Abstract

A method for determining vertical wind velocities (&ω) using the pressure-and temperature–change data from superpressure(constant–density) balloons was proposed in an earlier paper. In this paper, we present a case study using actual balloon data to test the method. Data from the Eole balloon experiment were used to estimate the sign of ω as well as the Horizontal velocity field, near 200 mb over Australia. The resulting patterns of vertical velocity compare favorably with satellite pictures and with current models of flow near jet-stream maxima. Additionally, this study provides evidence that areas of vertical motion in the troposphere extend upward into the lower stratosphere without changing sign.

Abstract

The turbulence or “gustiness” of the winds new the surface in a mountain valley increases sharply after the nocturnal inversion layer dissipates, normally in the late morning or early afternoon. This increase—almost all in the horizontal wind components-results from an increase in the mixed-layer velocity scaling parameter w _*. The increase in w _*, in turn, results mostly from the rapid increase in the mixed-layer inversion height z _i which occurs as the nocturnal inversion layer dissolves. The consistency between the turbulent energy jump and the increase in w _* agrees with findings over flatter terrain. It suggests that at least some of the thermal eddies in the mountain mixed layer are transient, i.e., advected by the mean wind and not permanently attached to terrain features.

Abstract

Analyses of Doppler lidar data reveal sea breezes occurring on two different depth and time scales at Monterey Bay, California, on a day with offshore gradient flow indicated before sunrise and after sunset. The lidar data used in this study consist of vertical cross sections and profiles of the westerly, onshore wind component u. In the morning after 0900 PST a shallow sea breeze formed, which reached a depth of 300 m by noon. Starting in early afternoon a deeper sea-breeze layer formed in the lowest kilometer, and by late afternoon the shallow sea breeze blended into the deeper sea breeze and was no longer evident. Maximum speeds of 6 m s⁻¹ in the shallow sea breeze occurred at the surface, whereas those in the deep sea breeze (also 6 m s⁻¹) were about 300 m above the surface. It is hypothesized that the shallow sea breeze is a local phenomenon responding to a more local temperature contrast between the sea and the region between the ocean and the mountain ranges. The deeper sea breeze, on the other hand, is seen as a more regional circulation, driven by the larger-scale contrast between the atmosphere over the ocean and that over the hot interior valleys of California, or perhaps even a larger continental scale. The lidar observations also included the evening transition, which began as a very shallow land breeze observed only by surface observing stations. In the deep sea-breeze layer between 250 m and 1 km AGL, the flow returned to offshore gradient flow simultaneously through the entire layer 2–4 h after sunset. The sea breeze was thus seen as a daytime interruption of the basic gradient offshore flow.

Abstract

Numerical simulators of upslope flow forming on the lee side of a heated mountain ridge showed this flow to be a transient phenomenon, in agreement with observations. The simulations, performed with a two-dimensional, dry version of the cloud model of Tripoli and Cotton, included a nocturnal inversion layer (cold pool) and ridgetop winds. Cross sections of potential temperature and vertical profiles of potential temperature and horizontal winds reproduced observations well. Runs with and without the inversion layer showed that this layer was important in the formation of upslope winds near the ground when ridgetop-level winds were present: the runs without an inversion layer never developed upslope flow on the lee slopes. Runs in which the upper-level winds were varied showed that the duration of the upslope flow on the lee slope bore an inverse relationship to upper-level wind speed: runs with stronger winds had shorter-lived upslope flow. A set of observations from South Park, Colorado supported this laser conclusion. The conclusion implies that, on days with strong ridgetop-level winds, the leeside convergence zone mechanism proposed in the earlier observational study will not be as effective in initiating and sustaining deep convective clouds in the mountains.

An evaluation of the terms in the horizontal equation of motion showed that the initial push starting the upslope winds along the lee slope in all of the runs came from the pressure-gradient force, as expected, and a period of steady-state upslope then followed this push. The subsequent transition to downslope flow described in the observational studies, however, occurred through different processes, depending on upper-level wind speed. With stronger winds aloft, downslope winds first mixed downwards and then advected horizontally from higher elevations. With weaker winds aloft the transition occurred mostly because of the downslope propagation of a surface pressure minimum (low). Following the transition in all of the runs, near-steady downslope existed until the end of the simulation.

Abstract

This paper presents the boundary layer structure which accompanies the development of daytime local wind systems in a broad mountain valley, as revealed by cross sections of potential temperature. It describes how this structure leads to the occurrence of a region of convergence to the downwind side of mountains. Previous studies, based primarily on one-dimensional sounding of potential temperature and horizontal winds, have shown that profiles of static stability and the presence of winds aloft have an important effect on the manner in which daytime, thermally-forced wind systems develop. In the present study, two-dimensional cross sections obtained from aircraft data, vertical soundings and surface mesonet data show several relevant features. In mid to late morning near the surface, for example, upslope winds form in a shallow mixed layer at the underside of the nocturnal inversion layer (cold pool); at elevations above the top of this cold pool, convectively-mixed surface winds exist. The spatial arrangement of these features in two dimensions is such that a region of convergence forms near the surface on the leeward side of mountains or mountain ranges.

This convergence region, called the leeside convergence zone, thus occurs at the upwind edge of the cold pool in a mountain valley. Evidence suggests that it is an important mechanism for the initiation of mountain- generated cumuli and their continued growth into cumulus congestus and cumulonimbus clouds.

Abstract

A series of trials are performed to evaluate the sensitivity of a 4DVAR algorithm for retrieval of microscale wind and temperature fields from single-Doppler lidar data. These trials use actual Doppler lidar measurements to examine the sensitivity of the retrievals to changes in 1) the prescribed eddy diffusivity profile, 2) the first-guess or base-state virtual potential temperature profile, 3) the phase and duration of the assimilation period, and 4) the grid resolution.

The retrieved fields are well correlated among trials over a reasonable range of variation in the eddy diffusivity coefficients. However, the retrievals are quite sensitive to changes in the gradients of the first-guess or base-state virtual potential temperature profile, and to changes in the phase (start time) and duration of the assimilation period. Retrievals using different grid resolutions exhibit similar larger-scale structure, but differ considerably in the smaller scales. Increasing the grid resolution significantly improved the fit to the radial velocity measurements, improved the convergence rate, and produced variances and fluxes that were in better agreement with tower-based sonic anemometers.

Horizontally averaged variance and heat flux profiles derived from the final time steps of all the retrievals are similar to typical large-eddy-simulation (LES) results for the convective boundary layer. However, all retrieved statistics show significant nonstationarity because fluctuations in the initial state tend to be confined within the boundaries of the scan.

Abstract

A four-dimensional variational data assimilation (4DVAR) algorithm for retrieval of spatially and temporally resolved velocity and thermodynamic fields within the atmospheric boundary layer (ABL) is described and applied to a coherent Doppler lidar dataset. The adjoint method is used to find the initialization of an ABL model that gives the best fit to radial velocity measurements from the Doppler lidar. The adjoint equations are derived by assuming that subgrid-scale fluxes can be represented as general functions of the resolved-scale rates of strain and potential temperature gradients. For this study, particular attention is paid to the treatment of real measurement error. Radial velocity precision as a function of the signal-to-noise ratio (SNR) is estimated from time series analysis of real fixed beam data, and this information is used in the evaluation of the cost function. The cost function is evaluated by interpolating the model output to the observation coordinates. As a result, the error covariance matrix retains its diagonal structure and the form of the cost function is simplified.

The retrieval method is applied to Doppler lidar data collected under convective conditions during the Cooperative Atmosphere/Surface Exchange Study (CASES-99) field program. The impact of the SNR-dependent measurement error is investigated by comparing a retrieval using equally weighted data to a retrieval using the estimated velocity precisions. At near range the fields are well correlated. However, at longer range, as the velocity precision exceeds the standard deviation of the measurements, the correlation decreases rapidly. Furthermore, retrievals using equally weighted data produce higher variances.

Abstract

This study investigates a shear-flow instability observed in the stably stratified nighttime boundary layer on 6 October 1999 during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) in south-central Kansas. A scanning Doppler lidar captured the spatial structure and evolution of the instability, and high-rate in situ sensors mounted on a nearby 60-m tower provided stability and turbulence data with excellent vertical resolution. Data from these instruments are analyzed and linear stability analysis (LSA) is employed to carefully characterize the wave field, its interaction with the mean flow, and its role in turbulence generation.

The event persisted for about 30 min and was confined within the shear zone between the surface and a low-level jet (LLJ) maximum. Eigenvalues corresponding to the fastest growing mode of the LSA showed good agreement with the basic wave parameters determined from the lidar data. Good qualitative agreement was also obtained between the eigenfunction of the fastest growing mode and the vertical profile of the dominant Fourier mode in wavenumber spectra from spatially resolved lidar data. The height of the measured momentum flux divergence associated with the wave motion was consistent with the LSA prediction of the height of the critical level.

Data show that the instability was triggered by an increase in shear due to a slowing of the flow below the LLJ maximum. This low-level slowing produced a local maximum in the shear profile, which was elevated above the surface. The speed and height of the LLJ remained relatively constant before, during, and after the event. Prior to the event turbulent momentum flux increased as the shear increased and as the gradient Richardson number decreased. With the onset of wave activity, a sudden increase in downward wave-momentum flux was accompanied by a sharp reduction in shear near the critical level.

Abstract

The depth h of the stable boundary layer (SBL) has long been an elusive measurement. In this diagnostic study the use of high-quality, high-resolution (Δz = 10 m) vertical profile data of the mean wind U(z) and streamwise variance σ_u ²(z) is investigated to see whether mean-profile features alone can be equated with h. Three mean-profile diagnostics are identified: h_J , the height of maximum low-level-jet (LLJ) wind speed U in the SBL; h ₁, the height of the first zero crossing or minimum absolute value of the magnitude of the shear ∂U/∂z profile above the surface; and h ₂, the minimum in the curvature ∂² U/∂z ² profile. Boundary layer BL here is defined as the surface-based layer of significant turbulence, so the top of the BL was determined as the first significant minimum in the σ_u ²(z) profile, designated as h_σ . The height h_σ was taken as a reference against which the three mean-profile diagnostics were tested. Mean-wind profiles smooth enough to calculate second derivatives were obtained by averaging high-resolution Doppler lidar profile data, taken during two nighttime field programs in the Great Plains, over 10-min intervals. Nights are chosen for study when the maximum wind speed in the lowest 200 m exceeded 5 m s⁻¹ (i.e., weak-wind, very stable BLs were excluded). To evaluate the three diagnostics, data from the 14-night sample were divided into three profile shapes: Type I, a traditional LLJ structure with a distinct maximum or “nose,” Type II, a “flat” structure with constant wind speed over a significant depth, and Type III, having a layered structure to the shear and turbulence in the lower levels. For Type I profiles, the height of the jet nose h_J , which coincided with h ₁ and h ₂ in this case, agreed with the reference SBL depth to within 5%. The study had two major results: 1) among the mean-profile diagnostics for h, the curvature depth h ₂ gave the best results; for the entire sample, h ₂ agreed with h_σ to within 12%; 2) considering the profile shapes, the layered Type III profiles gave the most problems. When these profiles could be identified and eliminated from the sample, regression and error statistics improved significantly: mean relative errors of 8% for h_J and h ₁, and errors of <5% for h ₂, were found for the sample of only Type I and II profiles.

Abstract

Observations taken during the February 1991 Atmospheric Studies in Complex Terrain (ASCOT) Winter Validation Study are used to describe the wind field associated with a terrain-forced mesoscale vortex and thermally forced canyon drainage flows along the Front Range of northeastern Colorado. A case study is presented of the night of 6/7 February 1991 when a weak vortex formed and propagated through the ASCOT domain.

The NOAA/ERL Environmental Technology Laboratory Doppler lidar, one of an ensemble of instruments participating in the ASCOT field experiment, obtained high-resolution measurements of the structure of both the vortex and the canyon drainage flows. The lidar observations documented the kinematic and structural changes in the cyclone and their relationship to a drainage jet exiting a nearby canyon. Lidar analyses clearly show the layering and stratification present during this case, specifically the drainage jet flowing under the cyclone. A period of strong intensification of the drainage flows occurred, following the apparent inhibition of the exit jet by southeasterly flow and the subsequent release of the exit jet, as north-northwesterly flow developed along the foothills.

Additional analyses of the mesoscale surface wind field reveal the movement and spatial variations of the cyclone from initiation to dissipation. The ambient flow remained weak and the cyclone propagated from north to south, which is opposite to previous modeled and observational studies, and on several occasions the cyclone split into two separate vortices. A tracer diffusion test performed during this case shows that the vortex changed the trajectories of the test release cloud from northerly to southerly due both to the movement of the cyclone and to the presence of northerly flow associated with the vortex. Estimates of Froude number are consistent with previous studies that showed Denver cyclones are associated with periods of low-Froude number flow.