Abstract

The skill of mixed-layer hindcasts of short (12–120 h) duration was investigated using data from Ocean Weather Station Papa for the years 1960–68, and from the Mixed Layer Experiment (MILE), which was conducted about 4O km southwest of Papa in late summer of 1977. The hindcasts were initialized and validated with observed temperature profiles and forced with surface wind stresses and heat fluxes calculated from meteorological observations. Mean and rms hindcast errors for sea surface temperature (SST) and mixed-layer depth (MLD) were compared with errors for persistence and climatology. Hindcast skill was calculated as the percent improvement of the hindcast rms error over the persistence rms error.

The hindcast skill was significantly higher for the MILE data than for the Papa data. Hindcast skill with the Papa data was generally higher in spring and summer than in fall and winter. The range of hindcast skill for hindcasts of 12–36-h duration was 39%–48% for SST and 28%–37% for MLD for the MILE data versus 14%–19% for SST and 22%–33% for MLD for the Papa data for the spring and summer. Smoothing the MILE temperature observations with a 2-h running mean resulted in an increase in hindcast skill of about 3% due to the reduction in small-scale noise. The persistence rms MLD error was found to exceed the climatological error after about 2 days for both the MILE hindcasts and for the Papa spring and summer hindcasts. Hindcasts initialized from climatology in spring and summer showed skill for MLD similar to that for hindcasts initialized from an observed profile for hindcast durations longer than about 2 days.

A synoptic forecast model of the oceanic mixed layer has been developed for operational use at the U.S. Navy's Fleet Numerical Oceanography Center (FNOC), Monterey, Calif. The potential success of this model depends critically on the quality of the Navy's operational environmental data base, which supplies the model with its initial and boundary conditions. In this paper we discuss the model and its applications and investigate the suitability of this data base for operational ocean forecasting through verification results from a 72 h forecast performed in real time for the entire Northern Hemisphere.

The model does not exhibit skill in forecasting changes in sea surface temperature in the tropics. This may be due to an inadequate representation of the surface fluxes of heat, moisture, and momentum provided by the operational data base for this region. In extratropical regions, however, the model does show skill in predicting changes in sea surface temperature on the time scale considered.

Abstract

The Mellor and Yamada (1974) Level II turbulence closure scheme is used to study the oceanic bottom boundary layer (BBL). The model is tested against observations of the BBL obtained on the western Florida Shelf reported in Weatherly and Van Leer (1977) and in turn conclusions about the BBL made in that paper are tested against the model. The agreement between the model and the observations is good. The predicted and observed BBL thickness is ∼10 m which is appreciably less than 0.4 u _*/f ≈ 30 m, where u _* is the friction velocity and f the Coriolis parameter. The reason for the discrepancy is attributed to the BBL being formed in water which initially was stably stratified and characterized by a Brunt Vasäilä frequency N₀. It is suggested that the oceanic BBL thickness should be identified with the height at which the turbulence generated in the BBL goes to zero and on dimensional grounds it is proposed that this thickness is A u _*/f(1 + N ₀ ²/f ²)^½, where A is a constant. The Level II model indicates that this is a good approximation over the range 0 ≤ N ₀/f ≲ 200 provided A ≈ 1.3. Other features common to the predicted results and observations are 1) the vertical profiles of temperature and current direction which are very similar, with most of the direction changes (Ekman veering) occurring at the top of the BBL where the density stratification is largest, 2) a jet-like structure in some of the speed and direction profiles; and 3) appreciably more total Ekman veering than expected for a comparable BBL formed in neutrally stratified water.

The one-dimensional BBL formed under an along-isobath current in a stably stratified ocean is investigated for the case when the bottom is inclined relative to the horizontal isotherms. It is found that the BBL may no longer have the signature of a simple, vertically well-mixed layer because of Ekman-veering-induced upwelling (downwelling) of cooler (warmer) water in the BBL.

The profile of down-the-pressure gradient velocity component in the BBL is found to closely resemble the downslope flow of a heavier fluid discussed in Turner (1973). The Froude number stability criteria given in Turner (1973) when applied to the Level II model results suggest that the BBL formed in a stably stratified ocean is, in a Froude number sense, stable or marginally stable on continental margins while it is unstable in the deep ocean.

Abstract

This study uses a unique combination of airborne and satellite observations to characterize narrow regions of strong horizontal water vapor flux associated with polar cold fronts that occurred over the eastern North Pacific Ocean during the winter of 1997/98. Observations of these “atmospheric rivers” are compared with past numerical modeling studies to confirm that such narrow features account for most of the instantaneous meridional water vapor transport at midlatitudes.

Wind and water vapor profiles observed by dropsondes deployed on 25–26 January 1998 during the California Land-falling Jets Experiment (CALJET) were used to document the structure of a modest frontal system. The horizontal water vapor flux was focused at low altitudes in a narrow region ahead of the cold front where the combination of strong winds and large water vapor content were found as part of a low-level jet. A close correlation was found between these fluxes and the integrated water vapor (IWV) content. In this case, 75% of the observed flux through a 1000-km cross-front baseline was within a 565-km-wide zone roughly 4 km deep. This zone contained 1.5 × 10⁸ kg s⁻¹ of meridional water vapor flux, the equivalent of ∼20% of the global average at 35°N.

By compositing polar-orbiting satellite Special Sensor Microwave Imager (SSM/I) data from 46 dates containing long, narrow zones of large IWV, it was determined that the single detailed case was representative of the composite in terms of both the IWV amplitude (3.09 cm vs 2.81 cm) and the width of the area where IWV ≥ 2 cm (424 km vs 388 km). The SSM/I composites also showed that the width scales (defined by the 75% cumulative fraction along a 1500-km cross-plume baseline) for cloud liquid water and rain rate were 176 and 141 km, respectively, which are narrower than the 417 km for IWV. Examination of coincident Geostationary Operational Environmental Satellite (GOES) and SSM/I satellite data revealed that GOES cloud-top temperatures were coldest and cloud-top pressures were lowest in the core of the IWV plumes, and that the core cloud tops became substantially colder and deeper for larger IWV. A strong latitudinal dependence of the satellite-derived cross-river characteristics was also found.

Atmospheric rivers form a critical link between weather and climate scales. They strongly influence both short-term weather and flood prediction, as well as seasonal climate anomalies and the global water cycle, through their cumulative effects. However, the rivers remain poorly observed by the existing global atmospheric observing system in terms of their horizontal water vapor fluxes.

Abstract

Dropsonde observations are used to document the mean vertical profiles of kinematic and thermodynamic conditions in the pre-cold-frontal low-level-jet (LLJ) region of extratropical cyclones over the eastern Pacific Ocean. This is the region within storms that is responsible not only for the majority of heavy rainfall induced by orography when such storms strike the coast, but also for almost all meridional water vapor transport at midlatitudes. The data were collected from NOAA’s P-3 aircraft in 10 storms during the California Land-falling Jets Experiment (CALJET) of 1998 and in 7 storms during the Pacific Land-falling Jets Experiment (PACJET) of 2001. The mean position of the dropsondes was 500 km offshore, well upstream of orographic influences. The availability of data from two winters that were characterized by very different synoptic regimes and by differing phases of ENSO—that is, El Niño in 1998 and La Niña in 2001—allowed examination of interannual variability.

The composite pre-cold-frontal profiles reveal a well-defined LLJ at 1.0-km altitude with a wind speed of 23.4 m s⁻¹ and a wind direction of 216.7°, as well as vertical shear characteristic of warm advection. The composite thermodynamic conditions were also documented, with special attention given to moist static stability due to the nearly saturated conditions that were prevalent. Although the dry static stability indicated very stable conditions (4.5 K km⁻¹), the moist static stability was approximately zero up to 2.8-km altitude. Although the composite winds, temperatures, and water vapor mixing ratios in 2001 differed markedly from 1998, the moist static stability remained near zero from the surface up to 2.8–3.0-km altitude for both seasons. Hence, orographic precipitation enhancement is favored in this sector of the storm, regardless of the phase of ENSO.

The dropsonde data were also used to characterize the depth and strength of atmospheric rivers, which are responsible for most of the meridional water vapor transport at midlatitudes. The vertically integrated along-river water vapor fluxes averaged 525 × 10⁵ kg s⁻¹ (assuming a 100-km-wide swath), while the meridional and zonal components were 387 × 10⁵ kg s⁻¹ and 302 × 10⁵ kg s⁻¹, respectively. Although the composite meridional transport in 2001 was less than half that in 1998 (230 × 10⁵ kg s⁻¹ versus 497 × 10⁵ kg s⁻¹), the characteristic scale height of the meridional water vapor transport remained constant; that is, 75% of the transport occurred below 2.25-km altitude.

Abstract

Correlations between range-corrected signal power S _rc and radial vertical velocity V_r , from the vertical beam of a UHF wind profiler can be used to distinguish between air- and precipitation-dominated echoes using an S _rc–V_r correlation diagram. While there is no clear correlation between vertical air motions and S _rc, there is a strong correlation between the precipitation fall velocity and S _rc in snow, and to a lesser extent, in rain. This is illustrated through intercomparison of three types of precipitation events, and two types of clear-air events.

Using a histogram of V_r , from an event where there is evidence of precipitation in its S _rc–V_r correlation diagram, and from other information, it is possible to objectively determine a threshold value of V_r , referred to as V_T , that approximately identifies which measurements are dominated by Rayleigh scattering from precipitation in that event. A method is introduced that uses the histogram of observed V_r , from that event to provide an estimate of how many measurements are incorrectly attributed to Bragg scattering or Rayleigh scattering as a function of V_T . The error estimates can be used to select V_T on a case-by-case basis and according to the needs of the particular application. An objective dual-optimization technique results in an estimated overall error of less than 6%, averaged over three case studies. In addition, it is shown that inclusion of velocity variance from the vertical beam in the S _rc–V_r , correlation diagrams can help distinguish between rain and snow, and between convective and stratiform precipitation.

Abstract

Two vertically pointing S-band radars (coastal and inland) were operated in western Washington during two winters to monitor brightband snow-level altitudes. Similar snow-level characteristics existed at both sites, although the inland site exhibited lower snow levels by ~70 m because of proximity to cold continental air, and snow-level altitude changes were delayed there by several hours owing to onshore translation of weather systems. The largest precipitation accumulations and rates occurred when the snow level was largely higher than the adjacent terrain. A comparison of these observations with long-term operational radiosonde data reveals that the radar snow levels mirrored climatological conditions. The inland radar data were used to assess the performance of nearby operational freezing-level forecasts. The forecasts possessed a lower-than-observed bias of 100–250 m because of a combination of forecast error and imperfect representativeness between the forecast and observing points. These forecast discrepancies increased in magnitude with higher observed freezing levels, thus representing the hydrologically impactful situations where a greater fraction of mountain basins receive rain rather than snow and generate more runoff than anticipated. Vertical directional wind shear calculations derived from wind-profiler data, and concurrent surface temperature data, reveal that most snow-level forecast discrepancies occurred with warm advection aloft and low-level cold advection through the Stampede Gap. With warm advection, forecasts were too high (low) for observed snow levels below (above) 1.25 km MSL. An analysis of sea level pressure differences across the Cascades indicated that mean forecasts were too high (low) for observed snow levels below (above) 1.25 km MSL when higher pressure was west (east) of the range.

Abstract

A 915-MHz wind profiler, a GPS receiver, and surface meteorological sites in and near California’s northern Central Valley (CV) provide the observational anchor for a case study on 23–25 October 2010. The study highlights key orographic influences on precipitation distributions and intensities across northern California during a landfalling atmospheric river (AR) and an associated Sierra barrier jet (SBJ). A detailed wind profiler/GPS analysis documents an intense AR overriding a shallow SBJ at ~750 m MSL, resulting in record early season precipitation. The SBJ diverts shallow, pre-cold-frontal, incoming water vapor within the AR poleward from the San Francisco Bay gap to the northern CV. The SBJ ultimately decays following the passage of the AR and trailing polar cold front aloft. A statistical analysis of orographic forcing reveals that both the AR and SBJ are crucial factors in determining the amount and spatial distribution of precipitation in the northern Sierra Nevada and in the Shasta–Trinity region at the northern terminus of the CV. As the AR and SBJ flow ascends the steep and tall terrain of the northern Sierra and Shasta–Trinity region, respectively, the precipitation becomes enhanced. Vertical profiles of the linear correlation coefficient quantify the orographic linkage between hourly upslope water vapor flux profiles and hourly rain rate. The altitude of maximum correlation (i.e., orographic controlling layer) is lower for the shallow SBJ than for the deeper AR (i.e., 0.90 versus 1.15 km MSL, respectively). This case study expands the understanding of orographic precipitation enhancement from coastal California to its interior. It also quantifies the connection between dry antecedent soils and reduced flood potential.

Abstract

Because knowledge of the melting level is critical to river forecasters and other users, an objective algorithm to detect the brightband height from profiles of radar reflectivity and Doppler vertical velocity collected with a Doppler wind profiling radar is presented. The algorithm uses vertical profiles to detect the bottom portion of the bright band, where vertical gradients of radar reflectivity and Doppler vertical velocity are negatively correlated. A search is then performed to find the peak radar reflectivity above this feature, and the brightband height is assigned to the altitude of the peak. Reflectivity profiles from the off-vertical beams produced when the radar is in the Doppler beam swinging mode provide additional brightband measurements. A consensus test is applied to subhourly values to produce a quality-controlled, hourly averaged brightband height. A comparison of radar-deduced brightband heights with melting levels derived from temperature profiles measured with rawinsondes launched from the same radar site shows that the brightband height is, on average, 192 m lower than the melting level. A method for implementing the algorithm and making the results available to the public in near–real time via the Internet is described. The importance of melting level information in hydrological prediction is illustrated using the NWS operational river forecast model applied to mountainous watersheds in California. It is shown that a 2000-ft increase in the melting level can triple run off during a modest 24-h rainfall event. The ability to monitor the brightband height is likely to aid in melting-level forecasting and verification.

Abstract

A multiscale analysis is conducted in order to examine the physical processes that resulted in prolonged heavy rainfall and devastating flash flooding across western and central Tennessee and Kentucky on 1–2 May 2010, during which Nashville, Tennessee, received 344.7 mm of rainfall and incurred 11 flood-related fatalities. On the synoptic scale, heavy rainfall was supported by a persistent corridor of strong water vapor transport rooted in the tropics that was manifested as an atmospheric river (AR). This AR developed as water vapor was extracted from the eastern tropical Pacific and the Caribbean Sea and transported into the central Mississippi Valley by a strong southerly low-level jet (LLJ) positioned between a stationary lee trough along the eastern Mexico coast and a broad, stationary subtropical ridge positioned over the southeastern United States and the subtropical Atlantic. The AR, associated with substantial water vapor content and moderate convective available potential energy, supported the successive development of two quasi-stationary mesoscale convective systems (MCSs) on 1 and 2 May, respectively. These MCSs were both linearly organized and exhibited back-building and echo-training, processes that afforded the repeated movement of convective cells over the same area of western and central Tennessee and Kentucky, resulting in a narrow band of rainfall totals of 200–400 mm. Mesoscale analyses reveal that the MCSs developed on the warm side of a slow-moving cold front and that the interaction between the southerly LLJ and convectively generated outflow boundaries was fundamental for generating convection.