Abstract

Analyses of proximity soundings and upper-air fields for 37–51 Utah severe wind cases (WIND), reported in the months of May–September and occurring within 3 h after upper-air observation time, are presented. In addition, a comparison of sample mean values between the WIND cases and a climatological sample (CLIM) is made using a standard t test to determine which variables are significantly different between the two samples. This study seeks to determine if the synoptic-scale-derived fields play a significant role in producing severe wind for a region in which subsynoptic effects, attributed to uneven terrain, are important. The WIND sample environment had the following important differences when compared to CLIM:

1. more convergent wind in the lower troposphere (700-mb moisture and wind convergence),

2. greater moisture at 500 mb (dewpoint, mixing ratio),

3. greater positive vorticity advection (500 mb) and differential vorticity advection (700–500 mb),

4. a larger lapse rate based on various stability indices,

5. more southerly component flow at levels from 500 to 200 mb,

6. higher absolute vorticity at levels from 300 to 200 mb,

7. greater 500-mb wind speeds, and

8. larger thermal advection (warm) at 200 mb.

Abstract

A Lagrangian numerical model is used to simulate upwelling in an idealized large lake. This simulation is carried out to test the model's potential for simulating lake and ocean circulations.

The model is based on the slippery sack (SS) method that was recently developed by the authors. It represents the lake as a pile of conforming sacks. The motions of the sacks are determined using Newtonian dynamics. The model uses gravity wave retardation to allow for long time steps and has pseudo-Eulerian vertical mixing.

The lake is exposed to northerly winds for 29 h. Upwelling develops in the eastern edge of the basin, and after the winds shut off, upwelling fronts propagate around the lake. This case was previously simulated using a height- and sigma-coordinate ocean model. The SS model produces circulations that are similar to those produced by the other models, but the SS simulation exhibits less mixing.

Abstract

Two recent severe extratropical storms, the “Halloween storm” of October 26–November 2 1991 (HOS) and the “storm of the century” (SOC) of March 12–15 1993, are characterized by measurements of sea states of unprecedented magnitude off the east coast of North America. A Canadian buoy moored in deep water south of Nova Scotia recorded peak significant wave heights (HS) exceeding 16 m in both storms. In SOC, a NOAA buoy moored southeast of Cape Hatteras recorded a peak HS of 15.7 m, a record high for NOAA buoys. These extreme storm seas (ESS) exceed existing estimates of the 100-yr estimated design wave in these regions by about 50%. The extensive wave measurements made in both storms from buoys moored in deep water provide a rare opportunity to validate modern ocean wave models in wave regimes far more severe than those used for model tuning. In this study, four widely applied spectral wave models (OWI1G, Resio2G, WAM4, and OWI3G) are adapted to the western North Atlantic basin on fine mesh grids and are driven by common wind fields developed for each storm using careful manual kinematic reanalysis. The alternative wave hindcasts are evaluated against time series of measured HS and dominant wave period obtained at nine U.S. and Canadian buoys moored in deep water between offshore Georgia and Newfoundland. In general, it was found that despite the large differences in model formulation, the hindcasts were almost uniformly skillful in specification of the evolution of wave height and period in these two storms. The skill was much greater than achieved routinely in real time wave analyses provided by some of these same models operating at U.S., Canadian, and European centers, confirming that at least for these particular models, typically large errors in operational surface marine wind field analyses are the dominant source of errors in operational wave analyses and forecasts. However, all models were found to systematically underpredict the magnitude of the peak sea states in both storms at buoys that recorded peak HS in excess of about 12 m (ESS). This bias in ESS wave heights was 3.2 m for OWI1G, 1.9 m for Resio2G, 2.2 m for OWI3G, and 1.5 m for WAM4. These results provide an interesting assessment of the Progress made in the past decade in ocean wave modeling, both in terms of improvements of 1G and 2G models, and the introduction of 3G models. The 2G and 3G models show a slight advantage over the 1G model in simulating the most extreme wave regimes. These results suggest strongly that, for applications where supercomputers are not available, and especially for most operational applications where only integrated properties of the spectrum (e.g., HS) are required or where errors in forcing wind fields are typical of real time objective analyses and forecasts, highly developed and validated 1G and 2G wave models may continue to be used. However, accurate specification of ESS is especially critical for application of wave models to determine the extreme wave climate for ship, offshore, and coastal structure design. Therefore, further study is required to isolate the contribution of remaining wind field errors and model physics and numerics to the underprediction of ESS in extreme storms. The common phenomenological link between these two storms in the regions of ESS appears to be wave generation along a dynamic fetch associated with intense surface wind maxima or jet streaks (JS), which maintain high spatial coherency over at least 24 h and propagate at speeds of 15–20 m s⁻¹. ESS were observed only at those buoys directly in the path of the core of such features. This finding suggests that high-resolution wave models are required to model ESS, but these are justified only if the small-scale JS phenomena can be resolved in operational analysis and forecast systems.

Abstract

Recent tests of all generations of numerical wave models indicate that extreme wave heights are significantly underpredicted by these models. This behavior is consistent with the finding by Ewing and Laing that fully developed wave spectra do not have the universal self-similar form postulated by Pierson and Moskowitz. This paper postulates that it is inappropriate to scale fully developed seas by winds taken from a fixed level above the mean sea surface. Instead, winds should be taken from a dynamically scaled height that is linearly related to the wavelength of the spectral peak. This alternative scaling is consistent with friction-velocity scaling and yields predicted wave heights and periods that are in better agreement with the data collected by Ewing and Laing and appear to explain some of the discrepencies in results from previous studies with numerical wave models in large storms.

Abstract

Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

Abstract

Boundary layer probing by multiple remote sensors can greatly improve the understanding of processes in this complex region. For this purpose one needs to know the unique information each individual sensor can provide. Two promising boundary layer remote sensors, a microwave, frequency-modulated, continuous-wave (FM-CW) radar and a laser radar (lidar), were operated simultaneously to probe a common volume. As expected, the lidar sometimes separately detected aerosol layers, notably cloud bases, and the radar sometimes separately detected refractive layers and insects. Boundaries of aerosol structures were often found to be regions of radar returns such as in layers, convective activity, and breaking waves. In contrast, however, a refractive layer was observed within an apparently well-mixed aerosol layer. The data indicate that the radar may have a characteristic echo which is coincident with cloud and fog tops. This experiment shows that FM-CW radars and lidars can separately sense layering in the boundary region and that they provide complementary information on boundary layer mixing processes.

Abstract

The OWLeS IOP2b lake-effect case is simulated using the Weather Research and Forecasting (WRF) Model with a horizontal grid spacing of 148 m (WRF-LES mode). The dynamics and microphysics of the simulated high-resolution snowband and a coarser-resolution band from the parent nest (1.33-km horizontal grid spacing) are compared to radar and aircraft observations. The Ice Spheroids Habit Model with Aspect-ratio Evolution (ISHMAEL) microphysics is used, which predicts the evolution of ice particle properties including shape, maximum diameter, density, and fall speed. The microphysical changes within the band that occur when going from 1.33-km to 148-m grid spacing are explored. Improved representation of the dynamics at higher resolution leads to a better representation of the microphysics of the snowband compared to radar and aircraft observations. Stronger updrafts in the high-resolution grid produce higher ice number concentrations and produce ice particles that are more heavily rimed and thus more spherical, smaller (in terms of mean maximum diameter), and faster falling. These changes to the ice particle properties in the high-resolution grid limit the production of aggregates and improve reflectivity compared to observations. Graupel, observed in the band at the surface, is simulated in the strongest convective updrafts, but only at the higher resolution. Ultimately, the duration of heavy precipitation just onshore from the collapse of convection is better predicted in the high-resolution domain compared to surface and radar observations.

Abstract

Wind-sea generation was observed during two experiments off the coast of North Carolina. One event with offshore winds of 9–11 m s⁻¹ directed 20° from shore normal was observed with eight directional stations recording simultaneously and spanning a fetch from 4 to 83 km. An opposing swell of 1-m height and 10-s period was also present. The wind-sea part of the wave spectrum conforms to established growth curves for significant wave height and peak period, except at inner-shelf stations where a large alongshore wind-sea component was observed. At these short fetches, the mean wave direction θ_m was observed to change abruptly across the wind-sea spectral peak, from alongshore at lower frequencies to downwind at higher frequencies. Waves from another event with offshore winds of 6–14 m s⁻¹ directed 20°–30° from shore normal were observed with two instrument arrays. A significant amount of low-frequency wave energy was observed to propagate alongshore from the region where the wind was strongest. These measurements are used to assess the performance of some widely used parameterizations in wave models. The modeled transition of θ_m across the wind-sea spectrum is smoother than that in the observations and is reproduced very differently by different parameterizations, giving insights into the appropriate level of dissipation. Calculations with the full Boltzmann integral of quartet wave–wave interactions reveal that the discrete interaction approximation parameterization for these interactions is reasonably accurate at the peak of the wind sea but overpredicts the directional spread at high frequencies. This error is well compensated by parameterizations of the wind input source term that have a narrow directional distribution. Observations also highlight deficiencies in some parameterizations of wave dissipation processes in mixed swell–wind-sea conditions.

Abstract

There is an urgent need to develop and optimize tools for designing large wind farm arrays for deployment offshore. This research is focused on improving the understanding of, and modeling of, wind turbine wakes in order to make more accurate power output predictions for large offshore wind farms. Detailed data ensembles of power losses due to wakes at the large wind farms at Nysted and Horns Rev are presented and analyzed. Differences in turbine spacing (10.5 versus 7 rotor diameters) are not differentiable in wake-related power losses from the two wind farms. This is partly due to the high variability in the data despite careful data screening. A number of ensemble averages are simulated with a range of wind farm and computational fluid dynamics models and compared to observed wake losses. All models were able to capture wake width to some degree, and some models also captured the decrease of power output moving through the wind farm. Root-mean-square errors indicate a generally better model performance for higher wind speeds (10 rather than 6 m s⁻¹) and for direct down the row flow than for oblique angles. Despite this progress, wake modeling of large wind farms is still subject to an unacceptably high degree of uncertainty.

Abstract

The contribution of turbulent mixing to heat and tracer transport in the tropical tropopause layer (TTL) is poorly constrained, partly owing to a lack of direct observations. Here, the authors use high-resolution (20 Hz) airborne measurements to study the occurrence and properties of small-scale (<100 m) wind fluctuations in the TTL (14–19 km) over the tropical Pacific. The fluctuations are highly intermittent and appear localized within shallow (100 m) patches. Furthermore, active turbulent events are more frequent at low altitude, near deep convection, and within layers of low gradient Richardson number. A case study emphasizes the link between the turbulent events and the occurrence of inertio-gravity waves having small horizontal or vertical scale. To evaluate the impact of the observed fluctuations on tracer mixing, their characteristics are examined. During active events, they are in broad agreement with inertial-range turbulence theory: the motions are close to 3D isotropic and the spectra follow a −5/3 power-law scaling. The diffusivity induced by turbulent bursts is estimated to be on the order of 10⁻¹ m² s⁻¹ and increases from the top to the bottom of the TTL (from ~2 × 10⁻² to ~3 × 10⁻¹ m² s⁻¹). Given the uncertainties involved in the estimate, this is in reasonable agreement (about a factor of 3–4 lower) with the parameterized turbulent diffusivity in ERA-Interim, but it disagrees with other observational estimates from radar and radiosondes. The magnitude of the consequent vertical transport depends on the altitude and the tracer; for the species considered, it is generally smaller than that induced by the mean tropical upwelling.