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P. Ola G. Persson and Thomas T. Warner

Abstract

A two-dimensional version of the Penn State–NCAR mesoscale model (MM4) has been used to simulate the life cycle of conditional symmetric instability (CSI) under conditions of no deformational or planetary boundary layer forcing with the model starting from idealized initial conditions. Detailed diagnostics from the growth, decay, and post-CSI stages of the life cycle are presented, and some of these features are compared to expectations from linear theory.

The life cycle features include local areas of potential and inertial instability and specific patterns of ageo-strophic zonal flow. Local areas of increased and decreased dry potential vorticity (q), including areas of negative q, develop from the initially everywhere-positive q field, principally because of the horizontally differential diabatic heating. Negative wet-bulb potential vorticity (qw) is principally advected into the upper troposphere by the CSI updraft, though some changes in qw do occur because of the diffusion of temperature. Model-output soundings along surfaces of constant absolute momentum (m) show that lower-tropospheric thermodynamic stabilization and a decrease in slantwise convective available potential energy occur during the simulation. Net changes produced by the CSI circulations include low-level frontogenesis, upper-level frontolysis, and local buoyant and inertial stabilization-destabilization.

The modeled updraft slope is between that of the surfaces of constant wet-bulb potential temperature (θw) and that of the surfaces of constant m, since the viscosity and finite grid spacing yield an unstable mode with a finite updraft width. Such a mode differs from the inviscid mode, which has an infinitely narrow updraft width and a slope along the θw surfaces. The cessation of the CSI is not due to the removal of the area of negative moist potential vorticity. Instead, linear stability analysis suggests that the cessation is due to the stabilization of modes with resolvable updraft widths and, possibly, to the depletion of the water vapor supply.

Idealized studies such as these do not attempt to achieve absolute realism but are necessary steps in the methodical process of linking simple theoretical treatment of CSI with the complex observations; they may be useful as aids in interpreting observational data or numerical model simulations of real-atmosphere cases.

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P. Ola G. Persson and Thomas T. Warner

Abstract

The importance of the consistency between the vertical and horizontal resolution of numerical models has been suggested in recent studies. In this context, consistency means that the vertical scales that are physically related to the resolvable horizontal scales are also resolved. In this study, gravity waves produced in a hydrostatic primitive-equation numerical simulation of conditional symmetric instability (CSI) are shown to be produced by the inconsistency of the model resolution, where the physical relationship between the vertical and horizontal scales is determined by the slope of the narrow thermal structures (loosely termed “fronts”) produced by the CSI.

The detailed examination of the spurious gravity waves in the numerical simulation and height perturbations in diagnostic experiments quantify the effects of this inconsistency in the resolution. It is shown that 1) spurious height perturbations of ∼1 m or less are produced, though these may be sufficient to cause significant gravity waves detectable in the fields of certain variables, 2) the amplitudes of the height perturbations increase with increasing AS ratio (defined as the grid aspect ratio Δpy, divided by the slope s of the front), 3) the amplitudes of the height perturbations increase with increasing horizontal temperature difference across the front, 4) the amplitudes of the height perturbations increase with decreasing horizontal grid-space width of the front, and 5) the wavelengths of the height perturbations, when measured in grid spaces, are numerically equal to the AS ratio for AS ⩾ 2 and can otherwise be determined for AS < 2. Only horizontal frontal widths ⩽7Δy and cross-frontal temperature differences ⩽2 K are examined. If sufficiently narrow and sufficiently strong sloping fronts exist initially or develop in a numerical simulation, AS ⩽ 1 will ensure that the amplitudes of spurious height perturbations and gravity waves caused by the resolution inconsistency will be small. The horizontal wavelengths of the gravity waves will be especially large if AS−1 is approximately an integer. Decreasing the vertical grid spacing is the most effective means of reducing the height-perturbation amplitudes, though increasing the horizontal grid spacing may reduce the perturbation amplitudes if the grid-space width of the front remains constant as the grid spacing is increased.

It is strongly recommended that the selection of a vertical grid spacing for a numerical simulation should not be done without considering the horizontal grid spacing, the likely occurrence of narrow sloping frontlike structures, and the AS ratio.

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P. Ola G. Persson and Thomas T. Warner

Abstract

A simplified two-dimensional version of the Pennsylvania State University-NCAR Mm scale Model (MM4) was used to investigate the nonlinear evolution of unforced conditional symmetric instability (CSI). Sensitivity tests examines the effects of the model resolution, the magnitudes and formulation of the vertical and horizontal diffusion, and the Prandtl number on various aspects of the CSI circulations. Aspects of the circulations that are considered include the transverse velocity, the growth rate, and the updraft slope, as well as the existence of a CSI circulation.

With the fairly unstable initial conditions used in these tests, unstable CSI modes are explicitly resolved with horizontal grid spacing of at most 30 km, using a fairly weak horizontal diffusion. The vertical grid spacing needs to be no greater than 340 m to be consistent with the horizontal grid spacing and the sloping thermal structures. To resolve the most unstable nonlinear CSI modes, horizontal resolutions of no more than 15 km and vertical resolutions less than 170 m are necessary. The magnitude of the horizontal diffusion, the magnitude and the formulation of the vertical diffusion, and the Prandtl number all affect the simulated CSI circulations.

Comparisons to linear theory allow the generalization of the results. These generalizations suggest that the combined effects of model resolution, model diffusion, scale height of the instability, and environmental conditions (e.g., N 2) determine whether CSI will be explicitly released in a numerical model, and also whether the resolved unstable modes include the desirable most unstable mode. Unstable CSI modes that do not correspond to the most unstable mode can be explicitly resolved with a grid spacing that is coarser than that necessary for the most unstable mode. If the resolved modes do not include the most unstable mode, the evolution of the CSI circulations will be substantially slower and the amplitudes reduced from what should be expected in the real atmosphere in which the most unstable mode should be present.

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Peter V. Hobbs and P. Ola G. Persson

Abstract

The organization and structure of a narrow cold-frontal rainband (NCFR) on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity, Doppler radar observations, airborne observations and surface measurements. The NCFR was composed of small mesoscale regions of heavy precipitation called “precipitation cores” (PCs) oriented at an angle to the synoptic-scale cold front; in horizontal cross section the PCs were roughly elliptical in shape. Areas of lighter precipitation called “gap regions” (GRs) separated the PCs. The PCs were so oriented that their loading edges were regions of strong low-level convergence.

The weather associated with the passage of a PC resembled that of a squall-line gust front, with concurrent windshifts and pressure checks occurring ∼5 min before heavy precipitation and a fall in temperature. The changes in surface weather that accompanied the passage of a GR were more variable but tended to be less marked than for PCs. Thus, the sequence of weather experienced by a ground station can be markedly affected by its position with respect to the small mesoscale structure of the cold front.

The loading edges of the PCs were generally marked by relatively strong updrafts and high liquid water contents. Ice particle concentrations were high, particularly in the upper regions of the updrafts and in the downdraft regions of the PCs. Considerable ice enhancement, probably due to ice splinter production during riming, was present in these two regions. Riming was the dominant mechanism for the growth of precipitation in the PCs.

Several aspects of the small mesoscale structure of cold fronts are reminiscent of features seen with gravity currents. Also, the velocity of motion predicted by gravity current theory is in good agreement with the observed motion of the cold front.

We now visualize a cold front on the small mesoscale as a series of parallel line segments, each passing through the long axes of a PC, connected by kinks in the GRs. In the vicinity of a kink, the circulation can form a meso-low which, in extreme cases, may be a preferred region for the development of tornadoes and downbursts.

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Edgar L. Andreas, P. Ola G. Persson, and Jeffrey E. Hare

Abstract

Sensible and latent heat can cross the air–sea interface by two routes: as interfacial fluxes controlled by molecular processes right at the interface, and as spray fluxes from the surface of sea spray droplets. Once the 10-m wind speed over the ocean reaches approximately 11–13 m s−1, the spray sensible and latent heat fluxes become significant fractions (i.e., 10% or greater) of the corresponding interfacial fluxes. The analysis here establishes that result by combining the Tropical Ocean-Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (COARE) version 2.6 bulk interfacial flux algorithm with a microphysical spray model to partition measured heat fluxes from two good high-wind datasets into spray and interfacial flux contributions. The measurements come from the Humidity Exchange over the Sea (HEXOS) experiment and the Fronts and Atlantic Storm-Tracks Experiment (FASTEX); wind speeds in these two datasets span 5 to 20 m s−1.

After the measured heat fluxes are separated into spray and interfacial contributions, the spray fluxes are used to develop a fast spray flux algorithm to combine with the COARE version 2.6 interfacial flux algorithm in a unified turbulent surface flux algorithm for use in large-scale and ocean storm models. A sensitivity analysis of the spray and interfacial components of this unified flux algorithm demonstrates how the two component fluxes scale differently with the mean meteorological variables and why they must therefore be parameterized separately in models intended to treat air–sea fluxes in high winds.

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P. Ola G. Persson, P. J. Neiman, B. Walter, J-W. Bao, and F. M. Ralph

Abstract

Analysis of the case of 3 February 1998, using an extensive observational system in the California Bight during an El Niño winter, has revealed that surface sensible and latent heat fluxes within 150 km of the shore contributed substantially to the destabilization of air that subsequently produced strong convection and flooding along the coast. Aircraft, dropsonde, and satellite observations gathered offshore documented the sea surface temperatures (SSTs), surface fluxes, stratification, and frontal structures. These were used to extrapolate the effects of the fluxes on the warm-sector, boundary layer air ahead of a secondary cold front as this air moved toward the coast. The extrapolated structure was then validated in detail with nearshore aircraft, wind profiler, sounding, and buoy observations of the frontal convection along the coast, and the trajectory transformations were confirmed with a model simulation. The results show that the surface fluxes increased CAPE by about 26% such that the nearshore boundary layer values of 491 J kg−1 were near the upper end of those observed for cool-season California thunderstorms.

The increased CAPE due to upward sensible and latent heat fluxes was a result of the anomalously warm coastal SSTs (+1°–3°C) typical of strong El Niño events. Applications of the extrapolation method using a surface flux parameterization scheme and different SSTs suggested that convective destabilization due to nearshore surface fluxes may only occur during El Niño years when positive coastal SST anomalies are present. The fluxes may have no effect or a stabilizing effect during non–El Niño years, characterized by zero or negative coastal SST anomalies. In short, during strong El Niños, it appears that the associated coastal SST anomalies serve to further intensify the already anomalously strong storms in southern California, thus contributing to the increased flooding. This modulating effect by El Niño–Southern Oscillation (ENSO) of a mesoscale process has not been considered before in attempts at assessing the impacts of ENSO on U.S. west coast precipitation.

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David P. Jorgensen, Zhaoxia Pu, P. Ola G. Persson, and Wei-Kuo Tao

Abstract

A NOAA P-3 instrumented aircraft observed an intense, fast-moving narrow cold frontal rainband (NCFR) as it approached the California coast on 19 February 2001 during the Pacific Coastal Jets Experiment. Airborne Doppler radar data obtained while the frontal system was well offshore indicated that a narrow ribbon of very high radar reflectivity convective cores characterized the rainband at low levels with echo tops to ∼4–5 km, and pseudo-dual-Doppler analyses showed the low-level convergence of the prefrontal air. The NCFR consisted of gaps of weaker reflectivity and cores of stronger reflectivity along its length, perhaps as a result of hydrodynamic instability along its advancing leading edge. In contrast to some earlier studies of cold frontal rainbands, density-current theory described well the motion of the overall front. The character of the updraft structure along the NCFR varied systematically along the length of the precipitation cores and in the gap regions. The vertical shear of the cross-frontal low-level ambient flow exerted a strong influence on the updraft character, consistent with theoretical arguments developed for squall lines describing the balance of vorticity at the leading edge. In short segments at the northern ends of the cores, the vertical wind shear was strongest with the updrafts and rain shafts more intense, narrower, and more erect or even downshear tilted. At the southern ends of the cores and just north of the gaps, the wind shear weakened with less intense updrafts that tilted upshear and contained a broader band of rainfall. Simulations using the nonhydrostatic nested grid version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are used to investigate the core and gap regions, focusing on the relationship between the character of the modeled updrafts and the balance between the cold-air-induced vorticity and the prefrontal ambient shear vorticity. The cold air behind the NCFR, which forces new convection along its leading edge, is probably maintained by large-scale advection of cold air plus evaporative cooling processes within the heavy rain region of the NCFR. Observations confirm the model results; that is, that the updraft character depends on the balance of vorticity at the leading edge. Downshear-tilted updrafts imply that convection at the northern ends of cores may weaken with time relative to the frontal segments at the southern ends, because inflow air would be affected by passage through the heavy rain region before ascent. A mechanism for line modification is thus implied.

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Michael Tjernström, Caroline Leck, P. Ola G. Persson, Michael L. Jensen, Steven P. Oncley, and Admir Targino

An atmospheric boundary layer experiment into the high Arctic was carried out on the Swedish icebreaker Oden during the summer of 2001, with the primary boundary layer observations obtained while the icebreaker drifted with the ice near 89°N during 3 weeks in August. The purposes of the experiment were to gain an understanding of atmospheric boundary layer structure and transient mixing mechanisms, in addition to their relationships to boundary layer clouds and aerosol production. Using a combination of in situ and remote sensing instruments, with temporal and spatial resolutions previously not deployed in the Arctic, continuous measurements of the lower-troposphere structure and boundary layer turbulence were taken concurrently with atmospheric gas and particulate chemistry, and marine biology measurements.

The boundary layer was strongly controlled by ice thermodynamics and local turbulent mixing. Near-surface temperatures mostly remained between near the melting points of the sea- and freshwater, and near-surface relative humidity was high. Low clouds prevailed and fog appeared frequently. Visibility outside of fog was surprisingly good even with very low clouds, probably due to a lack of aerosol particles preventing the formation of haze. The boundary layer was shallow but remained well mixed, capped by an occasionally very strong inversion. Specific humidity often increased with height across the capping inversion.

In contrast to the boundary layer, the free troposphere often retained its characteristics from well beyond the Arctic. Elevated intrusions of warm, moist air from open seas to the south were frequent. The picture that the Arctic atmosphere is less affected by transport from lower latitudes in summer than the winter may, thus, be an artifact of analyzing only surface measurements. The transport of air from lower latitudes at heights above the boundary layer has a major impact on the Arctic boundary layer, even very close to the North Pole. During a few week-long periods synoptic-scale weather systems appeared, while weaker and shallower mesoscale fronts were frequent. While frontal passages changed the properties of the free troposphere, changes in the boundary layer were more determined by local effects that often led to changes contrary to those aloft. For example, increasing winds associated with a cold front often led to a warming of the near-surface air by mixing and entrapment.

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Michael Tjernström, Caroline Leck, P. Ola G. Persson, Michael L. Jensen, Steven P. Oncley, and Admir Targino
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Edgar L. Andreas, Christopher W. Fairall, P. Ola G. Persson, and Peter S. Guest

Abstract

Defining the averaging time required for measuring meaningful turbulence statistics is a central problem in boundary layer meteorology. Path-averaging scintillation instruments are presumed to confer some time-averaging benefits when the objective is to measure surface fluxes, but that hypothesis has not been tested definitively. This study uses scintillometer measurements of the inner scale (l 0) and the refractive index structure parameter (C2n) to investigate this question of required averaging time. The first conclusion is that the beta probability distribution is useful for representing C2n and l 0 measurements. Consequently, beta distributions are used to set confidence limits on C2n and l 0 values obtained over various averaging periods. When the C2n and l 0 time series are stationary, a short-term average of C2n or l 0 can be as accurate as a long-term average. However, as with point measurements, when time series of path averaged C2n or l 0 values are nonstationary, turbulent surface fluxes inferred from these C2n and l 0 values can be variable and uncertain—problems that path averaging was presumed to mitigate. Because nonstationarity is a limiting condition, the last topic is quantifying the nonstationarity with a published nonstationarity ratio and also by simply counting zero crossings in the time series.

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