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Robert M. Rabin, Lynn A. McMurdie, Christopher M. Hayden, and Gary S. Wade

Abstract

Spatial and temporal changes of atmospheric water vapor and surface wind speeds are investigated for a period following an intrusion of cold continental air over the Gulf of Mexico, during the Gulf of Mexico Experiment (GUFMEX) in March 1988. Microwave and infrared satellite measurements from the Special Sensor Microwave/Imager (SSM/I) instrument aboard the Defense Meteorological Satellite Project (DMSP) F8 satellite and from the GOES VISSR Atmospheric Sounder (VAS) are used to augment the sparse coverage of rawinsonde sites and surface reports in the vicinity of the Gulf of Mexico. Total precipitable water is derived from both instruments and from rawinsonde measurements at coastal locations and auxiliary sites on ships and platforms over the Gulf. Accuracies of the precipitable water derived from SSM/I and GOES are comparable, though microwave data provide more uniform coverage, when they are available, than VAS since they are relatively free from contamination by most clouds. Also, the moisture fields derived from microwave data appear to be less noisy than those derived from the infrared. To illustrate possible use of satellite data in the forecast office, moisture fields from both SSM/I and VAS are blended together into imagery, which are compared to analyses from an operational model. Surface wind speeds are also obtained from the microwave data and are compared to the surface observations. Analyses from satellite data appear to add considerable information to the moisture and wind analysis over the Gulf of Mexico and should help in forecasting moisture changes, particularly moisture return near the surrounding coastal areas.

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Robert M. Rabin, Lynn A. McMurdie, Christopher M. Hayden, and Gary S. Wade

Abstract

The atmospheric water budget is examined for a 12-day period following an intense cold-air outbreak over the Gulf of Mexico. Budget terms are compared using analyses from the U.S. National Meteorological Center's operational Nested Grid Model (NGM) and using precipitable water and surface wind speed estimated from the Special Sensor Microwave/Imager (SSM/I) instrument aboard the defense meteorological satellite F8. The atmospheric-storage term, determined from the areal-averaged total precipitable water, does not differ significantly between that obtained from the NGM and that obtained from SSM/I data. The storage increases by a factor of more than 3 during the initial five days following the passage of the surface high over the Gulf. Horizontal flux divergence of water vapor computed from the full vertical structure in the NGM output is well approximated by the substitution of the surface-700-mb mean wind and the total precipitable water for the vertical profiles along the boundaries of the atmospheric volume. Evaporation from the sea surface is determined using GOES surface temperatures and NGM surface air conditions. The impact of satellite-derived surface winds on the areal-average evaporation is determined by replacing NGM wind speeds with those estimated from the SSM/I data. The relative importance of precipitation on the water budget is assessed from model estimates. During the onset of airmass modification, evaporation appears to be the dominant mechanism in producing the observed atmospheric moistening. As evaporation diminishes after one to two days, evaporation and flux convergence are of similar magnitude. Together, these terms underestimate the amount of moistening observed during the first five days.

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Robert M. Rabin, Lynn A. McMurdie, Christopher M. Hayden, and Gary S. Wade

Abstract

Spatial and temporal changes in the vertical distribution of atmospheric water vapor are investigated during a period following the intrusion of cold continental air over the Gulf of Mexico, during the Gulf of Mexico Experiment (GUFMEX) in February-March 1988. Infrared satellite measurements from the GOES (Geostationary Operational Environmental Satellite) VISSR (Visible-Infrared Spin Scan Radiometer) Atmospheric Sounder (VAS) are used to augment the sparse coverage of rawinsonde sites in the vicinity of the Gulf of Mexico. Precipitable water from two vertical layers, surface-850 and 850–250 mb, are estimated from the VAS and compared to those from rawinsonde observations. The accuracy of precipitable-water estimates in each vertical layer is less than that for the total precipitable water. However, improvements in the estimate of precipitable water for each layer are observed with respect to the profiles used in initializing the retrieval process. A consistent horizontal and temporal pattern of the vertical partition of water vapor between the lower and middle to upper troposphere is obtained from the analysis in both layers. A band of moist air that develops with return to southerly flow is common to both layers; however, the width of the band is more extensive in the lower layer. Drying to the rear of the band predominates in the upper layer while the lower layer remains quite moist.

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Aaron Donohoe, Eliza Dawson, Lynn McMurdie, David S. Battisti, and Andy Rhines

Abstract

We analyze the temporal structure of the climatological seasonal cycle in surface air temperature across the globe. We find that, over large regions of Earth, the seasonal cycle of surface temperature departs from an annual harmonic: the duration of fall and spring differ by as much as 2 months. We characterize this asymmetry by the metric ASYM, defined as the phase lag of the seasonal maximum temperature relative to the summer solstice minus the phase lag of the seasonal minimum temperature relative to winter solstice. We present a global analysis of ASYM from weather station data and atmospheric reanalysis and find that ASYM is well represented in the reanalysis. ASYM generally features positive values over land and negative values over the ocean, indicating that spring has a longer duration over the land domain whereas fall has a longer duration over the ocean. However, ASYM also features more positive values over North America compared to Europe and negative values in the polar regions over ice sheets and sea ice. Understanding the root cause of the climatological ASYM will potentially further our understanding of controls on the seasonal cycle of temperature and its future/past changes. We explore several candidate mechanisms to explain the spatial structure of ASYM including 1) modification of the seasonal cycle of surface solar radiation by the seasonal evolution of cloud thickness, 2) differences in the seasonal cycle of the atmospheric boundary layer depth over ocean and over land, and 3) temperature advection by the seasonally evolving atmospheric circulation.

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Hannah C. Barnes, Joseph P. Zagrodnik, Lynn A. McMurdie, Angela K. Rowe, and Robert A. Houze Jr.

Abstract

This study examines Kelvin–Helmholtz (KH) waves observed by dual-polarization radar in several precipitating midlatitude cyclones during the Olympic Mountains Experiment (OLYMPEX) field campaign along the windward side of the Olympic Mountains in Washington State and in a strong stationary frontal zone in Iowa during the Iowa Flood Studies (IFloodS) field campaign. While KH waves develop regardless of the presence or absence of mountainous terrain, this study indicates that the large-scale flow can be modified when encountering a mountain range in such a way as to promote development of KH waves on the windward side and to alter their physical structure (i.e., orientation and amplitude). OLYMPEX sampled numerous instances of KH waves in precipitating clouds, and this study examines their effects on microphysical processes above, near, and below the melting layer. The dual-polarization radar data indicate that KH waves above the melting layer promote aggregation. KH waves centered in the melting layer produce the most notable signatures in dual-polarization variables, with the patterns suggesting that the KH waves promote both riming and aggregation. Both above and near the melting layer ice particles show no preferred orientation likely because of tumbling in turbulent air motions. KH waves below the melting layer facilitate the generation of large drops via coalescence and/or vapor deposition, increasing mean drop size and rain rate by only slight amounts in the OLYMPEX storms.

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Zachary S. Bruick, Kristen L. Rasmussen, Angela K. Rowe, and Lynn A. McMurdie

Abstract

El Niño–Southern Oscillation (ENSO) is known to have teleconnections to atmospheric circulations and weather patterns around the world. Previous studies have examined connections between ENSO and rainfall in tropical South America, but little work has been done connecting ENSO phases with convection in subtropical South America. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has provided novel observations of convection in this region, including that convection in the lee of the Andes Mountains is among the deepest and most intense in the world with frequent upscale growth into mesoscale convective systems. A 16-yr dataset from the TRMM PR is used to analyze deep and wide convection in combination with ERA-Interim reanalysis storm composites. Results from the study show that deep and wide convection occurs in all phases of ENSO, with only some modest variations in frequency between ENSO phases. However, the most statistically significant differences between ENSO phases occur in the three-dimensional storm structure. Deep and wide convection during El Niño tends to be taller and contain stronger convection, while La Niña storms contain stronger stratiform echoes. The synoptic and thermodynamic conditions supporting the deeper storms during El Niño is related to increased convective available potential energy, a strengthening of the South American low-level jet (SALLJ), and a stronger upper-level jet stream, often with the equatorward-entrance region of the jet stream directly over the convective storm locations. These enhanced synoptic and thermodynamic conditions provide insight into how the structure of some of the most intense convection on Earth varies with phases of ENSO.

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Joseph P. Zagrodnik, Lynn A. McMurdie, Robert A. Houze Jr., and Simone Tanelli

Abstract

As midlatitude cyclones pass over a coastal mountain range, the processes producing their clouds and precipitation are modified, leading to considerable spatial variability in precipitation amount and composition. Statistical diagrams of airborne precipitation radar transects, surface precipitation measurements, and particle size distributions are examined from nine cases observed during the Olympic Mountains Experiment (OLYMPEX). Although the pattern of windward enhancement and leeside diminishment of precipitation was omnipresent, the degree of modulation was largely controlled by the synoptic environment associated with the prefrontal, warm, and postfrontal sectors of midlatitude cyclones. Prefrontal sectors contained homogeneous stratiform precipitation with a slightly enhanced ice layer on the windward slopes and rapid diminishment to a near-complete rain shadow in the lee. Warm sectors contained deep, intense enhancement over both the windward slopes and high terrain and less prominent rain shadows owing to downstream spillover of ice particles generated over terrain. Surface particle size distributions in the warm sector contained a broad spectrum of sizes and concentrations of raindrops on the lower windward side where high precipitation rates were achieved from varying degrees of both liquid and ice precipitation-generating processes. Spillover precipitation was rather homogeneous in nature and lacked the undulations in particle size and concentration that occurred at the windward sites. Postfrontal precipitation transitioned from isolated convective cells over ocean to a shallow, mixed convective–stratiform composition with broader coverage and greater precipitation rates over the sloping terrain.

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Gary M. Lackmann, Brian Ancell, Matthew Bunkers, Ben Kirtman, Karen Kosiba, Amy McGovern, Lynn McMurdie, Zhaoxia Pu, Elizabeth Ritchie, and Henry P. Huntington
Open access
Robert A. Houze Jr., Lynn A. McMurdie, Walter A. Petersen, Mathew R. Schwaller, William Baccus, Jessica D. Lundquist, Clifford F. Mass, Bart Nijssen, Steven A. Rutledge, David R. Hudak, Simone Tanelli, Gerald G. Mace, Michael R. Poellot, Dennis P. Lettenmaier, Joseph P. Zagrodnik, Angela K. Rowe, Jennifer C. DeHart, Luke E. Madaus, Hannah C. Barnes, and V. Chandrasekar

Abstract

The Olympic Mountains Experiment (OLYMPEX) took place during the 2015/16 fall–winter season in the vicinity of the mountainous Olympic Peninsula of Washington State. The goals of OLYMPEX were to provide physical and hydrologic ground validation for the U.S.–Japan Global Precipitation Measurement (GPM) satellite mission and, more specifically, to study how precipitation in Pacific frontal systems is modified by passage over coastal mountains. Four transportable scanning dual-polarization Doppler radars of various wavelengths were installed. Surface stations were placed at various altitudes to measure precipitation rates, particle size distributions, and fall velocities. Autonomous recording cameras monitored and recorded snow accumulation. Four research aircraft supplied by NASA investigated precipitation processes and snow cover, and supplemental rawinsondes and dropsondes were deployed during precipitation events. Numerous Pacific frontal systems were sampled, including several reaching “atmospheric river” status, warm- and cold-frontal systems, and postfrontal convection.

Open access
Adam C. Varble, Stephen W. Nesbitt, Paola Salio, Joseph C. Hardin, Nitin Bharadwaj, Paloma Borque, Paul J. DeMott, Zhe Feng, Thomas C. J. Hill, James N. Marquis, Alyssa Matthews, Fan Mei, Rusen Öktem, Vagner Castro, Lexie Goldberger, Alexis Hunzinger, Kevin R. Barry, Sonia M. Kreidenweis, Greg M. McFarquhar, Lynn A. McMurdie, Mikhail Pekour, Heath Powers, David M. Romps, Celeste Saulo, Beat Schmid, Jason M. Tomlinson, Susan C. van den Heever, Alla Zelenyuk, Zhixiao Zhang, and Edward J. Zipser

Abstract

The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign was designed to improve understanding of orographic cloud life cycles in relation to surrounding atmospheric thermodynamic, flow, and aerosol conditions. The deployment to the Sierras de Córdoba range in north-central Argentina was chosen because of very frequent cumulus congestus, deep convection initiation, and mesoscale convective organization uniquely observable from a fixed site. The C-band Scanning Atmospheric Radiation Measurement (ARM) Precipitation Radar was deployed for the first time with over 50 ARM Mobile Facility atmospheric state, surface, aerosol, radiation, cloud, and precipitation instruments between October 2018 and April 2019. An intensive observing period (IOP) coincident with the RELAMPAGO field campaign was held between 1 November and 15 December during which 22 flights were performed by the ARM Gulfstream-1 aircraft.

A multitude of atmospheric processes and cloud conditions were observed over the 7-month campaign, including: numerous orographic cumulus and stratocumulus events; new particle formation and growth producing high aerosol concentrations; drizzle formation in fog and shallow liquid clouds; very low aerosol conditions following wet deposition in heavy rainfall; initiation of ice in congestus clouds across a range of temperatures; extreme deep convection reaching 21-km altitudes; and organization of intense, hail-containing supercells and mesoscale convective systems. These comprehensive datasets include many of the first ever collected in this region and provide new opportunities to study orographic cloud evolution and interactions with meteorological conditions, aerosols, surface conditions, and radiation in mountainous terrain.

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