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C. C. Clark
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C. C. Clark
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Kevin C. Prince and Clark Evans

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Cold surges represent one of several phenomena by which midlatitude features can modulate the atmosphere, both dynamically and thermodynamically, deep into the tropics. This study involves the construction of a climatology of the strongest South American cold surges that follow along the Andes Mountains to quantify the extent to which these surges modulate the atmosphere from the midlatitudes to the tropics. Cold surges occurring during June–September (austral winter) from 1980 to 2017 are considered. In this study, cold-surge events are identified using standardized anomalies of 925-hPa meridional wind and 925-hPa temperature. As compared with previous cold-surge investigations, the use of standardized anomalies better enables spatial variation in cold-surge intensity and impacts to be quantified. A strong cold surge is defined as one in which the 925-hPa temperature is at least 3 standardized anomalies below 0 and the 925-hPa meridional wind is at least 3 standardized anomalies above 0 on the meso-α scale or larger. Using these criteria, 67 events are identified. The composite cold surge is characterized by highly anomalous cold, southerly flow that originates in northern Argentina and progresses northward, significantly modulating lower-tropospheric kinematic and thermodynamic fields across the entire Amazon basin over a period of 2 to as many as 8 days.

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Kevin C. Prince and Clark Evans

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While it is understood that a recurving tropical cyclone (TC) that interacts with the midlatitude flow can cause large changes to the midlatitude flow pattern, it is much less understood if, and how, such events could impact a downstream tropical cyclone. Here, an indirect TC interaction is defined as one in which a primary TC perturbs the downstream midlatitude waveguide within one synoptic-scale wavelength of a secondary TC. In this study, a climatology and composite analysis using ERA-Interim reanalysis data is completed for all indirect interactions occurring between two tropical and/or subtropical cyclones in the North Atlantic and western North Pacific basins between 1989 and 2018. In all, 26 cases are identified in the North Atlantic and 56 cases are identified in the western North Pacific. The composite-mean interaction between a primary TC and upstream trough amplifies the immediate downstream ridge, increasing the tropospheric-deep vertical wind shear on its poleward and, in the western North Pacific, eastern, and equatorward flanks. An amplified downstream trough is detectable farther downstream in the western North Pacific 1–2 days after interaction onset; however, the same is not true in the North Atlantic, in which some cases exhibit anticyclonic Rossby wave breaking of the immediate downstream ridge. Secondary TCs that weaken following the indirect-interaction events are primarily located along the gradient between the downstream ridge and trough (North Atlantic) or at high latitudes (western North Pacific); those that strengthen are primarily located equatorward of the downstream ridge, particularly in the western North Pacific.

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Marcia E. Clark and Eric C. Wildhagen

An information storage and retrieval system, designed for use on a minicomputer, was created to access an office collection of articles and reports on the topic of hail. Search capabilities of the system include author, keyword-in-title, year, subject, and location. Software description and suggestions concerning the input of information are detailed with the intent to encourage researchers to design systems for personal use.

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Martyn P. Clark and Mark C. Serreze

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At least four different modeling studies indicate that variability in snow cover over Asia may modulate atmospheric circulation over the North Pacific Ocean during winter. Here, satellite data on snow extent for east Asia for 1971–95 along with atmospheric fields from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis are used to examine whether the circulation signals seen in model results are actually observed in nature. Anomalies in snow extent over east Asia exhibit a distinct lack of persistence. This suggests that understanding the effects of east Asian snow cover is more germane for short- to medium-range weather forecasting applications than for problems on longer timescales. While it is impossible to attribute cause and effect in the empirical study, analyses of composite fields demonstrate relationships between snow cover extremes and atmospheric circulation downstream remarkably similar to those identified in model results. Positive snow cover extremes in midwinter are associated with a small decrease in air temperatures over the transient snow regions, a stronger east Asian jet, and negative geopotential height anomalies over the North Pacific Ocean. Opposing responses are observed for negative snow cover extremes. Diagnosis of storm track feedbacks shows that the action of high-frequency eddies does not reinforce circulation anomalies in positive snow cover extremes. However, in negative snow cover extremes, there are significant decreases in high-frequency eddy activity over the central North Pacific Ocean, and a corresponding decrease in the mean cyclonic effect of these eddies on the geopotential tendency, contributing to observed positive height anomalies over the North Pacific Ocean. The circulation signals over the North Pacific Ocean are much more pronounced in midwinter (January–February) than in the transitional seasons (November–December and March–April).

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Jackie C. May, Clark Rowley, and Charlie N. Barron

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The Naval Research Laboratory (NRL) ocean surface flux (NFLUX) system provides near-real-time satellite-based gridded surface heat flux fields over the global ocean within hours of the observed satellite measurements. NFLUX can serve as an alternative to current numerical weather prediction models—in particular, the U. S. Navy Global Environmental Model (NAVGEM)—that provide surface forcing fields to operational ocean models. This study discusses the satellite-based shortwave and longwave global gridded analysis fields, which complete the full suite of NFLUX-provided ocean surface heat fluxes. A companion paper discusses the production of satellite swath-level surface shortwave radiation and longwave radiation estimates. The swath-level shortwave radiation estimates are converted into clearness-index values. Clearness index reduces the dependency on solar zenith angle, which allows for the assimilation of observations over a given time window. An automated quality-control process is applied to the swath-level estimates of clearness index and surface longwave radiation. Then 2D variational analyses of the quality-controlled satellite estimates with background atmospheric model fields form global gridded radiative heat flux fields. The clearness-index analysis fields are converted into shortwave analysis fields to be used in other applications. Three-hourly shortwave and longwave analysis fields are created from 1 May 2013 through 30 April 2014. These fields are validated against observations from research vessels and moored-buoy platforms and compared with NAVGEM. With the exception of the mean bias, the NFLUX fields have smaller errors when compared with those of NAVGEM.

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Jackie C. May, Clark Rowley, and Neil Van de Voorde

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The Naval Research Laboratory ocean surface flux (NFLUX) system provides satellite-based surface state parameter and surface turbulent heat flux fields operationally over the global ocean. These products are presented as an alternative to using numerical weather prediction models—namely, the U.S. Navy Global Environmental Model (NAVGEM)—to provide the surface forcing to operational ocean models. NFLUX utilizes satellite sensor data records from the Special Sensor Microwave Imager/Sounder (SSMIS), the Advanced Microwave Sounding Unit-A (AMSU-A), the Advanced Technology Microwave Sounder (ATMS), and the Advanced Microwave Scanning Radiometer-2 (AMSR-2) sensors as well as satellite environmental data records from WindSat, the Advanced Scatterometers (ASCAT), and the Oceansat scatterometer (OSCAT). The satellite data are processed and translated into estimates of surface specific humidity, surface air temperature, and 10-m scalar wind speed. Two-dimensional variational analyses of quality-controlled satellite data, in combination with an atmospheric-model field, form global gridded surface state parameter fields. Bulk formulas are then applied to produce surface turbulent heat flux fields. Six-hourly analysis fields are created from 1 January 2013 through 31 December 2013. These fields are examined and validated against in situ data and NAVGEM. Overall, the NFLUX fields have a smaller bias, lower or similar root-mean-square error, and increased skill score relative to those of NAVGEM.

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T. L. Clark, F. I. Harris, and C. G. Mohr

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Simulations from a time-dependent model of moist convection have been used to assess magnitudes of errors in the estimates of derived wind fields from the synthesis of data from a network of Doppler radars. The two types of errors considered are, first, those due to temporal changes in the scales of deep convection resolved by the model, and second, those due to the random contributions of radial velocity estimates by scales smaller than model resolution (noise). Due to the coarse spatial resolution of the model, much of the assumed noise error is due to spatial scales between the model's resolution (∼1 km) and the Doppler radar sampling scale (∼100 m) and should not be considered in reality as “white” noise with respect to the radar sampling problem. The results presented in this paper must be interpreted only in terms of wind estimates derived by using radar sample volumes comparable to the models resolution. Much higher spatial resolution experiments with the model are necessary to clearly delineate the differences between temporal and noise errors for scales larger than the typical radar sampling volumes.

The temporal errors for the resolved scales of the model using a 3 min scan time were found to be less than those due to noise and in general quite tolerable in magnitude for three or more radars. A dual-Doppler analysis in x, y, z Cartesian space (as opposed to x, y, elevation angle coplane analysis) was considered. In this case the derived errors (in the steady state) were found to be significantly large.

The effects of scan time and number of radars were assessed and two methods of reducing temporal errors were investigated.

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Mark C. Serreze, Amanda H. Lynch, and Martyn P. Clark

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Calculations of a thermal front parameter using NCEP–NCAR reanalysis data over the period 1979–98 reveal a relative maximum in frontal frequencies during summer along northern Eurasia from about 60° to 70°N, best expressed over the eastern half of the continent. A similar relative maximum is found over Alaska, which is present year-round although best expressed in summer. These high-latitude features can be clearly distinguished from the polar frontal zone in the midlatitudes of the Pacific basin and collectively resemble the summertime“Arctic frontal zone” discussed in several early studies. While some separation between high- and midlatitude frontal activity is observed in all seasons, the summer season is distinguished by the development of an attendant mean baroclinic zone aligned roughly along the Arctic Ocean coastline and associated wind maxima in the upper troposphere. The regions of maximum summer frontal frequency correspond to preferred areas of cyclogenesis and to where the summertime contribution to annual precipitation is most dominant. Cyclones generated in association with the Eurasian frontal zone often track into the central Arctic Ocean, where they may have an impact on the sea-ice circulation. Development of the summertime Eurasian frontal zone and the summertime strengthening of the Alaskan feature appear to be largely driven by differential heating between the cold Arctic Ocean and warm snow-free land. Frontal activity also shows an association with orography. Several studies have argued that the location of the summer Arctic frontal zone may be in part determined by discontinuities in energy exchange along the tundra–boreal forest boundary. While such a linkage is not discounted here, a vegetation forcing is not required in this conceptual model.

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