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F. Martin Ralph

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F. Martin Ralph

Abstract

Satellite images of a decaying synoptic-scale cyclone over the North Pacific reveal two distinct types of multiple mososcale cyclonic disturbances. The approximate positions of these disturbances within the synoptic-scale cyclone are determined using analyses from an operational global-scale numerical model.

One type, a set of four 250-km-wavelength eddies, occurred in clear air and represent perturbations within the cyclonic shear side of a 40–45 m s−1 upper-level jet-front system. Their propagation at 28.5 ± 3.5 m s−1 roughly matched the wind speed and direction within the jet at their position and provided evidence of stretching deformation along their axis. Their growth is documented over 18 h, and is measured in terms of horizontal displacements of a preexisting moisture boundary in water vapor imagery. Their e-folding time increased from 6 to 9 h as horizontal displacements exceeded 100 km and horizontal billows indicative of wave breaking formed. The billow-like structures most likely represent areas of enhanced mixing of stratospheric and tropospheric air by means of a quasi-horizontal process acting in the vicinity of a tropopause fold. Because they developed in a region of significant horizontal shear and because the absence of clouds suggests vertical motions were small or absent, the behavior of these eddies is consistent with barotropic instability on a 50-km-wide shear zone or potential vorticity strip.

The other type, a set of five 7-km-wavelength cyclonic disturbances (mesocyclones), is also evident in the satellite images, but because they modulated the cloud field they appear in both the WV and infrared images. They wrapped fully around the synoptic-scale cyclone, forming a wavenumber 5 perturbation that later became a wavenumber 4 perturbation propagating cyclonically about its center at 19 ± 1 m s−1 and likely formed on an occluded front. Two of these waves are distinguished by the development of their own comma clouds, indicating they had deep vertical circulations and suggesting that moist baroclinic instability or CISK was active. These 750-km-wavelength mesocyclones most likely affected the deformation and shear along the upper front, which could have modified the barotropic stability of the region, and thus influenced where the 250-km-wavelength eddies formed.

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F. Martin Ralph

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Thresholds separating regimes for which Rayleigh scattering from precipitation is likely to dominate over Bragg scattering from clear air are established for several common radar wind profiler frequencies. The thresholds are first determined for radar reflectivity factor Z based on observed values of the structure parameter C n 2 in the troposphere. These thresholds for Z are then transformed to thresholds for modal terminal velocities V * of rain and snow for exponential particle size distributions. Measurements at 915,404, and 50 MHz in stratiform rain help substantiate the applicability of the calculated thresholds, even though fall velocities, rather than V *, are measured. Because the V * thresholds for rain at wind profiler frequencies (i.e., >2.5–5.6 m s−1 at 404 MHz) are substantially greater than typical vertical air motions outside convective cells, profiler-observed radial vertical velocities are a robust indicator of the presence of rain in profiler data. Although snow can also often be identified in this manner, the V * thresholds (i.e., 0.5–1.2 m s−1) are small enough to increase the probability that mesoscale clear-air vertical motions can mask or resemble its signature. The technique developed here allows rain, and to a lesser extent snow, to be identified in radar wind profiler data under most conditions without having to examine the entire Doppler power spectrum, even when the profiler is not calibrated to measure reflectivity factor.

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F. Martin Ralph

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Ralph F. Keeling and Martin Visbeck

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The suggestion is advanced that the remarkably low static stability of Antarctic surface waters may arise from a feedback loop involving global deep-water temperatures. If deep-water temperatures are too warm, this promotes Antarctic convection, thereby strengthening the inflow of Antarctic Bottom Water into the ocean interior and cooling the deep ocean. If deep waters are too cold, this promotes Antarctic stratification allowing the deep ocean to warm because of the input of North Atlantic Deep Water. A steady-state deep-water temperature is achieved such that the Antarctic surface can barely undergo convection. A two-box model is used to illustrate this feedback loop in its simplest expression and to develop basic concepts, such as the bounds on the operation of this loop. The model illustrates the possible dominating influence of Antarctic upwelling rate and Antarctic freshwater balance on global deep-water temperatures.

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Zhenhai Zhang and F. Martin Ralph

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Some extratropical cyclones (ETC) begin their development in close proximity to a pre-existing atmospheric river (AR). This study investigates the differences in the cyclogenesis stage between these cyclogenesis events and those that begin without an AR nearby. Well-established ETC and AR detection methods are applied to reanalysis over the North Pacific during the 1979-2009 cool seasons (November-March). Of the 3137 cyclogenesis cases detected, 35% are associated with a nearby AR at the time of initial cyclogenesis. Of all 448 cyclones that deepened explosively in the 24 h after their initiation, 60% began with a pre-existing AR nearby.

The roles of both dry and diabatic processes that contribute to cyclogenesis are examined, specifically, low-level baroclinicity, upper-level forcing, water vapor inflow and latent heating. ETCs that develop associated with a pre-existing AR receive nearly 80% more water vapor inflow on average, enhancing latent heating and intensifying cyclone deepening in the genesis stage. In contrast, neither low-level baroclinicity nor upper-level potential vorticity exhibit statistically significant differences between cyclogenesis events with and without an AR. Cyclogenesis events associated with an exceptionally strong AR at the ETC initial time deepen even more rapidly in the genesis stage, indicating that the intensity of an antecedent AR can modulate cyclogenesis. About half of the cyclogenesis cases off the U.S. West Coast are associated with ARs at their initial time.

These results imply that errors in initial conditions related to ARs can contribute to errors in both AR and ETC predictions, as well as their concomitant impacts.

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Jason M. Cordeira and F. Martin Ralph

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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. Martin Ralph and Thomas J. Galarneau Jr.

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Between North America’s Sierra Madre and Rocky Mountains exists a little-recognized terrain “gap.” This study defines the gap, introduces the term “Chiricahua Gap,” and documents the role of easterly transport of water vapor through the gap in modulating summer monsoon precipitation in southeastern Arizona. The gap is near the Arizona–New Mexico border north of Mexico and is approximately 250 km wide by 1 km deep. It is the lowest section along a 3000-km length of the Continental Divide from 16° to 45°N and represents 80% of the total cross-sectional area below 2.5 km MSL open to horizontal water vapor transport in that region. This study uses reanalyses and unique upper-air observations in a case study and a 15-yr climatology to show that 72% (76%) of the top-quartile (decile) monsoon precipitation days in southeast Arizona during 2002–16 occurred in conditions with easterly water vapor transport through the Chiricahua Gap on the previous day.

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Rebecca E. Morss and F. Martin Ralph

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Winter storms making landfall in western North America can generate heavy precipitation and other significant weather, leading to floods, landslides, and other hazards that cause significant damage and loss of life. To help alleviate these negative impacts, the California Land-falling Jets (CALJET) and Pacific Land-falling Jets (PACJET) experiments took extra meteorological observations in the coastal region to investigate key research questions and aid operational West Coast 0–48-h weather forecasting. This article presents results from a study of how information provided by CALJET and PACJET was used by National Weather Service (NWS) forecasters and forecast users. The primary study methodology was analysis of qualitative data collected from observations of forecasters and from interviews with NWS personnel, CALJET–PACJET researchers, and forecast users. The article begins by documenting and discussing the many types of information that NWS forecasters combine to generate forecasts. Within this context, the article describes how forecasters used CALJET–PACJET observations to fill in key observational gaps. It then discusses researcher–forecaster interactions and examines how weather forecast information is used in emergency management decision making. The results elucidate the important role that forecasters play in integrating meteorological information and translating forecasts for users. More generally, the article illustrates how CALJET and PACJET benefited forecasts and society in real time, and it can inform future efforts to improve human-generated weather forecasts and future studies of the use and value of meteorological information.

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Reuben Demirdjian, Joel R. Norris, Andrew Martin, and F. Martin Ralph

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The pre-cold-frontal low-level jet (LLJ) is an important contributor for water vapor transport within atmospheric rivers, though its dynamics are not completely understood. The present study investigates the LLJ using dropsonde observations from 24 cross-atmospheric river transects taken during the CalWater-2014, 2015 and the AR-Recon 2016, 2018 field campaigns. It is found that the LLJ, located at ~1-km elevation ahead of the cold front, has an average maximum wind speed of 30 m s−1 and is strongly supergeostrophic with an average ageostrophic component of 6 m s−1. The alongfront ageostrophy occurs within the atmospheric layer (750–1250 m) known to strongly control orographic precipitation associated with atmospheric rivers. The ERA5 reanalysis product is used to both validate the observed geostrophic winds and investigate the supergeostrophic jet dynamics. The comparison demonstrates that there is no systematic bias in the observed geostrophic wind but that the ERA5 LLJ total wind field is generally biased low by an amount consistent with the observed ageostrophy. One of the few cases in which the ERA5 produces an ageostrophic LLJ occurs on 13 February 2016, which is used to investigate the dynamical processes responsible for the ageostrophy. This analysis demonstrates that the isallobaric (pressure tendency) term serves to accelerate the ageostrophic jet, and the Coriolis torque and advective tendency terms serve to propagate the jet normal to the LLJ. Therefore, if a model is to accurately represent the LLJ, it must adequately resolve processes contributing toward the pressure tendencies along the cold front.

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