Abstract

Data from surface stations, profilers, long-range aircraft surveys, and satellites were used to characterize the large-scale structure of the marine boundary layer off of California and Oregon during June and July 1996. To supplement these observations, June–July 1996 averages of meteorological fields from the U.S. Navy’s operational Coupled Ocean–Atmospheric Mesoscale Prediction System (COAMPS) model were generated for the region. Model calculations show a broad band of fast northerly surface winds exceeding 7 m s⁻¹ extending along the California–Oregon coast. Buoy-measured peaks of 7.1 m s⁻¹ off Bodega Bay, 7.2 m s⁻¹ off Point Piedras Blancas, and 8.8 m s⁻¹ near Point Conception were reported. Mean winds at the buoys located 15–25 km offshore are generally faster than those at coastal stations, and all station winds are faster in the afternoon.

The aircraft and station observations confirm that an air temperature inversion typically marks the top of the marine boundary layer, which deepens offshore. Along the coast, the marine boundary layer thins between Cape Blanco and Santa Barbara. The inversion base height is at its lowest (195 m) at Bodega Bay in northern California and at its highest at Los Angeles and San Diego (416 m). The inversion strength is strongest between Bodega Bay and Point Piedras Blancas, exceeding 10.8°C. The June–July 1996 marine boundary layer depth from COAMPS shows a gradual deepening with distance offshore.

The model-averaged flow within the marine boundary layer is supercritical (Froude number > 1) in a region between San Francisco and Cape Mendocino that extends offshore to 126.4°W. Smaller isolated supercritical areas occur in the lee of every major cape, with the peak Froude number of 1.3 in the lee of Cape Mendocino. This is consistent with aircraft flights of Coastal Waves ’96, when extensive regions of supercritical flow off central California and downwind of major capes were recorded with highest Froude numbers around 1.5–2.0. A broad, wedge-shaped area of nearly critical flow (Froude number > 0.8) extends from Cape Blanco to Point Piedras Blancas and offshore to about 128.5°W in the model output.

The model wind stress has a broad maximum exceeding 0.3 N m⁻² between Cape Mendocino and San Francisco with the highest values found within 100 km of the coast. Stress calculated directly from low aircraft legs is highest in the lee of large capes with peak values exceeding 0.7 N m⁻². Overall aircraft magnitudes are similar to the model’s, but a direct comparison with the 2-month average from the model is not possible due to the lesser space and time coverage of the flights. The stress maxima along the California coast shown in the model results are spatially consistent with the region of coldest sea surface temperature observed by satellite.

Abstract

Decoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface-based mixed layer (SML) have frequently been observed and modeled. When such cumulus clouds penetrate the overlying stratocumulus layer, the cloud microphysics and hence the radiative properties of the cloud are altered locally. Observations made during a series of Lagrangian experiments in the Azores as part of the Atlantic Stratocumulus Transition Experiment (ASTEX, June 1992) have been analyzed to ascertain how the properties of a stratocumulus layer with which cumulus clouds are interacting differ from those of an unaffected cloud layer. The results suggest that in regions where cumulus clouds penetrate the cloud layer, the stratocumulus is thickened as the cumuli spread out into its base. Transport of air from the SML into the cloud by convective updrafts is observed, and the increase in available moisture within the penetrating cumulus clouds results in increased liquid water content and hence changes in the droplet size spectra. The greater liquid water path results in a larger cloud optical depth, so that regions where cumulus are interesting with the stratocumulus layer can be observed in satellite measurements. Therefore, it is likely that the surface energy budget may be significantly altered by this process, and it may be necessary to parameterize these effects in large-scale numerical models.

Abstract

An instrumented C-130 aircraft flew over water around Point Sur, California, on 17 June 1996 under strong northwest wind conditions and a strong marine inversion. Patterns were flown from 30- to 1200-m elevation and up to 120 km offshore. Nearshore, marine air accelerated past Point Sur, reaching a surface maximum of 17 m s⁻¹ in the lee. Winds measured over water in and above the marine layer were alongshore with no significant cross-shore flow. Sea level pressure, 10-m air temperature, and air temperature inversion base generally decreased toward the coast and were an absolute minimum just downcoast of the wind speed maximum. The sea surface temperature also decreased toward the coast, but was an absolute minimum directly off Point Sur. The near-coast, air temperature inversion base height was 400 m north of Point Sur, decreased to a minimum of 50 m in the lee of Point Sur, then increased farther to the south. Wind speeds were at a maximum centered along the air temperature inversion base; the fastest was 27 m s⁻¹ in the lee of Point Sur.

Using a Froude number calculation that includes the lower half of the capping layer, the marine layer in the area is determined to have been supercritical. Most of the marine layer had Froude numbers between 1.0 and 2.0 with the extreme range of 0.8–2.8. Temperatures in the air temperature inversion in the lee were substantially greater than elsewhere, modifying the surface pressure gradient. The overall structure was a hydraulic supercritical expansion fan in the lee of Point Sur under the influence of rotation and surface friction.

The Naval Research Laboratory nonhydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) indicated a broad, supercritical marine boundary layer moving to the south along central California and Point Sur during the aircraft flight. The marine boundary layer thinned and accelerated into the lee of Point Sur, which was the site of the fastest sea level wind speed along central California. Isotherms dip and speeds decreased in the lee of Point Sur in the capping inversion well above the marine layer. COAMPS forecasted a compression shock wave initiating off the upwind side of the topography behind Point Sur and other coastal points to the north. Evidence from the model and the aircraft supports the existence of an oblique hydraulic jump on the north side of Point Sur.

Abstract

A midlevel, coastally trapped atmospheric event occurred along the California coast 10–11 June 1994. This feature reversed the surface wind field along the coast in a northerly phase progression. Along the central California coast, the winds at the coastal stations reverse before the corresponding coastal buoy offshore, then followed hours later by passage of the leading edge of an overcast stratus cloud. The sea surface temperature was much colder in the narrow strip along the coast. The cloud characteristics may be accounted for by a sea surface mixed layer (SSML) model beginning with the wind reversal and growing with the square root of time. Heat is lost from the SSML to the sea surface. A cloud forms when the air temperature at the top of the SSML is equal to the dewpoint. It is suggested that a bore develops on the top of the SSML, increasing the thickness of the SSML and the progression speed of the cloud to 8 m s⁻¹. There is evidence that an undular bore with a leading cloud develops in the thinner inshore SSML.

Advancing beyond Monterey Bay, horizontal density contrast is believed to have caused the bore to change character to a gravity current with a narrower cloud that passed a point inshore before the winds reversed at the buoys. The last trace of a disturbed boundary layer ended at Point Arena where strong northerly winds prevented any further northerly progression and contributed to a cyclonic eddy that was formed in the lee of the point.

Caution is suggested in the interpretation of stratus cloud phase progression without coastal wind measurements.

Abstract

This study uses a recently developed airborne Doppler radar database to explore how vortex misalignment is related to tropical cyclone (TC) precipitation structure and intensity change. It is found that for relatively weak TCs, defined here as storms with a peak 10-m wind of 65 kt (1 kt = 0.51 m s⁻¹) or less, the magnitude of vortex tilt is closely linked to the rate of subsequent TC intensity change, especially over the next 12–36 h. In strong TCs, defined as storms with a peak 10-m wind greater than 65 kt, vortex tilt magnitude is only weakly correlated with TC intensity change. Based on these findings, this study focuses on how vortex tilt is related to TC precipitation structure and intensity change in weak TCs. To illustrate how the TC precipitation structure is related to the magnitude of vortex misalignment, weak TCs are divided into two groups: small-tilt and large-tilt TCs. In large-tilt TCs, storms display a relatively large radius of maximum wind, the precipitation structure is asymmetric, and convection occurs more frequently near the midtropospheric TC center than the lower-tropospheric TC center. Alternatively, small-tilt TCs exhibit a greater areal coverage of precipitation inward of a relatively small radius of maximum wind. Greater rates of TC intensification, including rapid intensification, are shown to occur preferentially for TCs with greater vertical alignment and storms in relatively favorable environments.

Abstract

During summer, significant changes in marine atmospheric boundary layer (MABL) speed and depth occur over small spatial scales (<100 km) downstream from topographic features along the California coast. In June and July 1996, the Coastal Waves 96 project collected observations of such changes at capes with an instrumented aircraft. This paper presents observations from the 7 June flight, when the layer-averaged speed increased 9 m s⁻¹ and depth decreased by 500 m over a 75-km downwind from Cape Mendocino, accompanied by enhanced surface fluxes and local cloud clearing. The acceleration and thinning are reproduced when the flow is modeled as a shallow transcritical layer of fluid impinging the bends of a coastal wall, leading to the interpretation that they are produced by an expansion fan. Model runs were produced with different coastlines and imposed pressure gradients, with the best match provided by a coastline in which the cape protruded into the flow and forced a response in the subcritical region upstream of the cape. A hydraulic jump was produced at a second bend, near where the aircraft's lidar observed the MABL height to increase. Light variable winds observed within Shelter Cove were replicated in model flows in which the flow separated from the coastline. Though highly idealized, the shallow-water model provided a satisfactory representation of the main features of the observed flow.

Abstract

Heterogeneous ice nucleation is a source of uncertainty in models that represent ice clouds. The primary goal of the Ice in Clouds Experiment–Layer Clouds (ICE-L) field campaign was to determine if a link can be demonstrated between ice concentrations and the physical and chemical characteristics of the ambient aerosol. This study combines a 1D kinematic framework with lee wave cloud observations to infer ice nuclei (IN) concentrations that were compared to IN observations from the same flights. About 30 cloud penetrations from six flights were modeled. The temperature range of the observations was −16° to −32°C. Of the three simplified ice nucleation representations tested (deposition, evaporation freezing, and condensation/immersion droplet freezing), condensation/immersion freezing reproduced the lee wave cloud observations best. IN concentrations derived from the modeling ranged from 0.1 to 13 L⁻¹ compared to 0.4 to 6 L⁻¹ from an IN counter. A better correlation was found between temperature and the ratio of IN concentration to the concentration of large aerosol (>500 nm) than between IN concentration and the large aerosol concentration or temperature alone.

Abstract

An experiment comparing balloon and profiler observations was carried out to evaluate the capability of Doppler radar wind profilers to remotely measure useful meteorological quantities other than wind. The site chosen was in Southern California during a time of year when it offers a natural laboratory for investigating extreme contrasts in temperature and humidity. To evaluate the new capabilities, it was found that new and additional treatment of the radar data was necessary. For example, the adequacy of the usual radar wind observations obtained by editing the Doppler spectral moments was found to be very questionable for short-term observations, so the authors extended the editing to the raw spectra, and substantial improvement was found. The advantages of the redundancy in five-beam systems are investigated and are also found to be very necessary to obtain the accuracy needed. A technique for minimizing the variances of the differences of the four redundant pairs of radials is described. The resulting improved vertical velocity estimates substantially improve the agreement between radio acoustic sounding system (RASS) temperature retrievals and balloon-measured temperatures. The ability of the profilers to measure turbulence intensity was tested, and the accuracy of techniques using the spectral width to measure turbulent dissipation rate when complicated spectra are present is examined. Two different techniques for optimizing the calculation of spectral width are compared and the errors assessed. One technique integrates over the uncontaminated range of the chosen spectral peak and then extrapolates a Gaussian function to infinity. The other method uses the slope of the log least squares best fit of the uncontaminated points to a Gaussian function. Profiler-measured length scales of wind and scalar quantities are measured and compared. Profiles of radar-measured gradients of refractive index are compared with gradients measured by balloon. It is shown how gradients of humidity can be calculated to about the same accuracy as refractive-index gradients by combining the temperature gradients from RASS with the refractive-index gradient observations from the radar.

Abstract

As part of the NASA Earth Venture-Instrument program, the Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission, to be launched in January 2022, will deliver unprecedented rapid-update microwave measurements over the tropics that can be used to observe the evolution of the precipitation and thermodynamic structure of tropical cyclones (TCs) at meso- and synoptic scales. TROPICS consists of six CubeSats, each hosting a passive microwave radiometer that provides radiance observations sensitive to atmospheric temperature, water vapor, precipitation, and precipitation-sized ice particles. In this study, the impact of TROPICS all-sky radiances on TC analyses and forecasts is explored through a regional mesoscale observing system simulation experiment (OSSE). The results indicate that the TROPICS all-sky radiances can have positive impacts on TC track and intensity forecasts, particularly when some hydrometeor state variables and other state variables of the data assimilation system that are relevant to cloudy radiance assimilation are updated. The largest impact on the model analyses is seen in the humidity fields, regardless of whether or not there are radiances assimilated from other satellites. TROPICS radiances demonstrate large impact on TC analyses and forecasts when other satellite radiances are absent. The assimilation of the all-sky TROPICS radiances without default radiances leads to a consistent improvement in the low- and midtropospheric temperature and wind forecasts throughout the 5-day forecasts, but only up to 36-h lead time in the humidity forecasts at all pressure levels. This study illustrates the potential benefits of TROPICS data assimilation for TC forecasts and provides a potentially streamlined pathway for transitioning TROPICS data from research to operations postlaunch.

Analyses and forecasts for the first 2 weeks of the Genesis of Atlantic Lows Experiment (GALE) are described. These fields were produced using the National Meteorological Center (NMC) Regional Analysis and Forecast System (RAFS). Two sets of analyses and forecasts were constructed: one using the NMC operational database only (Level IIIa), and one using the NMC data merged with high-density observations taken during GALE (Level IIIb).

During the first 14 days of GALE, supplemental data were collected throughout two Intensive Observing Periods (IOPs). Comparisons of the Level IIIa and IIIb analyses over the GALE observing region in the southeastern United States indicated a worsening of the geopotential height analysis at operational NWS rawinsonde sites using the supplemental IIIb data. This was caused by inconsistencies in the height measurements at the high-density GALE rawinsonde sites. Such patterns were not observed in the wind and temperature analyses.

During IOP No. 1, the Level IIIa and IIIb Nested Grid Model (NGM) forecasts were nearly identical. For IOP No. 2, one forecast cycle saw an improvement in the Level IIIb forecasts due to offshore GALE dropwindsonde data, while another was improved by the inclusion of late-arriving rawinsonde data in the IIIb analysis. The inland, high-density GALE soundings, however, had a negligible impact on NGM forecasts during the entire 12-day period.