Genesis Of Atlantic Lows Experiment Research Articles

Application of multilayer feedforward neural networks to precipitation cell-top altitude estimation

The use of passive 118-GHz O/sub 2/ observations of rain cells for precipitation cell-top altitude estimation is demonstrated by using a multilayer feedforward neural network retrieval system. Rain cell observations at 118 GHz were compared with estimates of the cell-top altitude obtained by optical stereoscopy. The observations were made with 2-4-km horizontal spatial resolution by using the millimeter-wave temperature sounder (MTS) scanning spectrometer aboard the NASA ER-2 research aircraft during the Genesis of Atlantic Lows Experiment (GALE) and the Cooperative Huntsville Meteorological Experiment (COHMEX) in 1986. The neural network estimator applied to MTS spectral differences between clouds, and nearby clear air yielded an rms discrepancy of 1.76 km for a combined cumulus, mature, and dissipating cell set and 1.44 km for the cumulus-only set. An improvement in rms discrepancy to 1.36 km was achieved by including additional MTS information on the absolute atmospheric temperature profile. An incremental method for training neural networks was developed that yielded robust results, despite the use of as few as 56 training spectra. Comparison of these results with a nonlinear statistical estimator shows that superior results can be obtained with a neural network retrieval system. Imagery of estimated cell-top altitudes was created from 118-GHz spectral imagery gathered from CAMEX, September through October 1993, and from cyclone Oliver, February 7, 1993.

Mesoscale Analysis of a Carolina Coastal Front

During the Intensive Observation Period (IOP) 7 (22 February 1986) of the Genesis of Atlantic Lows Experiment a persistent coastal front was observed along the Carolina coast in the eastern United States. An intensive baroclinic zone, associated with the cold air damming to the east of the Appalachian Mountains, and the warm marine atmospheric boundary layer over the Gulf Stream, resulted in a northeasterly low-level geostrophic wind maximum near the coast. Two convergence zones were observed, one near the shore and the other near the western edge of the Gulf Stream. The convergence zone near the coastline was relatively weaker than that near the Gulf Stream. The differential surface thermal forcing caused enhanced convergence associated with the frontogenesis, and a meso-low was observed over the offshore front. The terms in the frontogenesis equation are estimated, and the diabatic term is found to be larger than the frontogenetic confluence term along the shore.

Nutrients and chlorophyll at the shelf break off the southeastern United States during the Genesis of Atlantic Lows Experiment: Winter 1986

The outer shelf and upper slope off Charleston, South Carolina, were the site of oceanographic and meteorological measurements during the winter of 1986. The purpose of the study was to test ideas about front formation, heat transport, and stratification during cold air outbreaks. An ancillary part of the study was the observation of nutrients and chlorophyll concentrations. The observations extended across the shelf and sometimes crossed the Gulf Stream front. The results show slightly elevated nitrate concentrations in outer shelf waters (1–2μM NO3) with chlorophyll concentrations in the 1–1.8μg L−1 range. When effects of Gulf Stream frontal eddies are eliminated, the estimated wintertime wind‐driven transport of nutrients from the deep ocean to the shelf is comparable to that observed during spring and summer. Thus significant onshore nutrient transport can occur throughout the year in this region.

Effects of along-shore wind stress and surface cooling on the formation of shelf sea fronts in a simple air-sea interaction model

This paper presents numerical simulation results from a set of control and sensitivity experiments on the effects of winter-time air-sea interaction on the variability of sea surface temperature (SST) on a two-dimensional continental shelf which is uniform in the along-shore direction and is bounded to the west by a straight coast and to the east by a prescribed Gulf Stream front. In the control experiment, the model ocean circulation is driven by a time-dependent wind forcing which is parameterically coupled to the cross-shelf SST gradient. In the sensitivity experiments, wind stress, diabatic cooling and air-sea coupling are turned on separately to estimate the individual contribution of each effect to the cross-shelf SST variation. Experiments have also been carried out for different coupling strengths and diabatic cooling rates to examine the model sensitivity to these paramenters. The model results indicate that air-sea interaction could induce a secondary SST front on the shelf. Comparisons of model results with observations obtained during the Genesis of Atlantic Lows Experiment conducted off the east coasts of the Carolinas during January and February 1986 qualitatively confirm our finding.

Validity of similarity relations over the Gulf Stream

Observations obtained from aircraft, buoys, ships and vertical soundings made on 9, 10 and 22 February, 1986 during the Genesis of Atlantic Lows Experiment (GALE) are used to study the mean marine boundary-layer (MBL) structure over the cold coastal waters and the warm Gulf Stream. Different synoptic conditions prevailed on each day. The synoptic setting was characterized by prestorm conditions on 9 February and meso-low development on 10 and again on 22 February. Two convergence zones, one near the Gulf Stream and the other close to the coast were observed on both the days of cyclogenesis causing different stability conditions in the MBL near the coast and the Gulf Stream. The boundary-layer on 9 February was however well mixed. Temperature and humidity variances during prestorm conditions (9 February) generally followed the free convection relations. But on 10 and 22 February, in the presence of a meso-low, similarity relations do not appear to be valid for temperature and humidity variances and fluxes. DOI: 10.1034/j.1600-0870.1996.t01-2-00002.x

A Regional Model Intercomparison Using a Case of Explosive Oceanic Cyclogenesis

The authors evaluate the performance of current regional models in an intercomparison project for a case of explosive secondary marine cyclogenesis occurring during the Canadian Atlantic Storms Project and the Genesis of Atlantic Lows Experiment of 1986. Several systematic errors are found that have been identified in the refereed literature in prior years. There is a high (low) sea level pressure bias and a cold (warm) tropospheric temperature error in the oceanic (continental) regions. Though individual model participants produce central pressures of the secondary cyclone close to the observed during the final stages of its life cycle, systematically weak systems are simulated during the critical early stages of the cyclogenesis. Additionally, the simulations produce an excessively weak (strong) continental anticyclone (cyclone); implications of these errors are discussed in terms of the secondary cyclogenesis. Little relationship between strong performance in predicting the mass field and skill in predicting a measurable amount of precipitation is found. The bias scores in the precipitation study indicate a tendency for all models to overforecast precipitation. Results for the measurable threshold (0.2 mm) indicate the largest gain in precipitation scores results from increasing the horizontal resolution from 100 to 50 km, with a negligible benefit occurring as a consequence of increasing the resolution from 50 to 25 km. The importance of a horizontal resolution increase from 100 to 50 km is also generally shown for the errors in the mass field. However, little improvement in the prediction of the cyclogenesis is found by increasing the horizontal resolution from 50 to 25 km.

Small-Scale Structure of a Coastal Front as Revealed by Dual-Doppler Radar

Abstract The analysis of the rainband structure and wind fields associated with a coastal front along the North Carolina shoreline is described. Dual-Doppler radar and the augmented GALE (Genesis of Atlantic Lows Experiment) ensemble of in situ stations depict shallow, convective rainbands that overtake the front from the warm-air sector and intensify at the surface front location. Clockwise band rotation is shown to be caused by the difference in alignment between the echo motion and the rainband axes and by new development ahead of the front. Radar measurements depict the circulation systems associated with a portion of one rainband in the cold air ahead of the front. Here shallow precipitation cores are vertically tilted due to the frontal wind shear. Circulation cells and most precipitation cores are centered just above the frontal inversion, as inferred by the wind shift line aloft. This feature is nearly horizontal in the cross-frontal direction but slopes downward in a direction roughly parallel to...

Structure of the marine atmospheric boundary layer during two cold air outbreaks of varying intensities: GALE 86

Aircraft, surface, upper air and satellite measurements have been used to observe the evolution and growth of the convective Marine Atmospheric Boundary Layer (MABL) offshore of North Carolina in close proximity to the Gulf Stream, during the intense cold air outbreak of 28 January 1986 and the moderate event of 12 February 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the ocean-atmosphere exchanges of surface turbulent sensible and latent heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed near-surface total heat fluxes were 1045 and 811 W·m−2 over the core of the Gulf Stream, for 28 January and 12 February 1986, respectively. The observed changes in the overlying air mass occurred almost instantaneously as the ambient flow traversed different underlying SST conditions.

Scales and spectra of turbulence over the gulf stream during offshore cyclogenesis

Turbulence structure of the marine boundary layer (MBL) over the Gulf Stream and the adjacent coastal waters during the development of a storm is discussed. Prestorm conditions prevailed on 9 February and a meso-low formed on 10 February which intensified into an offshore cyclone on 11 February. Observations from aircraft, buoys and ships were made as part of the Genesis of Atlantic Lows Experiment (GALE, 86) during these three days. Analysis of the high frequency (20 Hz) turbulence data collected from low-level flights by the NCAR King Air and Electra indicates the effect of the storm development on the turbulence structure of the MBL.

Impact of Assimilating SSM/I Rainfall Rates on Numerical Prediction of Winter Cyclones

Abstract A series of observing system simulation experiments (OSSE) and real data assimilation experiments were conducted to assess the impact of assimilating Special Sensor Microwave/Imager (SSM/I)-estimated rainfall rates on limited-area model predictions of intense winter cyclones. For the OSSE, the slow-moving, fronto- and cyclogenesis along the cast coast of United States during the second intensive observation period (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE) (26-28 January 1986) was selected as the test case. The perfect “observed” rainfall rates were obtained by an integration of a version of the Naval Research Laboratory (NRL) limited-area model, whereas the “forecast” was generated by a degraded version of the NRL model. A number of OSSEs were conducted in which the “observed” rainfall rates were assimilated into the “forecast” model. Rainfall rates of various data frequencies, different vertical beating profiles, various assimilation windows, and prescribed systematic errors were...

Structure and evolution of a tropopause fold during GALE IOP-1: An eta model study

A fine-mesh regional model simulation of upper-level cyclogenesis is carried out to examine the structure and evolution of the accompanying tropopause fold and its relationship to the surface and upper-level cyclones. The initial state for the simulation, conducted using the 80-km, 16-level version of the National Meteorological Center Eta model, uses the Level III-b gridded dataset for 1200 UTC, 18 January 1986, during the First Intensive Observing Period (IOP-1) of the Genesis of Atlantic Lows Experiment (GALE) project.

Nonhydrostatic Simulations of Coastal Mesobeta-Scale Vortices and Frontogenesis

Abstract A nonhydrostatic version of the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, with a horizontal resolution of 5 km, is used with measurements taken during intensive observation period 2 of the Genesis of Atlantic Lows Experiment to study the offshore mesobeta-scale coastal front structure. Results from the 24-h model simulation and Doppler radar data indicate that precipitation bands, with embedded convective elements, are present along the coastal front in the vicinity of the Gulf Stream. As the frontogenesis evolves, the simulated surface frontal zone becomes fractured, and discontinuous lines of confluence and mesoscale ascent become apparent. A collapse of the cross-frontal thermal gradient is driven by intense gradients of the surface fluxes in the vicinity of the Gulf Stream. A mesoscale wave train, consisting of a series of shallow, weak vortices with horizontal scales between 50 and 100 km, forms along the front in agreement with the Doppler radar...

Continuous Data Assimilation Experiments with the NMC Eta Model: A GALE IOP 1 Case Study

The current major expansion in observational capability of the National Weather Service is principally in the volume of asynchronous data rather than synchronous observations at the standard synoptic times. Hence, the National Meteorological Center is considering a continuous data assimilation system to replace at some time the intermittent system now used by its regional and global operational models. We describe this system, based on the Newtonian relaxation technique, as developed for the eta model. Experiments are performed for the first intensive observing period of the Genesis of Atlantic Lows Experiment (GALE) in January 1986, when strong upper-level cyclogenesis occurred, with a pronounced tropopause fold but only modest surface development. The GALE level IIIb dataset was used for initializing and updating the model. Issues addressed in the experiments include choice of update variable, number, and length of update segments; need for updating moisture and surface pressure information; nudging along boundaries; and noise control. Assimilation of data from a single level was also studied. Use of a preforecast assimilation cycle was found to eliminate the spinup problem almost entirely. Multiple, shorter assimilation segments produced better forecasts than a single, longer cycle. Updating the mass field was less effective than nudging the windmore » field but assimilating both was best. Assimilation of moisture data, surprisingly, affected the spinup adversely, but nudging the surface pressure information reduced the spurious pillow effect. Assimilation of single-level information was ineffective unless accompanied by increased vertical coupling, obtained from a control integration. 52 refs., 19 figs., 1 tab.« less

Diabatic Initialization Tests Using the Naval Research Laboratory Limited-Area Numerical Weather Prediction Model

Diabatic forcing has been incorporated into a nonlinear normal-mode initialization scheme to provide more realistic initial conditions and to alleviate the problem of the spinup time of the Naval Research Laboratory Limited-Area Numerical Weather Prediction Model. Latent heating profiles are computed from the observed rainfall and from the model-generated convective rainfall at locations where there were no observations. The latent heating is distributed in the vertical according to the cumulus convective parameterization scheme (Kuo scheme) of the model. The results of a case study from the Genesis of Atlantic Lows Experiment indicated that model spinup of forecast rainfall can be reduced when diabatic initialization with merging of heat and/or rain is used.

Application of the Bratseth Scheme for the Analysis Of GALF, Data Using a Mesoscale Model

Abstract The successive correction scheme of Bratseth, which converges to optimum interpolation, is applied for the numerical analysis of data collected during the Genesis of Atlantic Lows Experiment. A first guess for the analysis is provided by a 12-h forecast produced by integrating a limited-area model from a prior coarse operational analysis. Initially, univariate analyses of the mass and wind fields are produced. To achieve the coupling of the mass and wind fields, additional iterations on the geopotential are performed by extrapolating the geopotential to grid points, using improving estimates of the geostrophic wind. This improved geostrophic wind is then used to update the geostrophic component of the initial univariate wind analysis. Use of a background forecast produces much improved mesoscale structures in the analysis. Enhanced gradients of the geopotential and larger wind shears are the result of the coupling of the mass and wind fields, particularly in regions of lower data density. Applica...

Characteristics of Rainbands, Radar Echoes, and Lightning near the North Carolina Coast during GALE

Characteristics of mesoscale rainbands and echoes in radar reflectivity data recorded during the field phase of the Genesis of Atlantic Lows Experiment (GALE) are presented. The primary sources of data were radar microfilm and manually digitized radar (MDR) reports from the operational National Weather Service (NWS) radars at Cape Hatteras (HAT) and Wilmington (ILM), North Carolina. The dataset also included cloud-to-ground lightning flashes that were recorded by the network operated by the State University of New York at Albany. The analyses included rainbands of at least 90-km length with lifetimes of at least 2 h. Nearly all of the rainbands were within 400 km of synoptic-scale or coastal fronts. Warm-sector rainbands predominated. Rain-bands were clarified by the location of their initial detection relative to the land. coastal shelf, and Gulf Stream. Rainbands were initially identified more frequently over the Gulf Stream and ten often over the coastal shelf than the corresponding fractional arms monitored by the radars. Statistical tests determined significant differences in the sample means of the MDR and lightning data between the Gulf Stream and land regions that were largely a consequence of many more hours with MDR and lightning over the Gulf Stream. Composites relative to the beginning and ending of the rainband cases indicated that differences between the Gulf Stream and land were small shortly after the initial detection of rainbands and large just before the final detection of rainbands. The largest Gulf Stream-land disparities occurred, average, during low-level cold and dry advection at HAT. Trunk and Bosart reported a convective echo maximum over the Gulf Stream near HAT and discussed physical processes that can account for the convective maximum. Analysis of one idealized distribution of convecton, however, supports the likely role of sampling limitations of the NWS radar network in determining the location of the convective echo maximum near HAT.

A Case Study Diagnosis of the Formation of an Upper-Level Cutoff Cyclonic Circulation over the Eastern United States

Abstract The synoptic-scale evolution during the formation phase of a midtropospheric cutoff cyclonic circulation over the eastern United States is diagnosed within the potential vorticity framework using the GALE (Genesis of Atlantic Lows Experiment) case of 18–19 January 1986. The study examines 1) the precursor flow evolution prior to cutoff cyclone formation; 2) the wind, mass, and potential vorticity evolution during the 2-day period encompassing cutoff formation; and 3) the relative contribution of upper-versus lower-tropospheric forcing on the quasigeostrophic height tendency field prior to and during cutoff formation. The primary large-scale features prior to cutoff cyclone formation are an amplifying ridge over the western United States and eastern North Pacific and a diffluent trough over the central United States. The primary smaller-scale feature prior to cutoff formation is a short-wave trough-jet streak system that propagates through the longer-wave-amplifying ridge, and then intensifies upo...

A Numerical Investigation of Coastal Frontogenesis and Mesoscale Cyclogenesis during GALE IOP 2

Abstract The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of coastal frontogenesis and mesoscale cyclogenesis observed during intensive observation period 2 (IOP 2) of the Genesis of Atlantic lows Experiment (GALE). The model accurately simulates many of the observed mesoscale Features including cold-air damming to the cast of the Appalachian Mountains, a coastal trough, coastal frontogenesis, and mesoscale cyclogenesis. The coastal front becomes apparent approximately 6 h after the formation of a coastal trough in the vicinity of the Gulf Stream. An analysis of the model results indicates that both latent beating from banded precipitation over the Gulf Stream and surface sensible heating contribute to trough development. The deformation resulting from the isallobaric accelerations, associated with the pressure changes that occur as the coastal trough forms, initiates the coastal frontogenesis. Numerical sensitivity tests reveal that the diabatic process...

A Three-Dimensional Numerical Investigation Of a Carolina Coastal Low-Level Jet during GALE IOP 2

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the structure and dynamics of three low-level jets (LLJs) observed during the second intensive observation period of the Genesis of Atlantic Lows Experiment: 1) a Piedmont LLJ along the east slope of the Appalachians, 2) a coastal LLJ (the focus of this study) along the Carolina coastline, and 3) an LLJ to the rein of a cold-frontal system positioned over the Gulf Stream. Geostrophic forcing was important for the formation of the LLJs. Shallow local baroclinity near the top of the cold dome associated with the cold air dammed to the east of the Appalachian Mountains forced the Piedmont LLJ. An analysis of the model momentum tendencies reveals that the coastal LLJ developed and was maintained by strong geostrophic forcing associated with the coastal baroclinic zone, and its strength was modulated by strong, inertial accelerations. Significant horizontal structure in the coastal LLJ developed during the daytime as a result of the different vertical mixing properties associated with continental and maritime parcel source regions. Model sensitivity experiments indicate that diabatic processes substantially influence the evolution of the coastal and cold-frontal LLJs, Latent heating associated with banded precipitation over the Gulf Strum to the rear of the front was the primary Forcing mechanism for the frontal LLJ. Sensible heating within the marine atmospheric boundary layer acted to enhance the coastal baroclinic zone and low-level geostrophic forcing, and to subsequently strengthen the coastal LLJ. Cold-air damming and strong lower-tropospheric sensible and latent heating in the vicinity of the Gulf Stream, which frequently occur during autumn and winter months along the East Coast, combine to produce a favorable mesoscale environment for LLJ formation with a wind direction parallel to the coastline.

The Impact of the Sea Surface Temperature Resolution on Mesoscale Coastal Processes during GALE IOP 2

The Pennsylvania State University-NCAR Mesoscale Model is used to examine the sensitivity of the structure and evolution of mesoscale coastal phenomena to the sea surface temperature (SST) distribution in the vicinity of the Gulf Stream during intensive observation period 2 (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE). Experiments are performed with three different SST analyses: A 1) a high-resolution 14-km analysis, 2) a medium-resolution 275-km analysis, and 3) a coarse-resolution 381-km analysis. The results indicate that numerical simulations of mesoscale phenomena embedded in the marine atmospheric boundary layer (MABL) in the vicinity of the Gulf Stream are very sensitive to the SST distribution. The total (sensible and latent) average heat fluxes differ by less than 15% among the three experiments; however, the mesoscale distributions of the oceanic surface heat fluxes differ substantially. As a result of large differences in the lower-tropospheric diabatic heating, significant dissimilarities occur among the three experiments in terms of the intensity and movement of the north-wall MABL front, MABL structure, coastal front, cyclone, and precipitation. The maximum values of diagnosed quantities (e.g., vorticity, divergence, thermal gradients, and frontogenesis) in the vicinity of the Gulf Stream vary by as much as a factor of 8 among the three simulations. Also, the lower-tropospheric geostrophic forcing along the coast is relatively weak in the two simulations that used lower-resolution SST analyses. This weak geostrophic forcing in the lower-resolution SST experiments results in the development of a low-level jet that is weaker than observed and simulated in the experiment with the high-resolution analysis. Among the three experiments, the high-resolution SST analysis simulation best captures the analyzed intensity, structure, and movement of the mesoscale coastal phenomena. Thus, the use of high-resolution SST analyses in research and operational mesoscale models may be essential in some cases for the accurate prediction of coastal cyclones and their associated mesoscale structures.