Abstract

Details of the structure of a moderate reflectivity microburst were provided by dual-Doppler radar measurements during the Phoenix II convective boundary layer experiment. The dated allowed high resolution of the descending microburst in both time and space. Thermodynamic fields of virtual potential temperature and buoyancy retrieved from the radar measurements indicated that the downdraft was associated with a minimum in virtual potential temperature, rather than coinciding with a maximum in precipitation loading. The physical separation of the downdraft from the reflectivity maximum was especially pronounced during the later stages of the microburst and was partly due to the tilled reflectivity core descending more rapidly than the downdraft. The downdraft corms also descended at a rate slower than the magnitude of the maximum downdraft so that air was continually converging and entraining into the downdraft above the level of its peak value and was detraining and diverging below it. The retrieved pressure fields and simple analytical calculations showed that this slower descent and internal circulation coincided with an upward-directed pressure form. Simple calculations also suggest that this influence of the pressure force on the vertical accelerations depends strongly on the aspect ratio of the negatively buoyant parce1; horizontally narrow and vertically deep negatively buoyant parcels result in stronger downdraft than wider and shallower parcels. Our study suggests the internal circulation and the relatively slow descent of the peak downdraft should be inherent characteristics of microbursts driven by corms of low virtual potential temperature air, while microbursts driven primarily by water loading could be expected to have a different structure. In the case of the microbursts driven by corms of cool air, observation and recognition of the convergence and divergence associated with the internal circulation provides important precursors to microburst activity. In this study, the Doppler measurements showed that the microburst descending into a stable layer may have enhanced the divergence pattern below the peak downdraft.

Abstract

Recent studies using data from the Tropical Ocean and Global Atmosphere program’s Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) have shown that synoptic-scale areas of extremely dry air can occur in the troposphere over the equatorial western Pacific. These layers of extremely dry air modify convective activity and the vertical profile of radiation in clear air. At the present time there is some disagreement as to the dynamic mechanism responsible for these events and a number of their characteristics are relatively unknown. In this study, the origin and characteristics of the dry air events were investigated through analysis of TOGA COARE rawinsonde data and examination of global analyses from two different forecast centers. These drying events were found to be very common and evidence was presented that their intensity was underestimated in the global analyses. These dry events were shown to most often originate in the Northern (winter) Hemisphere as troughs associated with baroclinic waves intensified and expanded equatorward, leading to a process analogous to Rossby wave breaking. In these cases, the dry air at the edge of the westerlies at upper levels was incorporated into the equatorward extension of thin NE–SW tropospheric troughs, where it subsided and was subsequently advected equatorward. If sufficient subsidence took place, the dry air continued flowing equatorward on the eastern edge of well-defined anticyclones in the lower troposphere. The dry air in one case originated in a Southern (summer) Hemisphere trough that was associated with midlatitude baroclinic waves that propagated equatorward and developed into a series of distinct disturbances along a subtropical jet. In both the Northern and Southern Hemisphere events, the subsiding dry air in the midtroposphere was injected into the fringes of the Tropics, where it was able to reach equatorial regions if it interacted with favorable meridional flow in the Tropics. Past studies have proposed that these intrusions of dry air could induce droughts in the Tropics through decreasing deep convective activity. The implication of this study is that these droughts are actually induced by midlatitude processes.

Abstract

This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

Abstract

Weak but persistent synoptic-scale ascent may play a role in the initiation or maintenance of nocturnal convection over the central United States. An analytical model is used to explore the nocturnal low-level jets (NLLJ) and ascent that develop in an idealized diurnally varying frictional (Ekman) boundary layer in a neutrally stratified barotropic environment when the flow aloft is a zonally propagating Rossby wave. Steady-periodic solutions are obtained of the linearized Reynolds-averaged Boussinesq-approximated equations of motion on a beta plane with an eddy viscosity that is specified to increase abruptly at sunrise and decrease abruptly at sunset. Rayleigh damping terms are used to parameterize momentum loss due to radiation of inertia–gravity waves. The model-predicted vertical velocity is (approximately) proportional to the wavenumber and wave amplitude. There are two main modes of ascent in midlatitudes, an afternoon mode and a nocturnal mode. The latter arises as a gentle but persistent surge induced by the decrease of turbulence at sunset, the same mechanism that triggers inertial oscillations in the Blackadar theory of NLLJs. If the Rayleigh damping terms are omitted, the boundary layer depth becomes infinite at three critical latitudes, and the vertical velocity becomes infinite far above the ground at two of those latitudes. With the damping terms retained, the solution is well behaved. Peak daytime ascent in the model occurs progressively later in the afternoon at more southern locations (in the Northern Hemisphere) until the first (most northern) critical latitude is reached; south of that latitude the nocturnal mode is dominant.

Abstract

A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-divergence flow structure; 6) the evolution of a traveling 700 mb shortwave; 7) the development of a rear-inflow jet; 8) the leading convective rainfall followed by a transition zone and trailing stratiform precipitation; 9) the observed configuration of front-to-rear relative flow at both upper and lower levels separated by the rear-to-front flow at midlevels; 10) the simulation of “onion-shaped” soundings; 11) the splitting of the wake low; 12) the maintenance and intensification of a mesovortex; 13) the distribution and magnitude of convective and stratiform rainfall; and 14) the diurnal cycle of the planetary boundary layer.

One of the encouraging results is that the model accurately simulates the rear-inflow jet as verified against Doppler windprofiler data after the 18-h integration from essentially synoptic-scale initial conditions. The results confirm the previously proposed hypothesis that the wake low develops hydrostatically as a consequence of adiabatic warming by descending flow entering the squall line within the rear-inflow jet The observed “onion-shaped” soundings are a manifestation of the warming and drying of air within the descending rear inflow jet. It is found that the present wake low is not a transient meso-β scale phenomenon, but has a time scale of more than 50% of the squall line lifetime. Another finding is that the present mesovortex is not produced by latent heat release associated with the squall line but was in existence prior to the model initialization time. The vortex appears to have a significant effect on the distribution of the rainfall associated with the squall line and on the intensity of the rear-inflow jet. Other mesoscale circulation features are also documented in this paper.

This study, along with previous investigations using the model, indicates that the meso-β scale structure and evolution of MCSs under certain synoptic-scale environmental conditions can be well simulated using the standard network observations if compatible grid resolution, reasonable model physics and initial conditions are utilized.

Abstract

On 30 June 1982, a multicellular storm in Colorado produced four downbursts, three misocyclones, a miso-anticyclone, and horizontal vortex circulations within a relatively small area of the storm. Weather events associated with this storm included hail, heavy rain, and strong winds. A sounding taken two hours before storm formation showed the mixed layer was characterized by a nearly dry adiabatic lapse rate to ∼2 km and was relatively moist for eastern Colorado. A hodograph showed the environment had weak to moderate vertical shear of the horizontal wind, a condition conducive to the formation of downdraft misocyclones. The four-dimensional structure of this storm is documented below cloud base using winds, reflectivity, and thermodynamic data derived front multiple Doppler analysis.

One misocyclone (<4 km scale) is particularly intense with a peak vorticity of ≈100 × 10⁻³ s⁻¹ near cloud base. Despite the intense rotation, no tornadoes or funnels were observed and no damage was reported. Radar characteristics of this misocyclone are similar to those of mesocyclones that produce tornadoes or funnels except that vorticity is a maximum near cloud base and the low-level divergence created by the downbursts weakens the low-level, positive vorticity. While the misocyclone is initially separated from the downdraft, the two features evolve to become collocated. Each misocyclone becomes associated with a local downdraft maximum, suggesting that the misocyclones are important to downdraft development.

Pressure perturbation analysis does not show any evidence for strong, downward-directed pressure gradient forces below cloud base that would act to accelerate a downdraft. Since the downdraft is observed to accelerate below cloud base, other forces must be important. Observations and buoyancy estimates calculated from radar reflectivity show negative buoyancy is playing a role in downdraft intensification. Despite the lack of dynamical forcing of the downdraft by the misocyclone below cloud base, dynamical forces may be playing a role in accelerating the downdraft above cloud base.

Horizontal vortex circulations, or rotors, form along the edge of the misocyclone and downdraft and propagate away from their source region. Strongest surface winds are associated with the rotors. Pressure perturbation analysis shows that a low forms at the center of the circulation that may cause an acceleration of the low-level outflow into the rotor and may explain the strong winds. Rotors may be an integral part of downburst outflows and perhaps multiple rotors are created by pulsating downdrafts. An explanation of these circulations is important since they seem to have been involved in the Dallas-Fort Worth Regional Airport crash of an L-1011 jet.

Abstract

The passage of shallow cold fronts during the late spring and early summer months over the island of Taiwan is often accompanied by heavy rainfall and occasional flash flood episodes. Previous studies have emphasized the weak baroclinicity of these fronts and their possible modification by fluxes from the air-sea interface. In this study a cold frontal passage in the vicinity of Taiwan is analyzed using data gathered during the Taiwan Area Mesoscale Experiment (TAMEX) on 8 June 1987. At the northern extent of the TAMEX network the cold front was shallow (1–2 km deep) and moderately baroclinic with 5°-7°C temperature contrasts at the surface. A Doppler radar cross section of radial velocity reveals a structure similar to that of a density current at the leading edge of the shallow front. The postfrontal air man was substantially modified by oceanic heat fluxes as it moved southward over the warm ocean waters. This led to a 60%–70% decrease in the temperature contrast across the front between ocean stations at the northern and southern ends of the island, a distance of ∼400 km.

Frontal passages across Taiwan are also influenced by the presence of the Central Mountain Range (CMR), which has an average ridge elevation of ∼2500 m, and is oriented NNE-SSW along the major axis of the island. In the case described in this paper the CMR, 1) acts as a barrier to both the pre- and postfrontal flows, and 2) is influential by inducing thermally-driven diurnal circulations associated with differential heating of the sloped terrain and the nearby ocean. Terrain influences on the kinematics of the flow in the vicinity of the front are also shown to locally modify the frontal intensity.

The inhomogeneous distribution of precipitation attending the frontal passage is related to strong regional variations in thermodynamic stability across the island. These variations in stability are linked to the mesoscale effects of terrain, and to the larger-scale influence of advection of an unstable tropical air mass into the region by a low-level wind maximum.

Abstract

Observations from the Oklahoma-Kansas Preliminary Regional Experiment for STORM-CENTRAL (OK PRE-STORM) have been used to study the evolution and propagation characteristics of a long-lived (≥16 h) mesoscale convective system (MCS) that produced locally heavy (50–100 mm) rainfall during 26–27 June 1985. The MCS formed in association with a synoptic-scale cold front and upper-level trough system. Mesoscale ascent contributed to an increase in convective available potential energy (CAPE) and a decrease in convective inhibition, facilitating the development of deep convection.

During the late morning and early afternoon hours convection was present along and within an ∼200-km zone in advance of the cold front. In advance of the main precipitation area, a series of nearly parallel rainbands formed from in situ boundary-layer cloud streets. The development and organization of these rainbands was aided by the moderate-to-large CAPE, small convective inhibition, and moderate unidirectional shear at low levels that characterized the preconvective environment over the ∼200-km region ahead of the cold front. The discrete eastward progression of convection afforded by the formation of the rainbands in advance of the main precipitation area represents a distinct departure from the propagation characteristics of many previously observed cases and idealized simulations of linearly oriented MCSs, where system propagation depends crucially on periodic regeneration of multicell convection along a storm-induced cold pool.

The MCS weakened over southern Kansas after the merger of the main precipitation area with the quasi-stationary presquall rainbands. During its dissipating stages, it exhibited circulation and surface pressure features commonly reported during the mature-to-decaying stages of previously observed systems. These features included a surface mesohigh to the rear of the leading edge of the precipitation, and regions of mesoscale ascent and subsidence associated with a trailing anvil and a sloping rear inflow jet. The presence of these features, despite a system evolution and precursor environment different from those of a more classical linearly oriented MCS supports the consensus from earlier studies that internal processes such as spatial variations in diabatic heating are likely responsible for the observed mesoscale flows in the mature-to-decaying stages of large MCSs.

Abstract

A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (∼1–5 m s⁻¹) ascent frequently creates an environment favorable to the initiation of convection, quite close (within ∼10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12–13 May 1985. The analysis presented here reveals that the mesoscale structure of the nocturnal dryline prior to the formation of deep convection is a gently sloping, slow-moving interface. The movement of the dryline at night was related to the evolution of the low-level jet within the moist air. Wavelike structures and evidence for vertical mixing were observed in the moist air as low Richardson numbers occurred below the height of the jet. The previously discussed strong ascent is largely lacking in the present nocturnal case so that the circulations inherent to an undisturbed dryline at night are far less favorable for the initiation of deep convection than in the afternoon and early evening.

In the present case, severe convection developed as a weak cold front approached and merged with the nocturnal dryline and the environment rapidly destabilized. Between soundings taken 2.5 h apart, the convective available potential energy increased from 524 to 3417 J kg⁻¹ and the absolute value of the convective inhibition decreased from 412 to 9 J kg⁻¹. The vertical shear of the horizontal wind also dramatically increased with time, so that the bulk Richardson number was within values normally associated with supercell convection. The timescale of the changes in stability and in the moisture field (∼1–2.5 h) has implications for the type of observing network needed to nowcast severe convection and for assessing the performance of research and operational numerical models.

Abstract

This study evaluates the predictions of radiative and cloud-related processes of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). It is based on extensive comparison of three-dimensional forecast runs with local data from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site collected at the Central Facility in Lamont, Oklahoma, over a seasonal timescale. Time series are built from simulations performed every day from 15 April to 23 June 1998 with a 10-km horizontal resolution. For the one single column centered on this site, a reasonable agreement is found between observed and simulated precipitation and surface fields time series. Indeed, the model is able to reproduce the timing and vertical extent of most major cloudy events, as revealed by radiative flux measurements, radar, and lidar data. The model encounters more difficulty with the prediction of cirrus and shallow clouds whereas deeper and long-lasting systems are much better captured. Day-to-day fluctuations of surface radiative fluxes, mostly explained by cloud cover changes, are similar in simulations and observations. Nevertheless, systematic differences have been identified. The downward longwave flux is overestimated under moist clear sky conditions. It is shown that the bias disappears with more sophisticated parameterizations such as Rapid Radiative Transfer Model (RRTM) and Community Climate Model, version 2 (CCM2) radiation schemes. The radiative impact of aerosols, not taken into account by the model, explains some of the discrepancies found under clear sky conditions. The differences, small compared to the short timescale variability, can reach up to 30 W m⁻² on a 24-h timescale.

Overall, these results contribute to strengthen confidence in the realism of mesoscale forecast simulations. They also point out model weaknesses that may affect regional climate simulations: representation of low clouds, cirrus, and aerosols. Yet, the results suggest that these finescale simulations are appropriate for investigating parameterizations of cloud microphysics and radiative properties, as cloud timing and vertical extension are both reasonably captured.