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Andrew Johnson Jr. and James J. O'brien

Abstract

A series of meteorological observations including aircraft, pilot balloon (pibal), rawinsonde, surface buoy, and special land-based surface observations was taken on 23–24 August 1972, on the central Oregon coast, to investigate the mesoscale thermal and kinematic responses of the lowest 4 km of the atmosphere during a sea breeze event.

A description of those field observations is given. Vertical cross sections of the wind field on a line perpendicular to the coast, extending 60 km inland from data obtained at three pibal stations, are presented and discussed. Time sections of the wind field and temperature fields at the coast are discussed. Mesoscale features are presented and related to prevailing synoptic-scale changes occurring aloft during the observational period.

The sea breeze event on 23 August exhibited the following important characteristics: 1) a sea breeze front, distinguishable in the zonal wind field, which penetrated more than 60 km inland; 2) a distinct wind maximum which followed the front inland; 3) the surface onshore flow at the coast which took place below the main inversion, deepening the marine layer at the onset; and 4) a return flow above the inversion which appeared in quasi-periodic surges in response to surges in the sea breeze flow.

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Eric P. James and Richard H. Johnson

Abstract

Climatological characteristics of mesoscale convective vortices (MCVs) occurring in the state of Oklahoma during the late spring and summer of four years are investigated. The MCV cases are selected based on vortex detection by an objective algorithm operating on analyses from the Rapid Update Cycle (RUC) model. Consistent with a previous study, true MCVs represent only about 20% of the mesoscale relative vorticity maxima detected by the algorithm. The MCVs have a broad range of radii and intensities, and their longevities range between 1 and 54 h. Their median radius is about 200 km, and their median midlevel relative vorticity is 1.2 × 10−4 s−1. There appears to be no significant relationship between MCV longevity and intensity. Similar to past estimates, approximately 40% of the MCVs generate secondary convection within their circulations.

The mean synoptic-scale MCV environment is determined by the use of a RUC-based composite analysis at four different stages in the MCV life cycle, defined based on vortex detection by the objective algorithm. MCV initiation is closely tied to the diurnal cycle of convection over the Great Plains, with MCVs typically forming in the early morning, near the time of maximum extent of nocturnal mesoscale convective systems (MCSs). Features related to the parent MCSs, including upper-level divergent outflow, midlevel convergence, and a low-level jet, are prominent in the initiating MCV composite. The most significant feature later in the MCV life cycle is a persistent mesoscale trough in the midlevel height field. The potential vorticity (PV) structure of the composite MCV consists of a midlevel maximum and an upper-level minimum, with some extension of elevated PV into the lower troposphere as the vortex matures. The environment immediately downshear of the MCV is more conducive to secondary convection than the environment upshear of the MCV.

This midlatitude MCV climatology represents an extension of past individual case studies by providing mean characteristics of a large MCV population; these statistics are suitable for the verification of MCV simulations. Also presented is the first high-resolution composite analysis of the MCV environment at different stages of the MCV life cycle, which will aid in identifying and forecasting these systems.

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Eric P. James and Richard H. Johnson

Abstract

Surface pressure manifestations of mesoscale convective vortices (MCVs) that traversed Oklahoma during the periods May–August 2002–05 are studied using the Weather Surveillance Radar-1988 Doppler (WSR-88D), the Oklahoma Mesonet, and the NOAA Profiler Network data. Forty-five MCVs that developed from mesoscale convective systems (MCSs) have been investigated, 28 (62%) of which exhibit mesolows detectable at the surface. Within this group, three distinct patterns of precipitation organization and associated mesolow evolution have been identified. The remaining 17 (38%) of the cases do not contain a surface mesolow. Two repeating patterns of precipitation organization are identified for the latter group.

The three categories of MCVs possessing a surface mesolow are as follows. Nineteen are classified as “rear-inflow-jet MCVs,” and tend to form within large and intense asymmetric MCSs. Rear inflow into the MCS, enhanced by the development of an MCV on the left-hand side relative to system motion, produces a rear-inflow notch and a distinct surface wake low at the back edge of the stratiform region. Hence, the surface mesolow and MCV are displaced from one another. Eight are classified as “collapsing-stratiform-region MCVs.” These MCVs arise from small asymmetric MCSs. As the stratiform region of the MCS weakens, a large mesolow appears beneath its dissipating remnants due to broad subsidence warming, and at the same time the midlevel vortex spins up due to column stretching. One case, called a “vertically coherent MCV,” contains a well-defined surface mesolow and associated cyclonic circulation, apparently due to the strength of the midlevel warm core and the weakness of the low-level cold pool. In these latter two cases, the surface mesolow and MCV are approximately collocated.

Within the group of MCVs without a surface mesolow, 14 are classified as “remnant-circulation MCVs” containing no significant precipitation or surface pressure effects. Finally, three are classified as “cold-pool-dominated MCVs;” these cases contain significant precipitation but no discernible surface mesolow.

This study represents the first systematic analysis of the surface mesolows associated with MCVs. The pattern of surface pressure and winds accompanying MCVs can affect subsequent convective development in such systems. Extension of the findings herein to tropical oceans may have implications regarding tropical cyclogenesis.

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Richard H. Johnson and James F. Bresch

Abstract

Characteristics of Mei-Yu precipitating cloud systems over Taiwan during the May–June 1987 Taiwan Area Mesoscale Experiment (TAMEX) have been studied using sounding, surface precipitation, and radar data. Vertical motion has been computed over the island at 6-h intervals from 13 May to 15 June using a modification of the kinematic method that takes into account the mountainous lower boundary within a four-station sounding polygon.

Two primary characteristics of the precipitation have been found. First, the major rainfall event were linked to the passage of midlatitude disturbances and typically consisted of both deep convective and stratiform components. Deep convection was primarily prefrontal or frontal, while the stratiform precipitation was postfrontal, presumably in association with overrunning and orographic lifting. Second, there was a pronounced diurnal variability in the rainfall.

Vertical motion, heating (Q 1), and moistening (Q 2) profiles have been used to define the character of the precipitating systems. During periods of deep convection (as indicated by radar and surface rainfall measurements), a separation of the Q 1 and Q 2 peaks is observed, whereas at times of stratiform precipitation, the Q 1 and Q 1 peaks are nearly coincident. The findings for Taiwan generally support those of Luo and Yanai, indicating a predominance of stratiform rainfall over the entire southern China and Yangtze regions (including Taiwan) during the Mei-Yu; however, they also suggest that in at least a portion of this region (Taiwan), precipitation may consist of a mixture of deep convective and stratiform components. The occurrence of coincident Q 1 and Q 1 peaks in the mid- to lower troposphere (600–800 mb) during moderate-to-heavy stratiform rain events indicates the importance of shallow cold-frontal and/or stable orographic lifting. Thus, it appears that in the Taiwan area, heavy rain in stable situations may depend critically on low-level forcing mechanisms.

The evolution of the sea breeze, the development of the afternoon mixed layer, and the diurnal cycle of Q 1 and Q 1 have been examined on a synoptically undisturbed day (24 May) when afternoon thunderstorms occurred over Taiwan. Moistening of the boundary layer by the daytime sea breeze was evident. A high-level heating peak and a midtropospheric drying peak were observed in the afternoon in association with the sea breeze and deep convection. In the evening, heating and drying aloft and cooling and moistening at low levels occurred, suggestive of stratiform precipitation during the decaying stage of the convection.

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James J. Toth and Richard H. Johnson

Abstract

Surface wind data from the program for Regional Observing and Forecasting Services (PROFS) have been analyzed to investigate the diurnal wind flow pattern over the broad drainage are of the South Platte River in northeast Colorado. A consistent diurnal pattern appears in monthly averages as well as on most undisturbed individual days, and is similar to the classic descriptions of mountain-valley wind flows. It is observed that rather than occurring simultaneously at all elevations, downslope-to-upslope and upslope-to-downslope surface flow transitions along the Front Range of northeast Colorado begin near the foothills of the Rocky Mountains and propagate eastward across the plains.

During the summer months, local confluence is found at midday along major east-west ridges in the region (e.g., Cheyenne Ridge and Palmer Lake Divide). Consequently, in addition to the north–south Continental Divide, these east–west ridges are preferred regions for initial afternoon thunderstorm development The late afternoon transition to downslope flow often appears to be associated with the propagation of thunderstorms from the mountains and ridges eastward to the plains.

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James H. Ruppert Jr. and Richard H. Johnson

Abstract

Atmospheric soundings, radar, and air–sea flux measurements collected during Dynamics of the Madden–Julian Oscillation (DYNAMO) are employed to study MJO convective onset (i.e., the transition from shallow to deep convection) in the tropical Indian Ocean. The findings indicate that moistening of the low–midtroposphere during the preonset stage of the MJO is achieved by simultaneous changes in the convective cloud population and large-scale circulation. Namely, cumuliform clouds deepen and grow in areal coverage as the drying by large-scale subsidence and horizontal (westerly) advection wane. The reduction of large-scale subsidence is tied to the reduction of column radiative cooling during the preonset stage, which ultimately links back to the evolving cloud population. While net column moistening in the preonset stage is tied to large-scale circulation changes, a new finding of this study is the high degree to which the locally driven diurnal cycle invigorates convective clouds and cumulus moistening each day. This diurnal cycle is manifest in a daytime growth of cumulus clouds (in both depth and areal coverage) in response to oceanic diurnal warm layers, which drive a daytime increase of the air–sea fluxes of heat and moisture. This diurnally modulated convective cloud field exhibits prominent mesoscale organization in the form of open cells and horizontal convective rolls. It is hypothesized that the diurnal cycle and mesoscale cloud organization characteristic of the preonset stage of the MJO represent two manners in which local processes promote more vigorous daily-mean column moistening than would otherwise occur.

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James H. Ruppert Jr., Richard H. Johnson, and Angela K. Rowe

Abstract

The diurnal cycle of the local circulation, rainfall, and heat and moisture budgets is investigated in Taiwan's heavy rain (mei-yu) season using data from the 2008 Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). Comparisons are made between an undisturbed (UNDIST; 22–29 May) and disturbed period (DIST; 31 May–4 June). Many aspects of the diurnal evolution in surface flows and rainfall were similar during both periods. At night and during early morning hours, the low-level southwesterly flow was deflected around Taiwan's main topographic barrier, the Central Mountain Range (CMR), with rainfall focused near areas of enhanced offshore confluence created by downslope and land-breeze flows. During the day, the flow switched to onshore and upslope, rainfall shifted inland, and deep convection developed along the coastal plains and windward slopes. Atmospheric budget analysis indicates a day-to-evening transition of convective structure from shallow to deep to stratiform. Evaporation associated with the evening/nighttime stratiform precipitation likely assisted the nocturnal katabatic flow.

Though the flow impinging on Taiwan was blocked during both periods, a very moist troposphere and strengthened low-level oncoming flow during DIST resulted in more widespread and intense rainfall that was shifted to higher elevations, which resembled a more weakly blocked regime. Correspondingly, storm cores were tilted upslope during DIST, in contrast to the more erect storms characteristic of UNDIST. There were much more lofted precipitation-sized ice hydrometeors within storms during DIST, the upslope advection of which led to extensive stratiform rain regions overlying the CMR peaks, and the observed upslope shift in rainfall.

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S. R. Johnson, James D. McQuigg, and Thomas P. Rothrock

Abstract

The electric power industry has long been known to be sensitive to weather events. In particular, daily temperatures in distribution areas are known to affect electric power consumption. In this paper the relationship between power consumption and daily temperatures is estimated using simple regression techniques. The resulting relationships permit an investigation of the consequences of temperature modification for 14 midwestern electric power production companies. Comparisons between power production costs for observed and modified historical and experimentally generated temperature series suggest that changes of 3–5F in average daily temperature can reduce costs substantially. Exact differentials in production cost which can be attributed to temperature modification are presented so as to be useful in firm, industry and public policy decisions.

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Andrew J. Heymsfield, Peter N. Johnson, and James E. Dye

Abstract

The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements.

The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.

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Richard H. Johnson, Sue Chen, and James J. Toth

Abstract

This study examines surface features associated with a mature-to-dissipating midlatitude mesoscale convective system that occurred on 23–24 June 1985 during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central. The primary data sources include a 400 × 500 km surface mesonetwork on a 50 km grid, rawinsonde observations from 12 supplementary sites in Kansas and Oklahoma and radar measurements from conventional as well as dual-Doppler networks.

The mesoscale convective system under investigation developed in an environment with weak vertical shear and had a lifetime of 9–12 h. It consisted in its mature stage of a southward-moving arc-shaped line of deep convective cells with a trailing stratiform precipitation region to the north. Thirty-three percent of the surface rain in the portion of the mesonetwork experiencing storm passage was from the stratiform region. An intense mesoscale downdraft developed beneath the stratiform cloud with a strong mesohigh at the surface. A wake low was positioned just to the rear of the trailing stratiform region. Local “heat bursts” were observed within the wake low. These phenomena am tentatively attributed to downbursts (which develop in a nearly dry-adiabatic environment created by the mesoscale downdraft) that penetrate a shallow, stable layer near the ground.

During the final dissipation of the stratiform precipitation (in a matter of 2 h), the surface mesohigh transformed into a mesolow. Observations suggest that at least part of this transformation process can be explained as a collapsing cold pool or spreading density current. This mechanism may also have contributed to the observed development or intensification of a midlevel mesovortex as the storm dissipated. Following the decay of the mesoscale convective system during the nighttime hours, new deep convection broke out in the region of the remnant midlevel circulation the next morning.

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