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Robert Tardif and Roy M. Rasmussen

Abstract

To gain insights into the poorly understood phenomenon of precipitation fog, this study assesses the evaporation of freely falling drops departing from equilibrium as a possible contributing factor to fog formation in rainy conditions. The study is based on simulations performed with a microphysical column model describing the evolution of the temperature and mass of evaporating raindrops within a Lagrangian reference frame. Equilibrium defines a state where the latent heat loss of an evaporating drop is balanced by the sensible heat flux from the ambient air, hence defining a steady-state drop temperature. Model results show that the assumption of equilibrium leads to small but significant errors in calculated precipitation evaporation rates for drops falling in continuously varying ambient near-saturated or saturated conditions. Departure from equilibrium depends on the magnitude of the vertical gradients of the ambient temperature and moisture as well as the drop-size-dependent terminal velocity. Contrasting patterns of behavior occur depending on the stratification of the atmosphere. Raindrops falling in inversion layers remain warmer than the equilibrium temperature and lead to enhanced moistening, with supersaturation achieved when evaporation proceeds in saturated inversions. Dehydration occurs in layers with temperature and water vapor increasing with height due to the vapor flux from the environment to the colder drops. These contrasts are not represented when equilibrium is assumed. The role of nonequilibrium raindrop evaporation in fog occurrences is further emphasized with simulations of a case study characterized by fog forming under light rain falling in a developing frontal inversion. Good agreement is obtained between fog water content observations and simulations representing only the effects of rainfall evaporation. This study demonstrates the need to take into account the nonequilibrium state of falling raindrops for a proper representation of an important mechanism contributing to precipitation fog occurrences.

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Robert Tardif and Roy M. Rasmussen

Abstract

An analysis of the environmental conditions associated with precipitation fog events is presented using 20 yr of historical observations taken in a region centered on New York, New York. The objective is to determine the preferred weather scenarios and identify physical processes influencing the formation of fog during precipitation. Salient synoptic-scale features are identified using NCEP–NCAR reanalyses. Local environmental parameters, such as wind speed and direction, temperature, and humidity, are analyzed using surface observations, while the vertical structure of the lower atmosphere is examined using available rawinsonde data. The analysis reveals that precipitation fog mostly occurs as a result of the gradual lowering of cloud bases as continuous light rain or light drizzle is observed. Such scenarios occur under various synoptic weather patterns in areas characterized by large-scale uplift, differential temperature advection, and positive moisture advection. Precipitation fog onset typically occurs with winds from the northeast at inland locations and onshore flow at coastal locations, with flows from the south to southwest aloft. A majority of the cases showed the presence of a sharp low-level temperature inversion resulting from differential temperature advection or through the interaction of warm air flowing over a cold surface in onshore flow conditions. This suggests a common scenario of fog formation under moistening conditions resulting from precipitation evaporating into colder air near the surface. A smaller number of events formed with cooling of the near-saturated or saturated air. Evidence is also presented of the possible role of shear-induced turbulent mixing in the production of supersaturation and fog formation during precipitation.

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Robert Tardif and Roy M. Rasmussen

Abstract

The character of fog in a region centered on New York City, New York, is investigated using 20 yr of historical data. Hourly surface observations are used to identify fog events at 17 locations under the influence of various physiographic features, such as land–water contrasts, land surface character (urban, suburban, and rural), and terrain. Fog events at each location are classified by fog types using an objective algorithm derived after extensive examination of fog formation processes. Events are characterized according to frequency, duration, and intensity. A quantitative assessment of the likelihood with which mechanisms leading to fog formation are occurring in various parts of the region is obtained. The spatial, seasonal, and diurnal variability of fog occurrences are examined and results are related to regional and local influences. The results show that the likelihood of fog occurrence is influenced negatively by the presence of the urban heat island of New York City, whereas it is enhanced at locations under the direct influence of the marine environment. Inland suburban and rural locations also experience a considerable amount of fog. As in other areas throughout the world, the overall fog phenomenon is a superposition of various types. Precipitation fog, which occurs predominantly in winter, is the most common type. Fog resulting from cloud-base lowering also occurs frequently across the region, with an enhanced likelihood in winter and spring. A considerable number of advection fog events occur in coastal areas, mostly during spring, whereas radiation fog occurs predominantly at suburban and rural locations during late summer and early autumn but also occurs during the warm season in the coastal plain of New Jersey as advection–radiation events.

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Roy M. Rasmussen and Piotr K. Smolarkiewicz

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This paper extends our earlier discussion of the flow past the island of Hawaii and the accompanying cloud band to smaller-scale effects occurring on the scale of the Hilo Bay region. The evolution of cloud bands forming upwind of the island on 1 August 1985 is studied using a high-resolution numerical model and available field observations. The current work provides further evidence in support of the view that the phenomenon of Hawaiian cloud bands is closely linked to the dynamics of strongly stratified flows past three-dimensional obstacles. In particular, results are presented that document the cloud interaction with a secondary, vertically propagating gravity wave and the formation of horizontally oriented vortices in the lower upwind flow—two characteristic features encountered in studies of idealized low Froude number flows. Quantification of the effects due to nocturnal thermal forcing is attempted, and it is shown that cooling along the volcano slope doubles the depth of the dynamically induced downslope flow as well as its maximum wind speed, whereas it has a little effect upon the position of the mesoscale convergence line and coinciding leading edge of the downslope current. Downslope surges of cold air from the volcano slope are shown to temporarily enhance the depth and strength of the downslope flow, leading to invigorated cloud development at the leading edge of the current. Analysis of the Hawaiian Rainband Project (HaRP) sounding data relates cloud bands to the theory of squall lines and suggests that the trade wind environment upstream of the island is favorable to the formation of cloud bands consisting of isolated cells advected by the local cloud-layer winds.

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Roy M. Rasmussen, Andrew Crook, and Cathy Kessinger

Abstract

The formation and evolution of convective rain and snow bands prior to and during the crash of Continental Airlines flight 1713 on 15 November 1987 at Denver Stapleton Airport are discussed. Convective rain occurred during the early stages of the storm in association with the approach of an upper-level trough from the west. Snow bands were observed following the passage of a shallow Canadian cold front from the north. These bands formed above the cold front and moved from southeast to northwest at 7 m s−1 with a horizontal spacing of 10–30 km. The winds within the cloud layer were southeasterly from 5 to 10 m s−1, suggesting that the bands were advected by the mean, cloud-layer flow. The most likely mechanism producing these bands was a convective instability in the shear layer above the cold front.

As the upper-level trough moved to the east, the winds in the cloud layer shifted to northerly, causing the bands to move southward with the major axis of the band oriented north–south. The high snowfall rate just prior to the takeoff of flight 1713 occurred as a result of one of these north–south–oriented bands moving over Denver Stapleton Airport from the north during the latter stages of the storm.

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Roy M. Rasmussen and Andrew J. Heymsfield

Abstract

The 1 August severe storm during the Cooperative Convective Precipitation Experiment (CCOPE) has been analyzed making use of T-28 aircraft data, CP-2 radar data, and a particle trajectory model in conjunction with a time-averaged Doppler-derived wind field. This analysis reveals that water drops are shed from wet hailstones in this storm, and that some of them are shed in favorable locations to grow back into hailstones. A particular region of the storm is identified as the most likely source region for shed drops, and particle trajectory calculations show that hailstones grown from this region fall out in locations consistent with the reflectivity structure of the storm. Some trajectories are shown to fall back through this same region while at the same time shedding, providing new hail embryos.

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Gregory Thompson, Roy M. Rasmussen, and Kevin Manning

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This study evaluates the sensitivity of winter precipitation to numerous aspects of a bulk, mixed-phase microphysical parameterization found in three widely used mesoscale models [the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), the Rapid Update Cycle (RUC), and the Weather Research and Forecast (WRF) model]. Sensitivities of the microphysics to primary ice initiation, autoconversion, cloud condensation nuclei (CCN) spectra, treatment of graupel, and parameters controlling the snow and rain size distributions are tested. The sensitivity tests are performed by simulating various cloud depths (with different cloud-top temperatures) using flow over an idealized two-dimensional mountain. The height and width of the two-dimensional barrier are designed to reproduce an updraft pattern with extent and magnitude consistent with documented freezing-drizzle cases. By increasing the moisture profile to saturation at low temperatures, a deep, precipitating snow cloud is also simulated. Upon testing the primary sensitivities of the microphysics scheme in two dimensions as reported in the present study, the MM5 with the modified scheme will be tested in multiple case studies and the results will be compared to observations in a forthcoming companion paper, Part II.

The key results of this study are 1) the choice of ice initiation schemes is relatively unimportant for deep precipitating snow clouds but more important for shallow warm clouds having cloud-top temperature greater than −13°C, 2) the assumed snow size distribution and associated snow diffusional growth along with the assumed graupel size distribution and method of transforming rimed snow into graupel have major impacts on the mass of cloud water and formation of freezing drizzle, and 3) a proper simulation of drizzle using a single-moment scheme and exponential size distribution requires an increase in the rain intercept parameter, thereby reducing rain terminal velocities to values more characteristic of drizzle.

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Roy M. Rasmussen, Piotr Smolarkiewicz, and John Warner

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This paper presents a detailed comparison study of three-dimensional model results with an aircraft wind field mapping for the island of Hawaii. Model runs were initialized using an aircraft sounding from 1 August 1985, and detailed predictions from the model are compared with observations from that day.

The strength and location of the upwind convergence zone were well simulated, as well as the strong deflection and deceleration of the flow around the island and the geometry and location of the upstream cloud bands. The good agreement between the model results and observations supports the results of our previous study in which we show that the flow pattern and associated cloud processes around the island of Hawaii can be understood by considering the flow of a stably stratified fluid around a large three-dimensional obstacle.

Model runs with different wind directions showed that increasing northerly tradewind flow resulted in the band clouds moving closer to the shore line, and the large scale flow pattern rotating counterclockwise. Model results were also compared with various aspects of the island climatology, and good agreement was found in both the temporal and spatial distribution of precipitation on the island.

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Roy M. Rasmussen and Andrew J. Heymsfield

Abstract

A sensitivity study on the melting and shedding behavior of individual graupel and hail is presented utilizing the detailed microphysical model presented in Part I. The influence of particle density and size, atmospheric temperature profile, relative humidity profile, liquid water content, shedding parameterization, and heat transfer rates are investigated. The results show that the melting of graupel and the melting and shedding behavior of hailstones are significantly affected by the initial particle density and size, temperature profile, and relative humidity.

When graupel and hail are grown in a Doppler-derived three-dimensional wind field, the results show that the melting and shedding behavior of the graupel and hail are relatively insensitive to changes as large as 25% in the heat transfer coefficient or to the differences between the Rasmussen et al. and the Chong and Chen shedding parameterizations.

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David B. Johnson and Roy M. Rasmussen

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The transition between wet and dry growth for graupel and hail is examined, and new figures are presented illustrating the critical water contents necessary for transitions into or out of the wet-growth regime. These figures are extended to smaller sizes and lower bulk densities than considered in previous studies. In addition, the possibility of hysteresis in the transitions is examined.

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