Brian A. Colle

CMIP5 Climate Model Analyses: Climate Extremes in the United States

This is the fourth in a series of four articles on historical and projected climate extremes in the United States. Here, we examine the results of historical and future climate model experiments from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) based on work presented at the World Climate Research Programme (WCRP) Workshop on CMIP5 Climate Model Analyses held in March 2012. Our analyses assess the ability of CMIP5 models to capture observed trends, and we also evaluate the projected future changes in extreme events over the contiguous Unites States. Consistent with the previous articles, here we focus on model-simulated historical trends and projections for temperature extremes, heavy precipitation, large-scale drivers of precipitation variability and drought, and extratropical storms. Comparing new CMIP5 model results with earlier CMIP3 simulations shows that in general CMIP5 simulations give similar patterns and magnitudes of future temperature and precipitation extremes in the Unite...

Sensitivity of Orographic Precipitation to Changing Ambient Conditions and Terrain Geometries: An Idealized Modeling Perspective

Abstract This paper utilizes the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) mesoscale model (MM5) in a two-dimensional (2D) configuration at 4-km horizontal grid spacing in order to better understand the relationship between orographic precipitation and the height and width of a barrier, as well as the ambient flow, uniform moist static stability, and freezing level. The focus is on how these parameters affect the orographic precipitation by changing the circulation and microphysical structures over the barrier. As the low-level flow becomes blocked for moist nondimensional mountain heights greater than 3.0, there is a rapid upstream shift in the precipitation maximum and a reduction in precipitation over the upper windward slope. For the terrain geometries used in this study (500 to 3500 m high and 25- to 50-km half-width), the maximum precipitation is a strong function of barrier slope for relatively weak upstream flow (U = 10 m s−1). For moderate ...

MM5 Precipitation Verification over the Pacific Northwest during the 1997–99 Cool Seasons*

Abstract Precipitation forecasts from the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) are verified for the 1997–98 and 1998–99 cool seasons over the Pacific Northwest. The MM5 precipitation at 36-, 12-, and 4-km horizontal resolution is compared with over 250 NOAA cooperative observer, snow telemetry (SNOTEL), avalanche, and National Weather Service sites in order to evaluate the effects of increasing horizontal resolution and to document spatial variations in model skill. A noticeable improvement in bias, equitable threat, and root-mean-square (rms) error scores occurs as horizontal resolution is increased from 36- to 12-km resolution; however, going from 12- to 4-km resolution improvements in skill are restricted to the heavy precipitation events (>5.08 cm in 24 h). For light to moderate precipitation events, both the 12- and 4-km domains have significant overprediction over the upper windward slopes of the higher terrain. In contrast, fo...

North American Climate in CMIP5 Experiments. Part I: Evaluation of Historical Simulations of Continental and Regional Climatology*

AbstractThis is the first part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the historical simulations of continental and regional climatology with a focus on a core set of 17 models. The authors evaluate the models for a set of basic surface climate and hydrological variables and their extremes for the continent. This is supplemented by evaluations for selected regional climate processes relevant to North American climate, including cool season western Atlantic cyclones, the North American monsoon, the U.S. Great Plains low-level jet, and Arctic sea ice. In general, the multimodel ensemble mean represents the observed spatial patterns of basic climate and hydrological variables but with large variability across models and regions in the magnitude and sign of errors. No single model stands out as being particularly better or worse across all analyses, although some models consistently outperform the others for certain variab...

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...

North American Climate in CMIP5 Experiments: Part III: Assessment of Twenty-First-Century Projections*

AbstractIn part III of a three-part study on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the authors examine projections of twenty-first-century climate in the representative concentration pathway 8.5 (RCP8.5) emission experiments. This paper summarizes and synthesizes results from several coordinated studies by the authors. Aspects of North American climate change that are examined include changes in continental-scale temperature and the hydrologic cycle, extremes events, and storm tracks, as well as regional manifestations of these climate variables. The authors also examine changes in the eastern North Pacific and North Atlantic tropical cyclone activity and North American intraseasonal to decadal variability, including changes in teleconnections to other regions of the globe. Projected changes are generally consistent with those previously published for CMIP3, although CMIP5 model projections differ importantly from those of CMIP3 in some aspects, inc...

The 5-9 February 1996 Flooding Event over the Pacific Northwest: Sensitivity Studies and Evaluation of the MM5 Precipitation Forecasts

This paper describes the flooding event of 5‐9 February 1996 in which a series of landfalling Pacific storms brought 30‐70 cm of rain to many mountain sites over southwest Washington and northwest Oregon. This event was simulated at 36-, 12-, 4-, and 1.33-km horizontal resolution using the Pennsylvania State University‐National Center for Atmospheric Research mesoscale model (MM5). The model precipitation was verified with over 300 rain gauges in western Washington and Oregon as well as WSR-88D radar data from Portland, Oregon. There was a significant improvement in the precipitation forecast skill as the grid spacing was decreased from 36 to 4 km; however, the 12- and 4-km resolutions had excessive precipitation shadowing in the lee of barriers. Although increasing resolution from 4 to 1.33 km did not produce a significant improvement in precipitation skill for the entire domain, the 1.33-km domain had more precipitation in the immediate lee of the Cascades, and thus verified better in those regions. Additional simulations explored the effects of changing vertical resolution. Enhancing the number of levels from 29 to 38 increased the model precipitation over the windward slopes of the Cascades by 10%‐30%; further increasing the number of levels to 57 decreased windward precipitation to the 29-level amounts. The leeside precipitation varied by 20%‐80% for the different vertical resolutions as a result of variations in mountain wave structure over the Cascades. Five different MM5 microphysical schemes were compared for a 24-h period during the February 1996 flooding event. The warm rain scheme dumped too much precipitation along the windward slopes, underlining the importance of ice microphysics during the cool season. The most sophisticated scheme (i.e., graupel scheme) also did not provide the best forecast. An additional microphysical sensitivity run, in which the snow fall speed was reduced by approximately 20%, improved model skill by advecting more precipitation advecting to the lee of the Cascades. Overall, these microphysical results suggest that further improvements to microphysical schemes are needed in order to more accurately predict precipitation.

The Structure and Evolution of Cold Surges East of the Rocky Mountains

Abstract Northerly surges of cold air often move southward along the eastern side of the Rockies from southern Canada into Mexico. The strongest surges, which generally develop in midwinter, are associated with temperature decreases and pressure rises of 20°–30°C and 15–30 mb, respectively, within 24 h. Surges are usually accompanied by a meridionally elongated pressure ridge and strong low-level ageostrophic winds that parallel the terrain. The width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters Mexico. This paper provides a detailed description of a northerly surge to the east of the Rocky Mountains that occurred on 12–14 November 1986. Using both observational and model data, the structural evolution of the surge is analyzed; in addition, the dynamics of the event is explored by diagnosing the momentum, thermodynamic energy, and vorticity equations. To determine the typical synoptic-scale evolution of these...

The Limitations of the WSR-88D Radar Network for Quantitative Precipitation Measurement over the Coastal Western United States

An objective assessment of the WSR-88D radar coverage for detection and quantitative measurement of precipitation over the U.S. west coast is presented. As a result of significant terrain blockage, shallow precipitation, and low freezing levels, only one-fourth to one-third of the land surface in the region has sufficient radar coverage for precipitation estimation. Furthermore, it was found that the radar coverage is not representative of the precipitation distribution, with poor radar coverage in the regions where the most rainfall occurs. Radar-derived storm-total precipitation estimates from the Portland, Oregon, radar for the catastrophic flood of February 1996 illustrate the limitations of the network, showing that the radar estimates in the heaviest precipitation regions to be less than 50% of the rain gauge values. A comparison of the WSR-88D coverage with the regional rain gauge network reveals that rain gauges will continue to be the major source of precipitation data over most of the region.

Electric power from offshore wind via synoptic-scale interconnection

World wind power resources are abundant, but their utilization could be limited because wind fluctuates rather than providing steady power. We hypothesize that wind power output could be stabilized if wind generators were located in a meteorologically designed configuration and electrically connected. Based on 5 yr of wind data from 11 meteorological stations, distributed over a 2,500 km extent along the U.S. East Coast, power output for each hour at each site is calculated. Each individual wind power generation site exhibits the expected power ups and downs. But when we simulate a power line connecting them, called here the Atlantic Transmission Grid, the output from the entire set of generators rarely reaches either low or full power, and power changes slowly. Notably, during the 5-yr study period, the amount of power shifted up and down but never stopped. This finding is explained by examining in detail the high and low output periods, using reanalysis data to show the weather phenomena responsible for steady production and for the occasional periods of low power. We conclude with suggested institutions appropriate to create and manage the power system analyzed here.