Stanley David Gedzelman

Modeling the isotopic composition of precipitation

The physics of the stable isotopes of water is incorporated into a two-dimensional, kinematic, bulk cloud microphysical model. The model is run for several idealized, classical stratiform and convective storm situations, and the resulting isotope ratios of precipitation and water vapor are diagnosed and compared to observations. For stratiform snow, the model produces low isotope ratios that decrease rapidly poleward of the warm front. The lowest isotope ratios occur when the atmosphere is cold and when the vertical velocity attains its maximum value high in the troposphere. For stratiform rains, the model produces much higher isotope ratios without a significant poleward gradient as a result of isotope exchange between the falling rain and the surrounding vapor. Isotope ratios of rain are lowest when the melting level is near the ground and isotope exchange is minimized. For air mass thunderstorms, isotope ratios are uniformly high in warm air, no matter what the cloud height, unless hail approaches or reaches the ground. The model also produces a significant amount effect for rain, in which isotope ratios decrease with increasing rainfall, totals. Isotope ratios are particularly low when the rain derives from a recirculation process in which air previously charged by vapor from falling rain subsequently rises. Under such conditions, the model sometimes produces isotope ratios that decrease from the periphery to the core of the precipitation shield. It is suggested that this recirculation process is responsible for extraordinarily low isotope ratios observed in some hurricanes and organized thunderstorms. The dominant cloud microphysical processes can sometimes be inferred from isotope ratios of precipitation. The model produces ice pellets with isotope ratios close to those of rain when the pellets are produced by homogeneous freezing of rain and close to those of snow when the pellets are produced by refreezing of partially melted snow. A climatology of isotope values that matches the main features of the observed global data set and of a seven-year record of storms at Mohonk Lake, New York is generated by running the model for a wide range of conditions. This includes the deuterium excess (d ≡ δD - 8*δ18O) for Antarctic snows that increases markedly as δD falls below −300‰ and the deuterium deficit observed for rain in warm, dry regions.

The Isotopic Composition of Cyclonic Precipitation

Abstract The deuterium/hydrogen ratios δD of sequential samples of precipitation at Palisades, New York were measured during four separate storms during January 1978. The values ranged from a low of −177‰ to a high of −30‰. For each of the storms there was a general increase of δD values with time. The changes of δD values are explained by means of detailed meteorological analysis assuming that the precipitation derives from a Rayleigh condensation process in which all condensation in the air column directly above the station is assumed to fall immediately to the ground. The resulting calculated values of δD usually agree to within 5‰ of the observed measured values.

Probing Hurricanes with Stable Isotopes of Rain and Water Vapor

Abstract Rain and water vapor were collected during flights in Hurricanes Olivia (1994), Opal (1995), Marilyn (1995), and Hortense (1995) and analyzed for their stable isotopic concentrations, or ratios, H218O:H2O and HDO:H2O. The spatial patterns and temporal changes of isotope ratios reflect details of a hurricanes structure, evolution, microphysics, and water budget. At all flight levels over the sea (850–475 hPa) the lowest isotope ratios occur in or near regions of stratiform rains between about 50 and 250 km from the eye. Isotope ratios are higher in the eyewall and were particularly high in the crescent-shaped eyewall of Hurricane Opal at a time when no rain was falling over a large area near the storm center. In Hurricane Olivia, isotope ratios decreased from 24 to 25 September after vertical and radial circulation weakened. A two-layer isotope model of a radially symmetric hurricane simulates these features. The low isotope ratios are caused by fractionation in extensive, thick, precipitating cl...

Stable isotope ratios of rain and vapor in 1995 hurricanes

Isotope ratios of rain and vapor samples collected at the surface from four tropical cyclones during the active 1995 Atlantic hurricane season were determined. A two-dimensional bulk microphysics isotope model was applied to steady symmetric tropical cyclones to explain the observed low mean values and inward decrease of isotope ratios of the rain and vapor. The low mean value is caused by the tropical cyclones relatively large size, longevity, and deep clouds. The inward decrease is due to diffusive isotope exchange between falling rain and converging vapor in the atmospheric boundary layer. Dean, a minimal tropical storm, produced relatively high isotope ratios because of its small size and youth. Rains from the extreme outer edge of Felix, a category 3 hurricane, exhibited high isotope ratios similar to normal summer rain. Isotope ratios of rains and vapors from Hurricane Luis in Puerto Rico decreased as the storm approached. Isotope ratios of rain exhibited an abrupt jump from low values in the eastern half of Puerto Rico to high values farther west which is linked to the storms rainbands. Isotope ratios of Hurricane Opals rains reflected the storms asymmetric structure, with lowest values west of the point of landfall. Record low isotope ratios from a squall line that struck eastern Texas two days before landfall are linked to low-level outflow from Opal and demonstrate that hurricanes can vent enormous quantities of vapor to the surroundings.

The Isotopic Composition of Precipitation from Two Extratropical Cyclones

Abstract Precipitation samples were collected at stations in the Eastern United States for two extratropical cyclones during the Genesis of Atlantic Lows Experiment (GALE) of 1986 and analyzed for their δ18O values. They represent the first synoptic-scale datasets of isotopic values. Measured isotope ratios are explained in terms of physical principles and meteorological processes. They are shown to be related to vertical profiles of ω cloud-top temperatures, evaporation beneath cloud base, isotope equilibration, and water vapor sources for the precipitation. Measured isotope ratios are then compared to values obtained from simple models of convective and stratiform precipitation. Both storms are shown to exhibit a consistent pattern of isotope ratios, with lowest δ18O values occurring in the stratiform precipitation well within the cold air, and highest values associated with the convective precipitation of the warm sector. A pronounced-amount effect, in which δ18O values decrease as rainfall totals incr...

Simulating coronas in color

Coronas are simulated in color by use of the Mie scattering theory of light by small droplets through clouds of finite optical thickness embedded in a Rayleigh scattering atmosphere. The primary factors that affect color, visibility, and number of rings of coronas are droplet size, width of the size distribution, and cloud optical thickness. The color sequence of coronas and iridescence varies when the droplet radius is smaller than approximately 6-microm. As radius increases to approximately 3.5 microm, new color bands appear at the center of the corona and fade as they move outward. As the radius continues to increase to approximately 6 microm, successively more inner rings become fixed in the manner described by classical diffraction theory, while outer rings continue their outward migration. Wave clouds or rippled cloud segments produce the brightest and most vivid multiple ringed coronas and iridescence because their integrated dropsize distributions along sunbeams are much narrower than in convective or stratiform clouds. The visibility of coronas and the appearance of the background sky vary with cloud optical depth tau. First the corona becomes visible as a white aureole in a blue sky when tau approximately 0.001. Color purity then rapidly increases to an almost flat maximum in the range 0.05 < or = tau < or = 0.5 and then decreases, so coronas are almost completely washed out by a bright gray background when tau > or = 4.

Subsuns, Bottlinger's rings, and elliptical halos

Subsuns, Bottlingers rings, and elliptical halos are simulated by the use of a Monte Carlo model; reflection of sunlight from almost horizontal ice crystals is assumed. Subsuns are circular or elliptical spots seen at the specular reflection point when one flies over cirrus or cirrostratus clouds. Bottlingers rings are rare, almost elliptical rings centered about the subsun. Elliptical halos are small rings of light centered around the Sun or the Moon that rarely occur with other halo phenomena. Subsuns and Bottlingers rings can be explained by reflection from a single crystal, whereas elliptical halos require reflection from two separate crystals. All three phenomena are colorless and vertically elongated with an eccentricity that increases with increasing solar zenith angle. For several cases of Bottlingers rings the simulations are compared with density scans of photographs. Clouds that consist of large swinging or gyrating plates and dendritic crystals, which form near -15 °C, seem the most likely ca didates to produce the rings and elliptical halos. Meteorological evidence is presented that supports these conditions for elliptical halos. Simulations suggest that the most distinct elliptical halos may be produced by hybrid clouds that contain both horizontal and gyrating crystals.

Short-period atmospheric gravity waves - A study of their statistical properties and source mechanisms

Abstract Gravity waves for the one year period beginning 19 October 1976 around Palisades, New York, are investigated to determine their statistical properties and sources. The waves have typical periods of 10 min, pressure amplitudes of 3 Pa and velocities of 30 m s−1. In general, the largest, amplitude waves occur during late fall and early winter when the upper tropospheric winds directly overhead are fastest and the static stability of the lower troposphere is greatest. Mean wave amplitudes correlate highly with the product of the mean maximum wind speed and the mean low level stratification directly aloft. A distinct diurnal variation of wave amplitudes with the largest waves occurring in the pro-dawn hours is also observed as a result of the increased static stability then. The majority of waves are generated by shear instability; however, a number of waves are generated by distant sources such as nuclear detonations or large thunderstorms. The waves with distant sources can be distinguished on the ...

Atmospheric optics in art

A brief historical overview of the atmospheric optical phenomena that appear in works of fine art is presented. It is shown that artists have recorded many features of the color and brightness of the sky and clouds, aerial perspective and visibility effects, and phenomena, including crepuscular rays, rainbows, halos, and coronas. Artistic biases resulting from prevailing styles and societal influences are noted. Attention is drawn to a number of phenomena recorded by artists that have not yet been explained or modeled.

Short-Period Atmospheric Gravity Waves: A Study of Their Dynamic and Synoptic Features

Abstract Atmospheric gravity wave records at Palisades, N.Y., during the two-mouth period November–December 1969 an investigated in the light of concurrent weather patterns and atmospheric soundings. Although it seems to be a characteristic of atmospheric gravity waves that no one mechanism affords a complete explanation for their behavior, it is shown that many of the waves seem to be generated by shear instability of the upper troposphere winds and that the pressure amplitudes of the waves are greatly increased by the presence of large static stability in the lower troposphere. A simple three-layer model is able to simulate these features. The synoptic applications of these observations are then investigated and it is shown that times of large-amplitude gravity waves often precede the onset of cyclonic precipitation by some 12–24 h while times of small-amplitude gravity waves usually indicate continuation of fair weather.