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Robert H. Johns

Abstract

A climatology of severe weather outbreaks occurring in areas of the contiguous United States where the mid-troposphere flow has a north of west component has been developed for the period 1962–77. During the 16 years, 163 outbreaks of severe weather have been identified that fit a specific set of criteria for “northwest flow”. Analyses of this data set reveal the diurnal, seasonal and geographical frequencies and characteristics of this phenomenon. The nature of “northwest flow” outbreaks is examined in relation to the effect on life and property.

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Robert H. Johns

Abstract

Most observational and numerical modeling investigations into the meteorological factors affecting bow echo development in the United States have concerned long-lived events occurring during the late spring and summer. As a result, the meteorological patterns and parameter values (conceptual models) typically associated with bow echo development are biased toward the larger-scale warm season events. This note discusses the spectrum of meteorological conditions observed with bow echo development and extends the classification of associated meteorological patterns to cool season cases.

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Robert H. Johns

Abstract

A climatology of meteorological parameters and synoptic patterns associated with severe weather outbreaks occurring in arm where the mid-tropospheric flow has a north of west component is presented. This climatology utilizes data and criteria previously described by Johns. A comparison of the northwest flow parameters and those associated with general severe weather is given. The importance of conditional instability and low-level warm advection in northwest flow situations is discussed. An explanation is offered for the location of the axes of highest frequency of northwest flow outbreaks. Furthermore, the varying nature of wind shear associated with severe weather is discussed and the importance of the directional contribution of wind shear to northwest flow severe weather is demonstrated.

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Robert H. Johns and William D. Hirt

Abstract

The derecho, a widespread convectively induced windstorm, is identified and defined in terms of current nomenclature. A comprehensive dataset consisting of 70 derecho cases has been developed from the warm season months of May through August for the 4-year period 1980–1983. Analyses of this dataset reveal that the warm season derecho typically emanates from a mesoscale convective system (MCS) moving along a quasistationary, low-level thermal boundary in an environment characterized by high potential instability and relatively strong midtropospheric winds. In the continental United States these windstorms are most frequent in a zone extending from eastern South Dakota to the Upper Ohio Valley, and typically commence during the afternoon and evening hours. Particular radar and satellite imagery characteristics are associated with the derecho-spawning MCS. Based upon the meteorological parameters and synoptic patterns associated with derecho events, a decision tree has been developed to assist the operational meteorologist in anticipating derecho development.

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Robert H. Johns and Charles A. Doswell III

Abstract

Knowledge of severe local storms has been increasing rapidly in recent years as a result of both observational studies and numerical modeling experiments. This paper reviews that knowledge as it relates to development of new applications for forecasting of severe local storms. Many of these new applications are based on physical understanding of processes taking place on the storm scale and thus allow forecasters to become less dependent on empirical relationships. Refinements in pattern recognition and severe weather climatology continue to be of value to the operational severe local storms forecasters, however.

Current methodology for forecasting severe local storms at the National Severe Storms Forecast Center is described. Operational uses of new forecast applications, new “real-time” data sources (such as wind profilers and Doppler radars), and improved numerical model products are discussed.

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Robert G. Lamb, Wen H. Chen, and John H. Seinfeld

Abstract

Numerico-empirical expressions for the particle displacement probability density function from which the mean concentration of material in turbulent fluid may be obtained are derived from the numerical planetary boundary layer model of Deardorff. These expressions are then used to compute profiles of the mean, cross-wind-integrated concentration of an inert pollutant issuing from a continuous point source below a stable layer. Profiles are derived for each of two conditions of atmospheric stability: zi/L=0 and –4.5, where zi is the inversion base height and L the Monin-Obukhov length. The resulting concentration profiles [referred to as the numerico-empirical (NE) profiles] are then used in two separate experiments designed to assess the adequacy of conventional atmospheric diffusion formulations.

First, the validity of the atmospheric diffusion equation is assessed by determining for each of the two stabilities cited above the profile of vertical eddy diffusivity that produces the closest fit of the mean concentration predicted by the atmospheric diffusion equation with the NE profiles.

Second, comparisons are made between the NE profiles and the corresponding concentration distributions predicted by the Gaussian plume formula with Pasquill-Gifford dispersion parameters, and the Gaussian puff equation with McElroy-Pooler travel-time-dependent dispersion parameters.

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John C. Swallow, Robert L. Molinari, John G. Bruce, Otis B. Brown, and Robert H. Evans

Abstract

Near-surface observations of temperature, salinity and current are used to describe the seasonal reversal of the Somali Current during 1979, in response to the onset of the southwest monsoon winds. During April, prior to the reversal of the winds north of the equator, the northward flowing East African Coastal Current (EACC) and the southward flowing Somali Current (SC) converged near the equator. The EACC was characterized by surface waters with salinities less than 35.1%, and the SC by salinities greater than 35.3%. The winds reversed north of the equator during the first week of May, and the boundary current intruded in the form of an anticyclonic gyre to 2.5°N. Most of the low-salinity water was recirculated back south of the equator by the offshore limb of the gyre. It did not flow continuously at the surface into the eastward equatorial jet, which was present farther offshore during May and June. That current was fed by high-salinity water from the region to the north of the low-latitude gyre. Surface winds increased dramatically in early June; and subsequently, the gyre intruded farther north and east; recirculation southward across the equator was still observed. A second gyre spun up north of the southern feature, apparently in response to the increase in winds. During July and early August the southern gyre intruded farther north, the northern gyre intensified and the equatorial jet disappeared. The data are inadequate to resolve the rapid changes which occurred in late August. The net result was the replacement of the offshore flow between the equator and 5°N by onshore flow along the equator and advection of low-salinity water from south of the equator to 12°N. The observations are discussed in the context of model results and implications for the redistribution and modification of local water masses.

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Addison L. Sears-Collins, David M. Schultz, and Robert H. Johns
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Alexander J. Roberts, John H. Marsham, and Peter Knippertz

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Reanalysis and operational analysis products are routinely used as the best estimates of the atmospheric state for operational and research purposes. However, different models, assimilation techniques, and assimilated datasets lead to differences between products. Here, such differences in the distribution of low-level water vapor over summertime West Africa are analyzed, as reflected in the zonal mean position of the leading edge of the West African monsoon [the intertropical discontinuity (ITD)] using five reanalyses [NCEP–NCAR, NCEP–Department of Energy (DOE), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), the Climate Forecast System Reanalysis (CFSR), and the Interim ECMWF Re-Analysis (ERA-Interim)] and two operational analyses [Global Forecast System (GFS) and ECMWF] during the 11 monsoon seasons (April–September) from 2000 to 2010. Specific humidity differences regularly reach 50% of the mean value over areas spanning hundreds of kilometers and often coincide with northward excursions of the ITD that last several days and bring unusual rainfall to the Sahel and Sahara. The largest disagreements occur during the southward retreat of the ITD and are connected with anomalously high values of aerosol optical depth, consistent with the production of haboob dust storms. The results suggest that known errors in the representation of moist convection and cold pools may contribute to the identified disagreements. A large reduction in disagreement occurs in 2006, when upper-air observations were enhanced during the African Monsoon Multidisciplinary Analysis (AMMA) campaign, pointing to an insufficient observational constraint of the (re)analyses in other years. It is hoped that this work will raise awareness of the limited reliability of (re)analysis products over West Africa during the summer, particularly during northward surges of the ITD, and will instigate further work to improve their quality.

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Ari-Juhani Punkka, Jenni Teittinen, and Robert H. Johns

Abstract

On 5 July 2002, a rapidly propagating bow echo formed over eastern Finland causing severe wind damage in an exceptionally large area. The Ministry of the Interior’s Emergency Response Centers received nearly 400 thunderstorm-related wind damage reports. The 5 July 2002 case is the highest-latitude derecho that has ever been documented. The bow echo developed ahead of a northeastward-moving 500-hPa trough inside of the warm sector of a secondary low and moved north-northwestward on the eastern (warm) side of the quasi-stationary front. The leading edge of the bow echo was oriented perpendicular to the low-level southerly wind shear and the convective system propagated along the 850-hPa equivalent potential temperature ridge with a speed that was close to the maximum wind throughout the troposphere. It is particularly noteworthy that the synoptic pattern was oriented about 90° counterclockwise when compared with the typical synoptic pattern associated with warm season derechos in the United States. This kind of synoptic situation associated along with the derecho mesoscale convective system’s (MCS’s) motion toward the north-northwest has not been mentioned in literature before. The MCS started as a cluster of thunderstorms and became a bow echo a few hours later. The leading edge of the bow echo had a strong reflectivity gradient and the region of stratiform precipitation was behind the strongest echoes. At the most intense stage, a rear-inflow notch was visible both in radar and satellite pictures. It was in good accordance with the location of an area of the most severe damage. Moreover, the storm-relative winds derived from the proximity sounding in the wake of the system showed the existence of rear-to-front flow above 850 hPa. The downdraft air appeared to originate from 4 km ASL, where the relative humidity was less than 50%. This probably led to enhanced evaporative cooling and the intense cold pool, which propagated faster than the mean wind. In the mesoscale, the 5 July 2002 derecho had many similarities to other derecho MCSs that have been described in the literature.

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