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Nicholas D. Metz, David M. Schultz, and Robert H. Johns

Abstract

Extratropical cyclones over the central United States and southern Canada from the years 1982 and 1989 were examined for the presence of two or more (multiple) warm-front-like baroclinic zones, hereafter called MWFL baroclinic zones. Of the 108 cyclones identified during this period, 42% were found to have MWFL baroclinic zones, where a baroclinic zone was defined as a magnitude of the surface temperature gradient of 8°F (4.4°C) 220 km−1 over a length of at least 440 km. The largest frequency of cyclones with MWFL baroclinic zones occurred during April, May, August, and September. Ninety-four percent of all baroclinic zones were coincident with a magnitude of the dewpoint temperature gradient of at least 4°F (2.2°C) 220 km−1, and 81% of all baroclinic zones possessed a wind shift of at least 20°, suggesting that these baroclinic zones were significant airmass and airstream boundaries. Although cyclones with MWFL baroclinic zones formed in a variety of ways, two synoptic patterns dominated. Thirty-eight percent of cyclones with MWFL baroclinic zones formed as a cold or stationary front from a previous cyclonic system was drawn into the circulation of a cyclone center, forming the southern baroclinic zone. Twenty-two percent of cyclones with MWFL baroclinic zones formed as a cold front to the north of the cyclone center was drawn into the circulation of the cyclone, forming the northern baroclinic zone. Other synoptic patterns included outflow boundaries (9%), chinook fronts (4%), return flow from the Gulf of Mexico (4%), and unclassified (22%). Although the frequency of severe weather in cyclones was roughly the same for cyclones with and without MWFL baroclinic zones, the presence of the southern baroclinic zone provided a mechanism to focus the location of severe weather, showing their utility for severe weather forecasting. Despite the potential for severe convective storms along these southern baroclinic zones, 51% were not identified on the National Meteorological Center (now known as the National Centers for Environmental Prediction) surface analyses, indicating the importance of performing real-time surface isotherm analysis.

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Addison L. Sears-Collins, David M. Schultz, and Robert H. Johns

Abstract

A climatology of nonfreezing drizzle is created using surface observations from 584 stations across the United States and Canada over the 15-yr period 1976–90. Drizzle falls 50–200 h a year in most locations in the eastern United States and Canada, whereas drizzle falls less than 50 h a year in the west, except for coastal Alaska and several western basins. The eastern and western halves of North America are separated by a strong gradient in drizzle frequency along roughly 100°W, as large as about an hour a year over 2 km. Forty percent of the stations have a drizzle maximum from November to January, whereas only 13% of stations have a drizzle maximum from June to August. Drizzle occurrence exhibits a seasonal migration from eastern Canada and the central portion of the Northwest Territories in summer, equatorward to most of the eastern United States and southeast Canada in early winter, to southeastern Texas and the eastern United States in late winter, and back north to eastern Canada in the spring. The diurnal hourly frequency of drizzle across the United States and Canada increases sharply from 0900 to 1200 UTC, followed by a steady decline from 1300 to 2300 UTC. Diurnal drizzle frequency is at a maximum in the early morning, in agreement with other studies.

Drizzle occurs during a wide range of atmospheric conditions at the surface. Drizzle has occurred at sea level pressures below 960 hPa and above 1040 hPa. Most drizzle, however, occurs at higher than normal sea level pressure, with more than 64% occurring at a sea level pressure of 1015 hPa or higher. A third of all drizzle falls when the winds are from the northeast quadrant (360°–89°), suggesting that continental drizzle events tend to be found poleward of surface warm fronts and equatorward of cold-sector surface anticyclones. Two-thirds of all drizzle occurs with wind speeds of 2.0–6.9 m s−1, with 7.6% in calm wind and 5% at wind speeds ⩾ 10 m s−1. Most drizzle (61%) occurs with visibilities between 1.5 and 5.0 km, with only about 20% occurring at visibilities less than 1.5 km.

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Miao-Ling Lu, Robert A. McClatchey, and John H. Seinfeld

Abstract

Significant enhancements in humidity around cumulus clouds, that is, the “cloud halos” observed in many aircraft penetrations, are simulated using a three-dimensional dynamic model. Five case studies show that humidity halos occur mainly near lateral cloud boundaries and also occur at cloud top and base when the cloud dissipates. The humidity halo broadens as the cloud ages and is also broader in the presence of wind shear than in its absence, especially on the downshear side of the cloud. The broadband calculation over the solar spectrum (0.2–4.0 μm) shows that the shortwave (SW) heating rate in the halo is about 11%–18% larger than the ambient environmental heating rate. The strongest halo-induced surface SW radiative forcing for all cases studied is about −0.2 W m−2, which is approximately a 0.02% change from the forcing without a halo.

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Stephen F. Corfidi, Robert H. Johns, and Mark A. Darrow

Abstract

A significant, convectively induced windstorm known as a derecho occurred over parts of Utah, Wyoming, Idaho, and Colorado on 31 May 1994. The event was unusual in that it occurred not only in an environment of relatively limited moisture, but also one with a thermodynamic profile favorable for dry microbursts in the presence of moderate midtropospheric flow. The development and evolution of the severe wind-producing convective system is described, with emphasis on the synoptic and mesoscale features that may have contributed to its strength and maintenance. A very similar derecho that affected much the same region on 1 June 2002 is more briefly introduced. Questions are raised regarding the unique nature of these events and their potential utility in achieving an increased understanding of the mechanics of derecho-producing convective systems in more moisture-rich environments.

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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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Katherine L. Horgan, David M. Schultz, John E. Hales Jr., Stephen F. Corfidi, and Robert H. Johns

Abstract

A 5-yr climatology of elevated severe convective storms was constructed for 1983–87 east of the Rocky Mountains. Potential cases were selected by finding severe storm reports on the cold side of surface fronts. Of the 1826 days during the 5-yr period, 1689 (91%) had surface fronts east of the Rockies. Of the 1689 days with surface fronts, 129 (8%) were associated with elevated severe storm cases. Of the 1066 severe storm reports associated with the 129 elevated severe storm cases, 624 (59%) were hail reports, 396 (37%) were wind reports, and 46 (4%) were tornado reports. A maximum of elevated severe storm cases occurred in May with a secondary maximum in September. Elevated severe storm cases vary geographically throughout the year, with a maximum over the south-central United States in winter to a central and eastern U.S. maximum in spring and summer. A diurnal maximum of elevated severe storm cases occurred at 2100 UTC, which coincided with the diurnal maximum of hail reports. The wind reports had a broad maximum during the daytime. Because the forecasting of hail from elevated storms typically does not pose as significant a forecast challenge as severe wind for forecasters and tornadoes from elevated storms are relatively uncommon, this study focuses on the occurrence of severe wind from elevated storms. Elevated severe storm cases that produce only severe wind reports occurred roughly 5 times a year. To examine the environments associated with cases that produced severe winds only, five cases were examined in more detail. Common elements among the five cases included elevated convective available potential energy, weak surface easterlies, and shallow near-surface stable layers (less than 100 hPa thick).

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Matthew S. van den Broeke, David M. Schultz, Robert H. Johns, Jeffry S. Evans, and John E. Hales

Abstract

During 9–11 November 1998 and 9–10 March 2002, two similar convective lines moved across the central and eastern United States. Both convective lines initiated over the southern plains along strong surface-based cold fronts in moderately unstable environments. Both lines were initially associated with cloud-to-ground (CG) lightning, as detected by the National Lightning Detection Network, and both events met the criteria to be classified as derechos, producing swaths of widespread damaging wind. After moving into areas of marginal, if any, instability over the upper Midwest, CG lightning production ceased or nearly ceased, although the damaging winds continued. The 9 March 2002 line experienced a second phase of frequent CG lightning farther east over the mid-Atlantic states. Analysis of these two events shows that the production of CG lightning was sensitive to the occurrence and vertical distribution of instability. Periods with frequent CG lightning were associated with sufficient instability within the lower mixed-phase region of the cloud (i.e., the temperature range approximately between −10° and −20°C), a lifting condensation level warmer than −10°C, and an equilibrium level colder than −20°C. Periods with little or no CG lightning possessed limited, if any, instability in the lower mixed-phase region. The current Storm Prediction Center guidelines for forecasting these convective lines are presented.

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Katrina S. Virts, John M. Wallace, Michael L. Hutchins, and Robert H. Holzworth
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H. Lee Kyle, Richard Hucek, Philip Ardanuy, Lanning Penn, Brian Groveman, John Hickey, and Robert Maschoff

Abstract

This paper describes the production calibration adjustment algorithms used to remove thermal perturbation and stray light noise signals from the Nimbus-7 earth radiation budget (ERB) measurements. Sunlight, both direct and scattered from the sensor baffles, contaminated the ERB measurements at satellite sunrise and sunset. The problem covered subsatellite solar zenith angles from 90° to 120° and reduced the usefulness of the longwave spectral radiation measurements. Scattered light corrections are made from 90° to 99° while orbit-by-orbit interpolation is used frown 99° to 121°. Tests indicate that in the mean the midpoint interpolation error is less than 1 W m−2 with a standard deviation of about 5 W m−2. Thermal perturbations on the total channel 12 (0.2–50 μm) appeared to be always less than 0.3%. However, the Suprasil-W domes on the otherwise similar shortwave channels 13 and 14 in some way helped produce thermal perturbations of up to 6% or more in channel 13 (0.2–3.8 μm and up to 3% or more in channel 14 (0.7–2.8 μm). These perturbations arose from variations in external radiant heating during the day, night, sunrise, and sunset. In addition, the on/off cycles of the ERB and neighboring experiments produced day-to-day variations. The algorithms described here helped produce a stable 9-year-long measurement set. No thermal corrections were made in channel 12 and the obvious thermal perturbations in channels 13 and 14 were corrected. The absolute accuracy of the calibrated measurements is difficult to determine. The remaining uncertainty depends on the perturbing functions that were greater at high latitudes, near satellite sunrise and sunset, than in the Tropics. In June and July, the corrections for the daytime thermal perturbations near the North Pole may be too large by 3–5 W m−2. In general, the Nimbus-7 ERB products show good agreement with the follow-on Earth Radiation Budget Experiment (ERBE) products.

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Oscar Martínez-Alvarado, Laura H. Baker, Suzanne L. Gray, John Methven, and Robert S. Plant

Abstract

Strong winds equatorward and rearward of a cyclone core have often been associated with two phenomena: the cold conveyor belt (CCB) jet and sting jets. Here, detailed observations of the mesoscale structure in this region of an intense cyclone are analyzed. The in situ and dropsonde observations were obtained during two research flights through the cyclone during the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) field campaign. A numerical weather prediction model is used to link the strong wind regions with three types of “airstreams” or coherent ensembles of trajectories: two types are identified with the CCB, hooking around the cyclone center, while the third is identified with a sting jet, descending from the cloud head to the west of the cyclone. Chemical tracer observations show for the first time that the CCB and sting jet airstreams are distinct air masses even when the associated low-level wind maxima are not spatially distinct. In the model, the CCB experiences slow latent heating through weak-resolved ascent and convection, while the sting jet experiences weak cooling associated with microphysics during its subsaturated descent. Diagnosis of mesoscale instabilities in the model shows that the CCB passes through largely stable regions, while the sting jet spends relatively long periods in locations characterized by conditional symmetric instability (CSI). The relation of CSI to the observed mesoscale structure of the bent-back front and its possible role in the cloud banding is discussed.

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