Christopher M. Godfrey

Diagnostic Verification of the IRI Net Assessment Forecasts, 1997-2000

The International Research Institute (IRI) for Climate Prediction produces operational outlooks for seasonal (3-month periods) average temperature and for total precipitation, at lead times of 0 and 3 months (Mason et al. 1999). These outlooks are probabilistic in nature and subjectively produced. During the 1997‐2000 period considered here, two seasonal forecasts, at 0- and 3month lead times, were produced 4 times per year. The forecast quantities are three-dimensional vectors specifying probabilities of temperature or precipitation outcomes falling in the lower (‘‘below normal’’), middle (‘‘near normal’’), and upper (‘‘above normal’’) thirds of the respective climatological distributions appropriate to particular seasons and locations; at global land (excluding Antarctica) and nearby ocean locations. For precipitation, these IRI forecasts began in late 1997, and are available for October‐November‐December (OND) 1997 and January‐February‐March (JFM) 1998 onward, for the 0- and 3-month leads, respectively. The temperature forecasts began one season later, and

Soil Temperature and Moisture Errors in Operational Eta Model Analyses

Proper partitioning of the surface heat fluxes that drive the evolution of the planetary boundary layer in numerical weather prediction models requires an accurate specification of the initial state of the land surface. The National Centers for Environmental Prediction (NCEP) operational Eta Model is used to produce land surface analyses by continuously cycling soil temperature and moisture fields. These fields previously evolved only in response to radiation budget constraints and modeled precipitation, but NCEP recently upgraded the self-cycling process so that soil fields respond instead to the radiation budget and observed precipitation. A comparison of 0000 and 1200 UTC Eta Model analyses of soil temperature and moisture at several soil depths with observations from the Oklahoma Mesonet during 2004 and 2005 shows that there are strong biases in soil temperature and a severe underestimation of soil moisture at all depths. After the change to a new assimilation scheme, there is notable improvement in the magnitude of the analyzed soil moisture fields, although a strong dry bias persists in the soil moisture field. A simple one-layer slab soil model quantifies the effect of such soil moisture errors on the diurnal cycle of soil temperature and reveals that these soil moisture errors alone may account for only 1.6°C increases in predicted maximum soil temperatures during the day and temperature reductions of the same magnitude at night. The much larger remaining soil temperature errors possibly stem from documented problems with the solar radiation and longwave parameterizations within the Eta Model.

An Empirical Latent Heat Flux Parameterization for the Noah Land Surface Model

Abstract Proper partitioning of the surface energy fluxes that drive the evolution of the planetary boundary layer in numerical weather prediction models requires an accurate representation of initial land surface conditions. Unfortunately, soil temperature and moisture observations are unavailable in most areas and routine daily estimates of vegetation coverage and biomass are not easily available. This gap in observational capabilities seriously hampers the evaluation and improvement of land surface parameterizations, since model errors likely relate to improper initial conditions as much as to inaccuracies in the parameterizations. Two unique datasets help to overcome these difficulties. First, 1-km fractional vegetation coverage and leaf area index values can be derived from biweekly maximum normalized difference vegetation index composites obtained from daily observations by the Advanced Very High Resolution Radiometer onboard NOAA satellites. Second, the Oklahoma Mesonet supplies multiple soil tempe...

Is the January Thaw a Statistical Phantom

The existence of the January thaw, a purported systematic anomalous warming in daily mean temperatures at northeastern U.S. stations during late January, is investigated quantitatively. A key idea in the analysis is that winter temperatures are intrinsically more variable, and this property must be accounted for when judging the unusualness of excursions of daily mean temperatures from a smooth climatic mean function. Accordingly the daily mean temperature departures are expressed nondimensionally by dividing by appropriate standard deviations that vary through the year. The warm excursion in observed records for late January is not always the most extreme such excursion in the nondimensionalized data, even when the definition of “excursion” is optimized to emphasize the late January event. Hypothesis tests based on time series models with smoothly varying climatologies (i.e., with no anomalous features such as the January thaw, by construction) are used to evaluate the statistical significance of the obs...

First Steps Toward Defining the Wind Disturbance Regime in Central Hardwoods Forests

Wind disturbance is one of the most prevalent natural disturbances in the Central Hardwoods Region (CHR). All ecoregions within the CHR are subject to a greater or lesser degree to tornado, derecho or thunderstorm wind damage, with an east-to-west increase in the importance of tornadoes and derechos. At the regional scale, hurricanes decrease in importance with distance from the Atlantic and Gulf coasts. The disturbed patch sizes created by these various storms include occasional very large (e.g., >25 ha) patches, but the great majority are a few ha or less, perhaps differing from common visual impressions. Hurricane and derecho disturbance patterns appear to be more predictable in relation to topographic features, whereas tornado damage is much more stochastic. All wind disturbance types cause greater damage to larger trees, and most studies reveal interspecific differences in levels of wind damage, although such patterns are not always consistent among studies or locations. Wind disturbance commonly advances succession in second-growth forest but may set succession back in primary forests. The greatest research needs are landscape-scale patterns of damage; relationships of damage to topography and soils; clarifying the tree characteristics (e.g., architecture, wood strength, rooting depth) that underlie interspecific differences in vulnerability; and documenting ecosystem effects of wind disturbance.

Estimating Enhanced Fujita Scale Levels Based on Forest Damage Severity

AbstractEnhanced Fujita (EF) scale estimates following tornadoes remain challenging in rural areas with few traditional damage indicators. In some cases, such as the 27 April 2011 tornadoes that passed through mostly inaccessible terrain in the Great Smoky Mountains National Park and the Chattahoochee National Forest in the southeastern United States, traditional ground-based tornado damage surveys are nearly impossible. This work presents a novel method to infer EF-scale categories in forests using levels of tree damage and a coupled wind and tree resistance model. High-resolution aerial imagery allows detailed analyses based on a field of nearly half a million trees labeled with their geographic location and fall direction. Ground surveys also provide details on the composition of tree species and tree diameters within each tornado track. A statistical resampling procedure randomly draws a sample of trees from this database of observed trees. The coupled wind and tree resistance model determines the per...

Severe Weather Field Experience: An Undergraduate Field Course on Career Enhancement and Severe Convective Storms

Abstract Undergraduate students acquire a deeper understanding of scientific principles through first-hand experience. To enhance the learning environment for atmospheric science majors, the University of North Carolina at Asheville has developed the severe weather field experience. Participants travel to Tornado Alley in the Great Plains to forecast and observe convective storms for two weeks. The objectives of the course encompass far more than observing severe storms. On days with non-threatening weather in the Great Plains, students participate in an array of activities that provide exposure to facilities and interaction with professionals in various sectors of meteorology. While the allure of chasing storms initially prompts the students to enroll in the course, the focused career-development aspect of the curriculum increases awareness for the varied career options in the atmospheric sciences and helps students discover where their own capabilities and interests might best suit the discipline. The course thus offers students a comprehensive career-development experience woven within a thrilling adventure.