The Rapid Update Cycle (RUC), an operational regional analysis–forecast system among the suite of models at the National Centers for Environmental Prediction (NCEP), is distinctive in two primary aspects: its hourly assimilation cycle and its use of a hybrid isentropic–sigma vertical coordinate. The use of a quasi-isentropic coordinate for the analysis increment allows the influence of observations to be adaptively shaped by the potential temperature structure around the observation, while the hourly update cycle allows for a very current analysis and short-range forecast. Herein, the RUC analysis framework in the hybrid coordinate is described, and some considerations for high-frequency cycling are discussed. A 20-km 50-level hourly version of the RUC was implemented into operations at NCEP in April 2002. This followed an initial implementation with 60-km horizontal grid spacing and a 3-h cycle in 1994 and a major upgrade including 40-km horizontal grid spacing in 1998. Verification of forecasts...

An Hourly Assimilation–Forecast Cycle: The RUC

Precipitation is a key link in the global water cycle and a proxy for changing climate; therefore, proper assessment of the urban envi- ronment's impact on precipitation (land use, aerosols, thermal properties) will be increasingly important in ongoing climate diagnostics and prediction, Glob- al Water and Energy Cycle (GWEC) analysis and modeling, weather forecast- ing, freshwater resource management, urban planning-design, and land- atmosphere-ocean interface processes. These facts are particularly critical if current projections for global urban growth are accurate. The goal of this paper is to provide a concise review of recent (1990-present) studies related to how the urban environment affects precipitation. In addition to providing a synopsis of current work, recent findings are placed in context with historical investigations such as Metropolitan Meteorological Experiment (METROMEX) studies. Both observational and modeling studies of urban- induced rainfall are discussed. Additionally, a discussion of the relative roles of urban dynamic and microphysical (e.g., aerosol) processes is presented. The paper closes with a set of recommendations for what observations and capa- bilities are needed in the future to advance our understanding of the processes.

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A Review of Current Investigations of Urban-Induced Rainfall and Recommendations for the Future

Data from the Tropical Rainfall Measuring Mission (TRMM) satellite's precipitation radar (PR) were employed to identify warm-season rainfall (1998–2000) patterns around Atlanta, Georgia; Montgomery, Alabama; Nashville, Tennessee; and San Antonio, Waco, and Dallas, Texas. Results reveal an average increase of about 28% in monthly rainfall rates within 30–60 km downwind of the metropolis, with a modest increase of 5.6% over the metropolis. Portions of the downwind area exhibit increases as high as 51%. The percentage changes are relative to an upwind control area. It was also found that maximum rainfall rates in the downwind impact area exceeded the mean value in the upwind control area by 48%–116%. The maximum value was generally found at an average distance of 39 km from the edge of the urban center or 64 km from the center of the city. Results are consistent with the Metropolitan Meteorological Experiment (METROMEX) studies of St. Louis, Missouri, almost two decades ago and with more recent stud...

/pdf/rainfall-modification-by-major-urban-areas-observations-from-1rzgu1oys6.pdf

Rainfall Modification by Major Urban Areas: Observations from Spaceborne Rain Radar on the TRMM Satellite

The International H2O Project (IHOP_2002) is one of the largest North American meteorological field experiments in history. From 13 May to 25 June 2002, over 250 researchers and technical staff from the United States, Germany, France, and Canada converged on the Southern Great Plains to measure water vapor and other atmospheric variables. The principal objective of IHOP_2002 is to obtain an improved characterization of the time-varying three-dimensional water vapor field and evaluate its utility in improving the understanding and prediction of convective processes. The motivation for this objective is the combination of extremely low forecast skill for warm-season rainfall and the relatively large loss of life and property from flash floods and other warm-season weather hazards. Many prior studies on convective storm forecasting have shown that water vapor is a key atmospheric variable that is insufficiently measured. Toward this goal, IHOP_2002 brought together many of the existing operational and new st...

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An Overview of the International H2O Project (IHOP_2002) and Some Preliminary Highlights

Herein preliminary findings are reported from a radar-based climatology of warm season precipitation ‘‘episodes.’’ Episodes are defined as time‐space clusters of heavy precipitation that often result from sequences of organized convection such as squall lines, mesoscale convective systems, and mesoscale convective complexes. Episodes exhibit coherent rainfall patterns, characteristic of propagating events, under a broad range of atmospheric conditions. Such rainfall patterns are most frequent under ‘‘weakly forced’’ conditions in midsummer. The longevity of episodes, up to 60 h, suggests an intrinsic predictability of warm season rainfall that significantly exceeds the lifetime of individual convective systems. Episodes are initiated primarily in response to diurnal and semidiurnal forcings. Diurnal forcing is dominant near the Rocky and Appalachian Mountains, whereas semidiurnal forcing is dominant between these cordilleras. A most common longitude of origin is at or near the east slope of the Continental Divide (1058W). These observations are consistent with a condition of continual thermal forcing, widespread hydrodynamic instability, and the existence of other processes that routinely excite, maintain, and regenerate organized convection. The propagation speed of major episodes is often in excess of rates that are easily attributable either to the phase speeds of large-scale forcing or to advection from low- to midlevel ‘‘steering’’ winds. It is speculated that wavelike mechanisms, in the free troposphere and/or the planetary boundary layer, may contribute to the rates of motion observed. Once understood, the representation of such mechanisms in forecast models offers the opportunity for improved predictions of warm season rainfall.

Inferences of Predictability Associated with Warm Season Precipitation Episodes

This paper reviews the status of forecasting convective precipitation for time periods less than a few hours (nowcasting). Techniques for nowcasting thunderstorm location were developed in the 1960s and 1970s by extrapolating radar echoes. The accuracy of these forecasts generally decreases very rapidly during the first 30 min because of the very short lifetime of individual convective cells. Fortunately more organized features like squall lines and supercells can be successfully extrapolated for longer time periods. Physical processes that dictate the initiation and dissipation of convective storms are not necessarily observable in the past history of a particular echo development; rather, they are often controlled by boundary layer convergence features, environmental vertical wind shear, and buoyancy. Thus, successful forecasts of storm initiation depend on accurate specification of the initial thermodynamic and kinematic fields with particular attention to convergence lines. For these reasons ...

https://opensky.ucar.edu/islandora/object/articles%3A3309/datastream/PDF/view

Nowcasting Thunderstorms: A Status Report

The Auto-Nowcast System (ANC), a software system that produces time- and space-specific, routine (every 5 min) short-term (0‐1 h) nowcasts of storm location, is presented. A primary component of ANC is its ability to identify and characterize boundary layer convergence lines. Boundary layer information is used along with storm and cloud characteristics to augment extrapolation with nowcasts of storm initiation, growth, and dissipation. A fuzzy logic routine is used to combine predictor fields that are based on observations (radar, satellite, sounding, mesonet, and profiler), a numerical boundary layer model and its adjoint, forecaster input, and feature detection algorithms. The ANC methodology is illustrated using nowcasts of storm initiation, growth, and dissipation. Statistical verification shows that ANC is able to routinely improve over extrapolation and persistence.

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NCAR Auto-Nowcast System

The kinematic and thermodynamic structures of a thunderstorm outflow are examined by means of dual Doppler radar analysis, mesonet, lower, and sounding data. The data were collected in the Denver, Colorado area during June 1984. The dual-Doppler analysis shows that the cold outflow is ducted beneath the PBL inversion. Along the gust front there is a narrow quasi-two-dimensional updraft. Kelvin-Helmholtz instability (KHI) developed along the top of the gust front head near the surface front, and propagated backwards, dissipating in the wake of the head region. An isothermal layer aloft appears to have limited billow amplification to the quasi-neutral layer below. The gust front's leading edge had numerous inflections which are believed to result from barotrophic instabilities. Small vortices develop at some of the inflection points. Detailed analysis of one such circulation shows evidence of the formation of two enhanced updrafts separated by an occlusion downdraft. These observations are the firs...

Dynamics of a Thunderstorm Outflow

The initiation of thunderstorms is examined through a combined observational and modeling case study. The study is based on Doppler radar, aircraft, mesonet, balloon sounding, and profiler and photographic data from the Convection Initiation and Downburst Experiment (CINDE) conducted near Denver, Colorado. The study examines the initiation of a line of thunderstorms that developed along a preexisting, quasi-stationary boundary-layer convergence line on 17 July 1987. The storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present. The primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection. The critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line. The effect was to cause the updrafts in the convergence line ...

https://opensky.ucar.edu:/islandora/object/articles%3A3268/datastream/PDF/view

The Role of Boundary-Layer Convergence Zones and Horizontal Rolls in the Initiation of Thunderstorms: A Case Study

This paper reports on experimental space-specific 30-min nowcasts of thunderstorm initiation, evolution, and movement. The experiments were conducted near Denver, Colorado, with the purpose of providing weather information for planning purposes to air traffic control managers. The nowcasts were based primarily on Doppler weather radar observations of the clear-air boundary layer, storm reflectivity, storm Doppler velocity structure, and visual observations of clouds. The forecasters found that they could often anticipate thunderstorm initiation by monitoring radar-detected boundary-layer convergence lines together with monitoring visual observations of cloud development in the vicinity of the convergence lines. Nowcast procedures and nowcast results for experiments in 1989 and 1990 are presented. The procedures are based on research experiments and exploratory field tests conducted since 1984. The forecaster results were better than persistence or extrapolation forecasts because of the ability to...

Cynthia K. Mueller

Papers

Nowcasting Thunderstorms: A Status Report

NCAR Auto-Nowcast System

Dynamics of a Thunderstorm Outflow

The Role of Boundary-Layer Convergence Zones and Horizontal Rolls in the Initiation of Thunderstorms: A Case Study

Nowcasts of Thunderstorm Initiation and Evolution