lagrangian models are used. Particular consideration is given to “deep” exchange events that, through fast ascent of tropospheric or fast descent of stratospheric air masses, bring into co...

Climate Research and Seasonal Forecasting for West Africans: Perceptions, Dissemination, and Use?

Abstract: Beginning in response to the disastrous drought of 1968–73, considerable research and monitoring have focused on the characteristics, causes, predictability, and impacts of West African Soudano–Sahel (10°–18°N) rainfall variability and drought. While these efforts have generated substantial information on a range of these topics, very little is known of the extent to which communities, activities at risk, and policy makers are aware of, have access to, or use such information. This situation has prevailed despite Glantz's provocative BAMS paper on the use and value of seasonal forecasts for the Sahel more than a quarter century ago. We now provide a systematic reevaluation of these issues based on questionnaire responses of 566 participants (in 13 communities) and 26 organizations in Burkina Faso, Mali, Niger, and Nigeria. The results reveal that rural inhabitants have limited access to climate information, with nongovernmental organizations (NGOs) being the most important source. Moreover, the pathways f...

The ARM program's water vapor intensive observation periods - Overview, initial accomplishments, and future challenges

Abstract: A series of water vapor intensive observation periods (WVIOPs) were conducted at the Atmospheric Radiation Measurement (ARM) site in Oklahoma between 1996 and 2000. The goals of these WVIOPs are to characterize the accuracy of the operational water vapor observations and to develop techniques to improve the accuracy of these measurements. The initial focus of these experiments was on the lower atmosphere, for which the goal is an absolute accuracy of better than 2% in total column water vapor, corresponding to ~1 W m−2 of infrared radiation at the surface. To complement the operational water vapor instruments during the WVIOPs, additional instrumentation including a scanning Raman lidar, microwave radiometers, chilled-mirror hygrometers, a differential absorption lidar, and ground-based solar radiometers were deployed at the ARM site. The unique datasets from the 1996, 1997, and 1999 experiments have led to many results, including the discovery and characterization of a large (> 25%) sonde-to-sonde variab...

Automated meteorological reports from commercial aircraft

Abstract: Commercial aircraft now provide over 130,000 meteorological observations per day, including temperature, winds, and in some cases, humidity, vertical wind gust, or eddy dissipation rate (turbulence). The temperature and wind data are used in most operational numerical weather prediction models at NCEP and at other centers worldwide. At nonsynoptic times, these data are often the primary source of upper-air information over the United States. Even at synoptic times, these data are critical in depicting the atmosphere along oceanic air routes. A Web site [http://acweb.fsl.noaa.gov/] has been developed that gives selected users access to these data. Because the data are proprietary to the airlines, real-time access is restricted to entities such as government agencies and nonprofit research institutions (although sample past data are available to all). Data can be displayed in a variety of ways and can be downloaded for local processing. These data are described here, and examples of how they have been usefu...

A review of positive and bipolar lightning discharges

Abstract: Characteristics of lightning discharges that transport either positive charge or both positive and negative charges to the ground are reviewed. These are termed positive and bipolar lightning discharges, respectively. Different types of positive and bipolar lightning are discussed. Although positive lightning discharges account for 10% or less of global cloud-to-ground lightning activity, there are five situations that appear to be conducive to the more frequent occurrence of positive lightning. These situations include 1) the dissipating stage of an individual thunderstorm, 2) winter thunderstorms, 3) trailing stratiform regions of mesoscale convective systems, 4) some severe storms, and 5) thunderclouds formed over forest fires or contaminated by smoke. The highest directly measured lightning currents (near 300 kA) and the largest charge transfers (hundreds of coulombs or more) are thought to be associated with positive lightning. Two types of impulsive positive current waveforms have been obse...

Identifying Underserved End-User Groups in the Provision of Climate Information

Abstract: Despite improvements in the science of climate forecasting, the application of forecasts faces key challenges. Prominent among such challenges is the fact that certain subgroupings of end users of climate information remain excluded from its potential benefits, or underserved. This paper suggests that such an omission may occur in part due to a lack of sophistication in the way the end user is viewed in the field of forecast applications research. End-user studies working both at generic and finer scales are cited, identifying reasons why certain user groups may be excluded from potential benefits of the forecast system. A case study in the Limpopo Province, South Africa, shows more specifically how one characteristic, namely gender, may determine such exclusion. The paper concludes by considering recommendations to improve inclusivity of climate information systems.

The New Digital Forecast Database of the National Weather Service

Abstract: The National Weather Service (NWS) has entered a new era in the production and dissemination of weather information and service to the nation. No longer are textual forecasts the primary medium of dissemination, but a digital database of official forecasts will be available to partners and customers by the end of 2003. The Interactive Forecast Preparation System (IFPS) is the means through which the forecasts are produced at Weather Forecast Offices (WFOs). Rather than typing forecasts, forecasters graphically prepare grids of sensible weather elements, such as surface temperature and probability of precipitation. These grids from the WFOs are then mosaicked on a central server intoa National Digital Forecast Database (NDFD), and this NDFD is provided to all who desire it. In the new forecasting paradigm, the National Centers for Environmental Prediction (NCEP) are playing a much larger role. In particular, the Hydrometeorological Prediction Center (HPC) collaborates with WFOs, and together HPC a...

Real-Time Mesoscale Modeling Over Antarctica: The Antarctic Mesoscale Prediction System*

Abstract: In support of the United States Antarctic Program (USAP), the National Center for Atmospheric Research and the Byrd Polar Research Center of The Ohio State University have created the Antarctic Mesoscale Prediction System (AMPS): an experimental, real-time mesoscale modeling system covering Antarctica. AMPS has been designed to serve flight forecasters at McMurdo Station, to support science and operations around the continent, and to be a vehicle for the development of physical parameterizations suitable for polar regions. Since 2000, AMPS has been producing high-resolution forecasts (grids to 3.3 km) with the “Polar MM5,” a version of the fifth-generation Pennsylvania State University-NCAR Mesoscale Model tuned for the polar atmosphere. Beyond its basic mission of serving the USAP flight forecasters at McMurdo, AMPS has assisted both in emergency operations to save lives and in programs to explore the extreme polar environment. The former have included a medical evacuation from the South Pole and a marin...

Regional Environmental Prediction Over the Pacific Northwest

Abstract: This paper examines the potential of regional environmental prediction by focusing on the local forecasting effort in the Pacific Northwest. A consortium of federal, state, and local agencies have funded the development and operation of a multifaceted numerical prediction system centered at the University of Washington that includes atmospheric, hydrologic, and air quality models, the collection of real-time regional weather data sources, and a number of real-time applications using both observations and model output. The manuscript reviews northwest modeling and data collection systems, describes the funding and management system established to support and guide the effort, provides some examples of regional real-time applications, and examines the national implications of regional environmental prediction.

THE JET2000 PROJECT Aircraft Observations of the African Easterly Jet and African Easterly Waves

Abstract: Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight C130 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ. Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale ...

Upward Electrical Discharges From Thunderstorm Tops

Abstract: A variety of storm top electrical discharges have been observed using several types of low-light imagers, film, and the human eye. Recently, a video recorded an unprecedented, bright blue upward discharge from a tropical thunderstorm top near Puerto Rico. The event reached the base of the ionosphere. The horizontal dimensions of cloud top discharges can range from 100 m to several kilometers. Upward extents vary from 100 m to 70 km. Shapes include “points” of light, upwardly flaring trumpets, and narrow, vertical, lightning-like channels, some topped with expanding blue, flame-like features. Visual appearances range from brilliant white lightning-like channels to a grainy, almost particulate appearing jets of dim blue light, and sometimes as a blue flame within which a brilliant white channel appears. The classical blue jet is at the lower limit of human night vision whereas some upward discharges have been clearly seen during daylight. Cloud top “pixies” last no longer than 16.7 ms, whereas upward lightn...

Climate-Change Scenarios for Water Planning Studies: Pilot Applications in the Pacific Northwest

Abstract: NOVEMBER 2003 AMERICAN METEOROLOGICAL SOCIETY | espite uncertainties in predictions of the magnitude and timing of anthropogenic climate change, many Pacific Northwest (PNW) water managers now recognize climate change as a significant issue that should be addressed in water resources planning. This represents a significant change over the last several years. These managers are requesting detailed information about potential climate impacts on their systems, in a form suitable for inclusion in planning. Academic climate-change assessments, which have been available since the mid-1980s, often produce information that is inconsistent with the specific periods of the historic streamflow record and/or the internally developed water management models used by individual water management agencies in formal water planning studies. While water managers may find an academic assessment of a system’s sensitivity to climate change (produced by coupling academically developed climate, hydrologic, and water management models) to be perfectly believable in the abstract, they are unlikely to consider such an assessment directly relevant to the decisions they make about the system they manage. This is one reason that water-planning studies have rarely addressed the implications of climate change in any more than a very cursory manner. We therefore believe that water management agencies would be more apt to incorporate climate change in their planning in the short term if they had access to scenarios of future streamflow that could easily and inexpensively be used within the confines of their existing planning frameworks. A few water management agencies in the PNW have taken the lead in incorporating climate change in their planning efforts. The Portland (Oregon) Water Bureau, for example, recently completed a study in partnership with the University of Washington’s Climate Impacts Group (CIG) that evaluated several long-range planning alternatives under four different scenarios of future climate. Seattle Public Utilities (which manages Seattle’s water supply) has begun a similar planning process. This paper describes how we are working to expand the availability of climate-change streamflow scenarios that would allow more planners to assess the impacts of climate change on PNW water systems. In the pilot project described here, streamflow scenarios are being generated to support two large-scale PNW water-planning studies.

CCI/CLIVAR Workshop to Develop Priority Climate Indices

Abstract: Africa remains an area of relatively little analysis of long-term variability and trends in climate extremes. In February 2001 a workshop on developing climate indices using daily climate data was held in Casablanca, Morocco. The workshop was sponsored by the World Meteorological Organization and included representatives from more than 20 African nations. Each person brought daily climate data from stations within their countries and received a series of lectures and training on climate data issues including data quality, homogeneity, and methods of analysis of temperature and precipitation extremes. The workshop produced some of the first analyses of changes in observed climate extremes for this part of the world.

An Improved Strategy for the Evaluation of Cloud Parameterizations in GCMS

Abstract: The parameterization schemes used to represent clouds in general circulation models have significantly evolved in their complexity over the last 10 yr. This increases the demand for a thorough evaluation of their performance. Several techniques ranging from the evaluation of the model climate to single column modeling have been proposed for that purpose. This paper aims to provide a strategy for an improved, more coherent use of these techniques. An overview of the different techniques is given using examples from the evaluation of the global model of the European Centre for Medium-Range Weather Forecasts. Advantages and disadvantages of the individual methods are highlighted. The paper closes by proposing a strategy to join the different techniques into a coherent procedure of cloud parameterization evaluation.