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F. M. Ralph
and
M. D. Dettinger
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Jason M. Cordeira
and
F. Martin Ralph

Abstract

The ability to provide accurate forecasts and improve situational awareness of atmospheric rivers (ARs) is key to impact-based decision support services and applications such as forecast-informed reservoir operations. The purpose of this study is to quantify the cool-season water year skill for 2017–20 of the NCEP Global Ensemble Forecast System forecasts of integrated water vapor transport along the U.S. West Coast commonly observed during landfalling ARs. This skill is summarized for ensemble probability-over-threshold forecasts of integrated water vapor transport magnitudes ≥ 250 kg m−1 s−1 (referred to as P 250). The P 250 forecasts near North-Coastal California at 38°N, 123°W were reliable and successful at lead times of ~8–9 days with an average success ratio > 0.5 for P 250 forecasts ≥ 50% at lead times of 8 days and Brier skill scores > 0.1 at a lead time of 8–9 days. Skill and accuracy also varied as a function of latitude and event characteristics. The highest (lowest) success ratios and probability of detection values for P 250 forecasts ≥ 50% occurred on average across Northern California and Oregon (Southern California), whereas the average probability of detection of more intense and longer duration landfalling ARs was 0.1–0.2 higher than weaker and shorter duration events at lead times of 3–9 days. The potential for these forecasts to enhance situational awareness may also be improved, depending on individual applications, by allowing for flexibility in the location and time of verification; the success ratios increased 10%–30% at lead times of 5–10 days allowing for flexibility of ±1.0° latitude and ±6 h in verification.

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F. M. Ralph
,
V. Venkateswaran
, and
M. Crochet

Abstract

This paper reports coordinated observations of a mesoscale gravity wave made during the FRONTS 84 field experiment conducted in southwestern France in the summer of 1984. The observations were unique in the sense that all relevant wave characteristics, including its detailed vertical structure, could be directly ascertained and shown to be mutually consistent. The wave was observed to be ducted. The static stability structure of the atmosphere was favorable for the existence of such a duct. A critical level (where the phase velocity of the wave matches the wave-parallel background wind speed) was present in or just above a layer of low static stability that capped a stabler layer at a still lower level. The observed vertical structure was that of a standing wave with the top of the stable lower layer corresponding roughly to an antinode and one-half the vertical wavelength below the critical level. The wave was observed when there was extensive activity associated with two line of deep convection. No define conclusion could be drawn, however, about the interaction between the wave and the convection.

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F. M. Ralph
,
C. Mazaudier
,
M. Crochet
, and
S. V. Venkateswaran

Abstract

Observations from two Doppler sodars and a radar wind profiler have been used in conjunction with data from a rawinsonde station and a mesoscale surface observation network to conduct a case study of a gravity current entering into an environment containing a nocturnal inversion and an elevated neutral layer. On the basis of synoptic and mesoscale analyses, it is concluded that the gravity current might have originated either as a scale-contracted cold front or as a gust front resulting from thunderstorm outflows observed very near the leading edge of a cold front. Despite this ambiguity, the detailed vertical structure of the gravity current itself is well resolved from the data. Moreover, the vertical velocity measurements provided by the sodars and the radar wind profiler at high time resolution have given unique information about the height structure of gravity waves excited by the gravity current. Although only wave periods, and not phase speeds or wavelengths, are directly measured, it is possible to make reasonable inferences about wave excitation mechanisms and about the influence and control of ambient stratification on wave-field characteristics. Both Kelvin-Helmholtz waves generated in the regions of high wind shear found in association with the gravity current and lee-type waves forced by the gravity current acting as an obstacle to opposing prefrontal flow are identified. It is also found that the propagation speed of the gravity current and the relative depths of the prefrontal inversion and the postfrontal cold air were not favorable for the formation of either internal bores or solitary waves at the time of day at which the gravity current was being observed.

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Toshio M. Chin
,
Ralph F. Milliff
, and
William G. Large

Abstract

A numerical technique sensitive to both spectral and spatial aspects of sea surface wind measurements is introduced to transform the irregularly sampled satellite-based scatterometer data into regularly gridded wind fields. To capture the prevailing wavenumber characteristics (power-law dependence) of sea surface wind vector components, wavelet coefficients are computed from the scatterometer measurements along the satellite tracks. The statistics of the wavelet coefficients are then used to simulate high-resolution wind components over the off-track regions where scatterometer data are not available. Using this technique, daily wind fields with controlled spectral features have been produced by combining the low-wavenumber wind fields from ECMWF analyses with the high-wavenumber measurements from the ERS-1 scatterometer. The resulting surface wind fields thus reflect nearly all available measurements affecting surface wind, including the synoptic surface pressure. The new surface wind forces a basin-scale quasigeostrophic ocean model such that the average circulation and energetics are consistent with the previous studies, in which purely synthetic high-wavenumber wind forcing was used.

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Kettyah C. Chhak
,
Andrew M. Moore
, and
Ralph F. Milliff

Abstract

At middle and high latitudes, the magnitude of stochastic wind stress forcing of the ocean by atmospheric variability on synoptic time scales (i.e., “weather” related variability) is comparable to that of the seasonal cycle. Stochastic forcing may therefore have a significant influence on the ocean circulation, climate, and ocean predictability. Here, the influence of stochastic forcing associated with the North Atlantic Oscillation on the subtropical gyre circulation of the North Atlantic is explored in an eddy-permitting quasigeostrophic framework. For the North Atlantic winds used in this study, the root-mean-square of the annual average Ekman pumping velocity of the seasonal cycle between 35° and 52°N is 1.3 × 10−7 m s−1, while the wintertime standard deviation of the stochastic component of the North Atlantic Oscillation over the same latitude band is 2.2 × 10−7 m s−1. Significant stochastically induced variability in the ocean circulation occurs near the western boundary region and along the western margins of the abyssal plains associated with vortex stretching, energy release from the mean flow, and the generation of topographic Rossby waves. Variability arises from a combination of two effects, depending on the measure of variance used: growth of unstable modes of the underlying circulation and modal interference resulting from their nonnormal nature, which dominates during the first 10 days or so of perturbation growth. Near the surface, most of the variability is associated with large-scale changes in the barotropic circulation, although more than 20% of the energy and enstrophy variability is associated with small-scale baroclinic waves. In the deep ocean, much of the stochastically induced variability is apparently due to topographic Rossby wave activity along the continental rise and ocean ridges. Previous studies have demonstrated that rectification of topographic Rossby wave–induced circulations in the western North Atlantic may contribute to the western boundary current recirculation zones. The authors suggest that a source of topographic Rossby wave energy, significant enough to rectify the mean ocean circulation, may arise from stochastic forcing by large-scale atmospheric forcing, such as the North Atlantic Oscillation and other atmospheric teleconnection patterns.

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Stephen J. Lord
,
Xingren Wu
,
Vijay Tallapragada
, and
F. M. Ralph

Abstract

The impact of assimilating dropsonde data from the 2020 Atmospheric River (AR) Reconnaissance (ARR) field campaign on operational numerical weather forecasts was assessed. Two experiments were executed for the period from 24 January to 18 March 2020 using the National Centers for Environmental Prediction (NCEP) Global Forecast System version 15 (GFSv15) with a four-dimensional hybrid ensemble–variational (4DEnVar) data assimilation system. The control run (CTRL) used all of the routinely assimilated data and included data from 628 ARR dropsondes, whereas the denial run (DENY) excluded the dropsonde data. Results from 17 intensive observing periods (IOPs) indicate a mixed impact for mean sea level pressure and geopotential height over the Pacific–North American (PNA) region in CTRL compared to DENY. The overall local impact over the U.S. West Coast and Gulf of Alaska for the 17 IOPs is neutral (−0.45%) for integrated vapor transport (IVT), but positive for wind and moisture profiles (0.5%–1.0%), with a spectrum of statistically significant positive and negative impacts for various IOPs. The positive dropsonde data impact on precipitation forecasts over U.S. West Coast domains appears driven, in part, by improved low-level moisture and wind fields at short-forecast lead times. Indeed, data gaps, especially for accurate and unbiased moisture profiles and wind fields, can be at least partially mitigated to improve U.S. West Coast precipitation forecasts.

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Stephen J. Lord
,
Xingren Wu
,
Vijay Tallapragada
, and
F. M. Ralph

Abstract

The impact of assimilating dropsonde data from the 2020 Atmospheric River (AR) Reconnaissance (ARR) field campaign on operational numerical precipitation forecasts was assessed. Two experiments were executed for the period from 24 January to 18 March 2020 using the NCEP Global Forecast System, version 15 (GFSv15), with a four-dimensional hybrid ensemble–variational (4DEnVar) data assimilation system. The control run (CTRL) used all the routinely assimilated data and included ARR dropsonde data, whereas the denial run (DENY) excluded the dropsonde data. There were 17 intensive observing periods (IOPs) totaling 46 Air Force C-130 and 16 NOAA G-IV missions to deploy dropsondes over targeted regions with potential for downstream high-impact weather associated with the ARs. Data from a total of 628 dropsondes were assimilated in the CTRL. The dropsonde data impact on precipitation forecasts over U.S. West Coast domains is largely positive, especially for day-5 lead time, and appears driven by different model variables on a case-by-case basis. These results suggest that data gaps associated with ARs can be addressed with targeted ARR field campaigns providing vital observations needed for improving U.S. West Coast precipitation forecasts.

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F. M. Ralph
,
P. J. Neiman
, and
T. L. Keller

Abstract

The water vapor channel of the Geostationary Operational Environmental Satellite GOES-8 reveals narrow (30 km wide), elongated (500–1000 km) bands that propagate southward on the eastern side of the Rocky Mountains from Colorado to Texas. Two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail, and several other cases are summarized. The wind profilers observe vertical motions exceeding 1 m s−1 in narrow plumes at the leading edge of the fronts, in broader zones in the upper troposphere, and in the lower stratosphere. These cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images.

The bands occur when the Rocky Mountains block either arctic leeside cold fronts coming from the north or northeast or Pacific cold fronts coming from the northwest. The blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain-modified fronts. This imbalance is manifested as strong (20 m s−1) prefrontal, front-relative, cross-front flow V r . Observations and numerical simulations are presented showing that deep-tropospheric gravity waves are produced in this region by the obstacle effect of the surface leeside cold front. Farther east, V r is near zero, and the waves are weak or absent.

Along the western portion of the front the waves propagate with the front and resemble trapped lee waves; however, farther east the waves appear ahead of the surface front by up to 100 km. These prefrontal gravity waves occur when the wave forcing decays along the eastern portion of the front and the trapped waves that had developed there become decoupled from the front and propagate away. Numerical simulations of a well-observed event confirm that trapped waves would have developed, and profiler data confirm the trapped nature of the observed gravity wave’s vertical structure. Such waves could create convection, including prefrontal squall lines, and can be seen in real-time satellite imagery before the convection is triggered.

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Michael D. Sierks
,
Julie Kalansky
,
Forest Cannon
, and
F. M. Ralph

Abstract

The North American monsoon (NAM) is the main driver of summertime climate variability in the U.S. Southwest. Previous studies of the NAM have primarily focused on the Tier I region of the North American Monsoon Experiment (NAME), spanning central-western Mexico, southern Arizona, and New Mexico. This study, however, presents a climatological characterization of summertime precipitation, defined as July–September (JAS), in the Lake Mead watershed, located in the NAME Tier II region. Spatiotemporal variability of JAS rainfall is examined from 1981 to 2016 using gridded precipitation data and the meteorological mechanisms that account for this variability are investigated using reanalyses. The importance of the number of wet days (24-h rainfall ≥1 mm) and extreme rainfall events (95th percentile of wet days) to the total JAS precipitation is examined and shows extreme events playing a larger role in the west and central basin. An investigation into the dynamical drivers of extreme rainfall events indicates that anticyclonic Rossby wave breaking (RWB) in the midlatitude westerlies over the U.S. West Coast is associated with 89% of precipitation events >10 mm (98th percentile of wet days) over the Lake Mead basin. This is in contrast to the NAME Tier I region where easterly upper-level disturbances such as inverted troughs are the dominant driver of extreme precipitation. Due to the synoptic nature of RWB events, corresponding impacts and hazards extend beyond the Lake Mead watershed are relevant for the greater U.S. Southwest.

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