A Simulation and Diagnostic Study of Water Vapor Image Dry Bands

A Limited Area Mesoscale Prediction System (LAMPS) model simulation and special 3-hour radiosonde dataset are used to investigate warm (dry) bands in 6,7 μm water vapor satellite imagery on 6–7 March 1982. The purpose is to reveal processes resulting in the formation and evolution of the dry features that appear as curving dark streaks in the imagery. Model soundings are input to a radiative transfer algorithm to generate synthetic 6.7 μm equivalent blackbody temperatures (TB) which are compared with those from the Visible infrared Spin Scan Radiometer Atmospheric Sounder aboard the Geostationary Operational Environmental Satellite. Simulated and radiosonde-derived vertical velocity and humidity also are compared with the images. Finally, trajectories are calculated from both radiosonde data and LAMPS output. The model reproduces major characteristics of the observed TB field. A “development” dry image feature occurs in conjunction with an upper level shortwave trough, and an “advective” feature is associated with a polar jet streak. Both model and observed TB features are associated with vorticity maxima. The development feature forms as moisture gradients are enhanced by differential subsidence early in the study period. Horizontal wind shear then narrows the incipient dry area into its streak-like shape. Trajectories reveal that air parcels ending in the development streak move with it, in northwesterly, subsiding flow throughout the study period. Near the leading edge of the streak, ahead of the short-wave trough, flow is southwesterly and ascending. Air parcels in the advective image feature sink in the wake of the vorticity maximum, move through it in the jet flow, and finally ascend ahead of it. Thus, warm TB regions do not equate with instantaneous subsidence patterns, but reflect a long history of parcel motions which can include ascent as well

A Swirl in the Clouds Near Santa Cruz Island (Images of Note)

The authors discuss a rare photograph of an atmospheric eddy produced by marine boundary layer flow past terrain

Simulations of the Effects of Water Vapor, Cloud Liquid Water, and Ice on AMSU Moisture Channel Brightness Temperatures

Radiative transfer simulations are performed to determine how water vapor and nonprecipitating cloud liquid water and ice particles within typical midlatitude atmospheres affect brightness temperatures T-B\u27s of moisture sounding channels used in the Advanced Microwave Sounding Unit (AMSU) and AMSU-like instruments. The purpose is to promote a general understanding of passive top-of-atmosphere T-B\u27s for window frequencies at 23.8, 89.0, and 157.0 GHz, and water vapor frequencies at 176.31, 180.31, and 182.31 GHz by documenting specific examples. This is accomplished through detailed analyses of T-B\u27s for idealized atmospheres, mostly representing temperate conditions over land. Cloud effects are considered in terms of five basic properties: droplet size distribution phase, liquid or ice water content, altitude, and thickness. Effects on T-B of changing surface emissivity also are addressed. The brightness temperature contribution functions are presented as an aid to physically interpreting AMSU T-B\u27s. Both liquid and ice clouds impact the T-B\u27s in a variety of ways. The T-B\u27s at 23.8 and 89 GHz are more strongly affected by altostratus liquid clouds than by cirms clouds for equivalent water paths. In contrast, channels near 157 and 183 GHz are more strongly affected by ice clouds. Higher clouds have a greater impact on 157- and 183-GHz T-B\u27s than do lower clouds. Clouds depress T-B\u27s of the higher-frequency channels by suppressing, but not necessarily obscuring, radiance contributions from below. Thus, T-B\u27s are less closely associated with cloud-top temperatures than are IR radiometric temperatures. Water vapor alone accounts for up to 89% of the total attenuation by a midtropospheric liquid cloud for channels near 183 GHz. The Rayleigh approximation is found to be adequate for typical droplet size distributions; however, Mie scattering effects from liquid droplets become important for droplet size distribution functions with modal radii greater than 20 mu m near 157 and 183 GHz, and greater than 30-40 mu m at 89 GHz. This is due mainly to the relatively small concentrations of droplets much larger than the mode radius. Orographic clouds and tropical cumuli have been observed to contain droplet size distributions with mode radii in the 30-40-mu m range. Thus, as new instruments bridge the gap between microwave and infrared to frequencies even higher than 183 GHz, radiative transfer modelers are cautioned to explicitly address scattering characteristics of such clouds

An Unusual Aerial Photograph of an Eddy Circulation in Marine Stratocumulus Clouds (Picture of the Month)

An aerial photograph of a cyclonic, von Kármán–like vortex in the marine stratocumulus clouds off the California coast, taken by a commercial pilot near Grover Beach, is presented. It is believed that this is the first photograph of such an eddy, taken from an airplane, to appear in publication. The eddy occurred with a strong inversion above a shallow marine boundary layer, in the lee of high, inversion-penetrating terrain. Tower and surface wind measurements plotted on satellite imagery demonstrate that the Grover Beach eddy was not just a cloud-level feature, but extended through the marine atmospheric boundary layer (MABL) to the surface. Evolution of the flow during the formation of the eddy appears similar to idealized numerical simulations of blocked MABL flow from the literature. The tower measurements sampled the northern part of the eddy circulation during its formation just offshore. The 2°–3°C temperature increases and then decreases during and after the eddy passage may be indicative of warmer air, from sheltered locations to the southeast, and/or downslope flow, being advected by and included into the eddy circulation. Satellite data compared with sequences of wind reversals at two different levels of the meteorological tower suggest that the eddy is tilted with height, at least during its formation stage. Formation mechanisms are discussed, but the subsynoptic observations are inadequate to resolve basic questions about the flow; ultimately a high-resolution model simulation is needed

An Alternative Representation of the Ice Canopy for Calculating Microwave Brightness Temperatures Over a Thunderstorm

Passive microwave brightness temperatures (T(B)\u27s) at 92 and 183 GHz from an aircraft thunderstorm overflight are compared with values calculated from radar-derived hydrometeor profiles and a modified proximity sounding. Two methods for modeling particles in the ice canopy are contrasted. The first is a \u27\u27traditional\u27\u27 approach employing Marshall-Palmer ice spheres. The second, or \u27\u27alternative,\u27\u27 method partitions 20% of the ice water content into a Marshall-Palmer component for graupel and hail, and 80% into a modified gamma spherical particle size distribution function representing ice crystals. Results from the alternative approach are superior to those from the traditional method in the anvil and mature convective core. In the decaying convective region, the traditional approach yields better agreement with observed magnitudes. Neither method, however, matches the geometry of the observed TB depression associated with the decaying convective core. This is likely due to the presence of graupel, which is not detected as a special signature in radar reflectivity, but does diminish T(B)\u27s through scattering. Brightness temperatures at the relatively high microwave frequencies considered are shown to be very sensitive to the ice-particle size distribution

Analysis and use of VAS satellite data

A series of interrelated investigations has examined the analysis and use of VAS (VISSR Atmospheric Sounder) satellite data. A case study of VAS-derived mesoscale stability parameters suggested that they would have been a useful supplement to conventional data in the forecasting of thunderstorms on the day of interest. However, the meteorological significance of small or short lived stability features was uncertain. A second investigation examined the roles of first guess and VAS radiometric data in producing sounding retrievals. The radiance data often did not have a decisive influence on the final satellite soundings. Broad-scale patterns of the first guess, radiances, and retrievals frequently were similar, whereas small scale retrieval features, especially in the dew points, were often of uncertain origin

Analysis and use of VAS satellite data

Four interrelated investigations have examined the analysis and use of VAS satellite data. A case study of VAS-derived mesoscale stability parameters suggested that they would have been a useful supplement to conventional data in the forecasting of thunderstorms on the day of interest. A second investigation examined the roles of first guess and VAS radiometric data in producing sounding retrievals. Broad-scale patterns of the first guess, radiances, and retrievals frequently were similar, whereas small-scale retrieval features, especially in the dew points, were often of uncertain origin. Two research tasks considered 6.7 micron middle tropospheric water vapor imagery. The first utilized radiosonde data to examine causes for two areas of warm brightness temperature. Subsidence associated with a translating jet streak was important. The second task involving water vapor imagery investigated simulated imagery created from LAMPS output and a radiative transfer algorithm. Simulated image patterns were found to compare favorably with those actually observed by VAS. Furthermore, the mass/momentum fields from LAMPS were powerful tools for understanding causes for the image configurations

Transitioning Traditional Aviation Weather Instruction to a Space Launch Weather Support Course: Operational Considerations

Weather support to space launch operations, while similar to that for traditional aviation, presents significant additional challenges. Embry-Riddle Aeronautical University (ERAU) currently offers three courses in traditional aviation meteorology, two needed by aeronautical science students seeking private pilot through airline transport pilot (ATP) FAA certifications, and one to provide meteorology majors with operational experience supporting an actual air race. However, with the advent of a new degree program in Commercial Space Operations (CSO), the need to modify and extend traditional aviation weather instruction to include space launch weather requirements has become increasingly important. While the traditional aviation meteorology coursework is beneficial, it does not cover the full-spectrum of weather impacts on space launch and suborbital space flight operations that both CSO and meteorology students will need. To address this challenge, ERAU (with guidance from the 45th Weather Squadron at Patrick Air Force Base) is working to create a new course focused on weather support to these operations, utilizing the new suborbital space flight simulator and lab housed in the Department of Applied Aviation Sciences. This presentation explores the operational considerations of transitioning traditional terrestrial aviation weather instruction to a suborbital space flight weather support course. These considerations include: more stringent spacecraft and system weather sensitivities, triggered lightning, vertical wind-shear profile analysis, attention to atmospheric conditions above the troposphere, and space weather impacts

Diurnal Distribution of Very Heavy Precipitation Over the Central and Eastern United States

A climatology of heavy precipitation events for the states of Nebraska, Missouri, Illinois, Kentucky, Tennessee, West Virginia, Pennsylvania, and Virginia is developed from 10 years (1968-1977) of Hourly Precipitation Data. Hourly precipitation events are categorized by severity, ranging from 1 to 2 inches (2.5 to 5.1 cm) in one hour or less (Type 1) to 4 or more inches (greater than 10.2 cm) in eight hours or less (Type 4). Hourly distributions of heavy precipitation events indicate that intense rainfalls of short duration occur most frequently during the afternoon and evening hours; whereas Type 4 events, which pose the most serious threat of flash flooding occur most often during the night and early morning hours in the central and eastern United States

Aviation Bird Hazard in NEXRAD Dual Polarization Weather Radar Confirmed by Visual Observations

Birds represent a significant hazard to flying aircraft as illustrated by the “Miracle on the Hudson” encounter in 2009 between U.S. Airways Flight 1549 and a flock of Canada Geese, forcing the flight to ditch in the river. Birds are common in the skies over Florida during the spring migration season, and often appear in the National Weather Service’s (NWS) NEXRAD weather radar imagery as an easily recognizable signature known as a “roost ring.” This paper presents a NEXRAD roost ring case in central Florida in a rare instance where the signatures were confirmed by visual observations of the birds. In 2013 the NWS completed an upgrade of its NEXRAD systems to dual polarization, a technology designed to improve target classification. Use of new dual polarization weather radar variables to better discriminate birds from precipitation for the current case is demonstrated. It is shown that the dual polarization capability, and specifically, the correlation coefficient product, allows for greater confidence in identifying radar echoes due to birds, and therefore could lead to better situational awareness for aviation operations personnel able to recognize these signatures