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Showing papers in "Journal of Applied Meteorology in 1996"


Journal ArticleDOI
TL;DR: In this article, an empirical stratiform-convective classification method based on N 0 and R (rainfall rate) is presented. But, the occurrence of precipitation was found to be 74% (stratiform) and 26% (convection) but total rainfall, on the other hand, was...
Abstract: An analysis of temporal variations in gamma parameters of raindrop spectra is presented utilizing surface-based observations from the Tropical Ocean Global Atmosphere Couple Ocean-Atmosphere Experiment. An observed dramatic change in the N0 parameter, found to occur during rainfall events with little change in rainfall rate, is suggestive of a transition from rain of convective origin to rain originating from the stratiform portion of tropical systems. An empirical stratiform-convective classification method based on N0 and R (rainfall rate) is presented. Properties of the drop size spectra from the stratiform classification are consistent with micro-physical processes occurring within an aggregation/melting layer aloft, which produces more large raindrops and fewer small to medium size raindrops than rain from the convective classification, at the same rainfall rate. The occurrence of precipitation was found to be 74% (stratiform) and 26% (convective), but total rainfall, on the other hand, was ...

685 citations


Journal ArticleDOI
TL;DR: In this paper, an algorithm based on the Magnus's form equations is proposed to minimize the difference between several relationships between temperature and water vapor pressure at saturation that are commonly used in archiving data.
Abstract: Algorithms, based on Magnus's form equations, are described that minimize the difference between several relationships between temperature and water vapor pressure at saturation that are commonly used in archiving data. The work was initiated in connection with the development of a unified upper-air dataset that will use measurements gathered from the late 1930s to the present and archived in several data centers. The conversion of field measurements to archived humidity values within the databases that are being used have not been consistent and in some cases are unknown. A goal of this work was to develop a uniform and accurate method to convert these data to various humidity variables without regard to the equations used in archiving the original data. Archived temperature values are recorded to 0.1°C. This precision creates a temperature dependent range in uncertainty in saturation vapor pressure. A procedure was developed to take this into account when the error minimizing equations were derived.

596 citations


Journal ArticleDOI
TL;DR: In this paper, the amount of soluble material on these particles is found to be related to their surface area, suggesting that the deposition process could be surface dependent, and the mechanism by which sulfate is found on some of the mineral dust particles is believed to originate from evaporating cloud drops, which were originally nucleated on sulfate cloud condensation nuclei (CCN) and subsequently collected dry interstitial mineral Dust particles.
Abstract: Measurements of aerosol composition in the eastern Mediterranean reveal that sulfate is found in most aerosol particles. Some of the large particles contain mixtures of chemicals such as sulfate and sea salt. The most striking observation is the discovery that mineral dust particles often get coated with sulfate and other soluble materials. The amount of soluble material on these particles is found to be related to their surface area, suggesting that the deposition process could be surface dependent. The mechanism by which sulfate is found on some of the mineral dust particles is believed to originate from evaporating cloud drops, which were originally nucleated on sulfate cloud condensation nuclei (CCN) and subsequently collected dry interstitial mineral dust particles. The presence of soluble material on mineral dust particles, converts the latter into effective giant CCN. This is further corroborated by the fact that the few large drops near the bases of convective clouds near the coast of Isr...

527 citations


Journal ArticleDOI
TL;DR: In this article, a simple approach to estimating vertically integrated atmospheric water vapor, or precipitable water, from Global Positioning System (GPS) radio signals collected by a regional network of ground-based geodetic GPS receiver is illustrated and validated.
Abstract: A simple approach to estimating vertically integrated atmospheric water vapor, or precipitable water, from Global Positioning System (GPS) radio signals collected by a regional network of ground-based geodetic GPS receiver is illustrated and validated. Standard space geodetic methods are used to estimate the zenith delay caused by the neutral atmosphere, and surface pressure measurements are used to compute the hydrostatic (or “dry”) component of this delay. The zenith hydrostatic delay is subtracted from the zenith neutral delay to determine the zenith wet delay, which is then transformed into an estimate of precipitable water. By incorporating a few remote global tracking stations (and thus long baselines) into the geodetic analysis of a regional GPS network, it is possible to resolve the absolute (not merely the relative) value of the zenith neutral delay at each station in the augmented network. This approach eliminates any need for external comparisons with water vapor radiometer observation...

462 citations


Journal ArticleDOI
TL;DR: In this paper, high-resolution thermal data derived from Landsat's thematic mapper are evaluated for their correspondence to building geometry and landscape features in Singapore`s high-rise housing estates.
Abstract: High-resolution thermal data derived from Landsat`s thematic mapper are evaluated for their correspondence to building geometry and landscape features in Singapore`s high-rise housing estates. The image data are sufficiently detailed to reveal that differences in solar azimuth on images taken at different times of year create different thermal patterns due to building geometry and surface materials. Field measurements of surface and adjacent air temperatures in urban canyons at different orientations and at different elevations above ground demonstrate that for Singapore conditions satellite-derived surface temperature patterns are a good indicator of the daytime urban heat island. 18 refs., 11 figs., 2 tabs.

244 citations


Journal ArticleDOI
TL;DR: A coupled atmosphere-fire model is described and demonstrated: a three-dimensional, time-dependent wildfire simulation model that can represent the finescale dynamics of convective processes and capture ambient meteorological conditions.
Abstract: The object of this paper is to describe and demonstrate the necessity and utility of a coupled atmosphere-fire model: a three-dimensional, time-dependent wildfire simulation model, based on the primitive equations of motion and thermodynamics, that can represent the finescale dynamics of convective processes and capture ambient meteorological conditions. In constructing this coupled model, model resolution for both the atmosphere and the fuel was found to be important in avoiding solutions that are physically unrealistic, and this aspect is discussed. The anelastic approximation is made in the equations of motion, and whether this dynamical framework is appropriate in its usual form for simulating wildfire behavior is also considered. Two simple experiments-the first two in a series of numerical simulations using the coupled atmosphere- fire model-are presented here, showing the effect of wind speed on fire-line evolution in idealized and controlled conditions. The first experiment considers a 42...

197 citations


Journal ArticleDOI
TL;DR: A neural network has been designed to diagnose which circulations detected by the NSSL MDA yield tornados, and it is shown that the network outperforms the rule-based algorithm existing in the MDA, as well as statistical techniques such as discriminant analysis and logistic regression.
Abstract: The National Severe Storms Laboratory's (NSSL) mesocyclone detection algorithm (MDA) is designed to scotch for patterns in Doppler velocity radar data that are associated with rotating updrafts in severe thunderstorms. These storm-scale circulations are typically precursors to tornados and severe weather in thunderstorms, yet not all circulations produce such phenomena. A neural network has been designed to diagnose which circulations detected by the NSSL MDA yield tornados. The data used both for the training and the testing of the network are obtained from the NSSL MDA. In particular, 23 variables characterizing the circulations are selected to be used as the input nodes of a feed-forward neural network. The output of the network is chosen to be the existence/nonexistence of tornados, based on ground observations. It is shown that the network outperforms the rule-based algorithm existing in the MDA, as well as statistical techniques such as discriminant analysis and logistic regression. Additio...

180 citations


Journal ArticleDOI
TL;DR: In this article, satellite composite images in visible and near-infrared wavelengths confirm seasonal trends in surface exchange properties of a red oak forest in central Massachusetts have been ongoing since the summer of 1991.
Abstract: Temperate deciduous forests exhibit dramatic seasonal changes in surface exchange properties following on the seasonal changes in leaf area index. Nearly continuous measurements of turbulent and radiative fluxes above and below the canopy of a red oak forest in central Massachusetts have been ongoing since the summer of 1991. Several seasonal trends are obvious. Global solar albedo and photosynthetically active radiation (PAR) albedo both are good indicators of the spring leaf emergence and autumnal defoliation of the canopy. The solar albedo decreases throughout the summer, a change attributed to decreasing near-infrared reflectance since the PAR reflectance remains the same. Biweekly satellite composite images in visible and near-infrared wavelengths confirm these trends. The thermal emissions from the canopy relative to the net radiation follow a separate trend with a maximum in the midsummer and minima in spring and fall. The thermal response number computed from the change in radiation tempe...

169 citations


Journal ArticleDOI
TL;DR: In this paper, three-dimensional tropical squall line simulations from the Goddard cumulus ensemble (GCE) model are used as input to radiative computations of upwelling microwave brightness temperatures and radar reflectivities at selected microwave sensor frequencies.
Abstract: Three-dimensional tropical squall-line simulations from the Goddard cumulus ensemble (GCE) model are used as input to radiative computations of upwelling microwave brightness temperatures and radar reflectivities at selected microwave sensor frequencies. These cloud/radiative calculations form the basis of a physical cloud/precipitation profile retrieval method that yields estimates of the expected values of the hydrometeor water contents. Application of the retrieval method to simulated nadir-view observations of the aircraft-borne Advanced Microwave Precipitation Radiometer (AMPR) and NASA ER-2 Doppler radar (EDOP) produce random errors of 23%, 19%, and 53% in instantaneous estimates of integrated precipitating liquid, integrated precipitating ice, and surface rain rate, respectively. On 5 October 1993, during the Convection and Atmospheric Moisture Experiment (CAMEX), the AMPR and EDOP were used to observe convective systems in the vicinity of the Florida peninsula. Although the AMPR data alon...

157 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined some effects of drop size distribution and shape on the rainfall-rate estimates obtained from the specific differential phase and presented an algorithm that uses exclusively the specific discrete phase.
Abstract: This paper examines some effects of drop size distribution and shape on the rainfall-rate estimates obtained from the specific differential phase. An algorithm that uses exclusively the specific differential phase is presented, and performance of this algorithm is examined by applying it to 15 storm events in Oklahoma that include heavy and light rainfalls. Radar-derived cumulative rainfall is compared to total rain measured by gauges in a dense gauge network in Oklahoma for each of the events.

150 citations


Journal ArticleDOI
TL;DR: In this paper, two stochastic weather simulation models (USCLIMATE and CLIGEN) were compared for their performance in replicating observed precipitation, temperature, and solar radiation variables at six locations in the United States.
Abstract: Two stochastic weather simulation models (USCLIMATE and CLIGEN) were compared for their performance in replicating observed precipitation, temperature, and solar radiation variables at six locations in the United States. Statistical tests of significance were performed on means and standard deviations of a variety of standard and derived daily weather variables over monthly and annual time periods. Model replication of extreme events also was evaluated. In general, mean monthly and annual values were well replicated by both models, but variance replication of temperature and solar radiation were found superior in USCLIMATE. Extreme-value replication, especially over short (24-h) time intervals, was highly a function of climatic element, location, and time of year but, in general, was judged only fair in both models. Suggestions for model improvement and enhancement are given, as are suggestions for model applications.

Journal ArticleDOI
TL;DR: In this article, a simple parameterization of gravitational drainage for land surface schemes describing soil water transfers according to the force-restore method of Deardorff is presented, and the simulation of the annual cycle of soil moisture by the land surface scheme ISBA is in this way greatly improved.
Abstract: This paper presents a simple parameterization of gravitational drainage for land surface schemes describing soil water transfers according to the force-restore method of Deardorff. A one-year time series of observed soil moisture period from HAPEX-MOBILHY (Hydrological Atmospheric Pilot Experiment-Mobilisation du Bilan Hydrique) 1986 revealed the importance of subsurface drainage during the wintertime period. This physical process is accounted for through a Newtonian restore to field capacity when soil moisture is above it. Simulation of the annual cycle of soil moisture by the land surface scheme ISBA (interactions soil biosphere atmosphere) is in this way greatly improved.

Journal ArticleDOI
TL;DR: In this article, the Nested Grid Model wind fields were used to forecast and analyze trajectories from six sites over North America at three altitudes (500, 1000, and 1500 m above ground) twice a day for a one-year period using Nested grid models.
Abstract: Forecast and “analysis” (reference) trajectories were computed from six sites over North America at three altitudes (500, 1000, and 1500 m above ground) twice a day for a one-year period using Nested Grid Model wind fields. The reference meteorology was a series of short-term forecasts. Absolute error (distance between reference and forecast trajectory), relative error (absolute error divided by forecast trajectory travel distance), and the angle between the reference and forecast trajectory were also computed. The mean relative error for all the forecast trajectories for a travel time of 36 h is about 35%; the 90th percentile of the relative error is about 65%. The forecast is slightly biased to the left of the reference early in the forecast period. Absolute error and travel distance both are larger in winter than summer, so that the relative error is generally constant throughout the year. Differences in mean error among the three starting altitudes, among the six origin sites, and between the...

Journal ArticleDOI
TL;DR: In this paper, the authors used kinematic air parcel trajectory analysis to determine patterns of horizontal air transport in 2000 km × 2000 km areas over southern Africa and derived composite zonal and meridional transport fields for the subcontinent to estimate the extent to which recirculation of air and aerosols may take place in the lower troposphere between the surface and 500 hPa.
Abstract: Kinematic air parcel trajectory analysis is used to determine patterns of horizontal air transport in 2000 km × 2000 km areas over southern Africa. From these, composite zonal and meridional transport fields are derived for the subcontinent to estimate the extent to which recirculation of air and aerosols may take place in the lower troposphere between the surface and 500 hPa. The nature and degree of recirculation beneath the persistent 500-hPa absolutely stable layer is demonstrated, and transport by recirculation in discrete streams is shown to constitute 44% of the total transport over the region. From a determination of air volume fluxes and estimates of aerosol concentrations, the total mass flux of aerosols by direct transport and by recirculation in conditions during which semipermanent, subtropical, continental anticyclones prevail is estimated to be about 51 Mton yr−1 in the surface-to-hPa layer. Recirculation comprises approximately 22 Mton yr−1 of this amount. Of the recirculated tran...

Journal ArticleDOI
TL;DR: In this article, a semi-empirical algorithm, making use of readily available meteorological variables and total column ozone, is presented for inferring historical UV levels at a particular location, including cloud cover effects.
Abstract: Recent extratropical ozone depletion and the concomitant increase in surface ultraviolet (UV) radiation may be expected to adversely influence the biosphere. Since few long-term, high quality datasets of surface UV are available for assessing these effects, there is a need to develop techniques for estimating past levels of biologically harmful UV at a particular location and thus derive long-term trends. This paper presents a semiempirical algorithm, making use of readily available meteorological variables and total column ozone, for inferring historical UV levels at a particular location, including cloud cover effects. Where input data are available for a network of locations, the technique can be used to generate geographical distributions of surface UV. Measurements made at Lauder (45.04°S, 169.68°E), from November 1993 to October 1994, were used to establish the relationship between cloud-induced reductions of erythemal UV and broadband irradiance, as a function of solar zenith angle—termed ...

Journal ArticleDOI
TL;DR: In this paper, single-source and dual-source models of the surface energy transfer across the soil-vegetation-atmosphere interface were used in conjunction with remotely sensed surface temperature for computing surface energy balance over heterogeneous surfaces.
Abstract: Single- and dual-source models of the surface energy transfer across the soil-vegetation-atmosphere interface were used in conjunction with remotely sensed surface temperature for computing the surface energy balance over heterogeneous surfaces. Both models are relatively simple so that only a few parameters are specified, making them potentially useful for computing surface fluxes with operational satellite observations. The models were tested with datasets collected from a semiarid rangeland environment with canopy cover generally less than 50% and a subhumid tallgrass prairie environment having canopy cover typically greater than 50%. For the semiarid site, differences between the single-source and dual-source model estimates of the sensible heat flux (H) and the observations averaged about 25%. For the tallgrass prairie, the disagreement between observations and single-source model estimates of H was significantly larger, averaging nearly 55%. The average difference between observations and t...

Journal ArticleDOI
TL;DR: In this paper, an automated road ice prediction model was developed for the purpose of nowcasting (up to 3h) ahead, which is validated against observations from 41 road surface sensors in seven countries.
Abstract: Winter road maintenance is an important application field of meteorology in western and northern Europe, North America, and many other parts of the world. In order to provide timely short-period high-accuracy forecasts of road surface temperature and state (dry, wet, frost, or ice), an automated road ice prediction model is developed for the purpose of nowcasting (up to 3 h ahead). The model was validated against observations from 41 road surface sensors in seven countries. As far as the authors are aware, this model is unique in that it is the only fully automated physical road ice prediction model that requires no external meteorological input data other than automatically collected sensor measurements of surface temperature, air temperature, dewpoint, and wind speed from the forecast site. The results show that (a) the projection of primary meteorological parameters by the model itself as input is acceptable for this purpose and (b) model performance becomes poorer as nowcast period gets longe...

Journal ArticleDOI
TL;DR: In this paper, a method developed in the former Soviet Union for predicting cloud amounts is supplemented with a new method of determining the base and tops of clouds. But this test was found from an analyses of United States radiosonde data.
Abstract: A method developed in the former Soviet Union for predicting cloud amounts is supplemented with a new method of determining the base and tops of clouds. Criteria for predicting a cloud layer are 0 ≤ T″(z) and R″(z) ≤ 0, where T″ is the second derivative of the vertical profile of temperature and R″ is the second derivative of the relative humidity. This test was found from an analyses of United States radiosonde data. Cloud amount (sky cover) is predicted from a relationship between cloud amount and dewpoint depression within the predicted cloud layer and the temperature at that level. This relationship is based on data from the former Soviet Union and data from the Indian 0cean and divides cloud amount into four categories: 0%–20%, 20%–60%, 60%–80%, and 80%–100% coverage. The new composite method is evaluated using data from several United States radiosonde stations within different climates. Evaluation data was selected to include only situations in which the observer (providing the “truth”) co...

Journal ArticleDOI
TL;DR: In this paper, a surface energy balance model is used, in association with remotely sensed and in situ data, to examine issues of measurement, scaling, and aggregation of high-frequency spatial variation in land surface properties and fluxes over regional scales.
Abstract: Numerous recent field experiments have examined the use of remote sensing to estimate land surface fluxes of latent and sensible heat using combinations of thermal, visible, and near-infrared data. While substantial progress has been made, significant problems remain unresolved with respect to both spatial aggregation of land surface fluxes over heterogeneous land surfaces and the use of thermal data for estimating sensible heat fluxes. In this paper a surface energy balance model is used, in association with remotely sensed and in situ data, to examine issues of measurement, scaling, and aggregation of high-frequency spatial variation in land surface properties and fluxes over regional scales. Results from this analysis show that instantaneous land surface fluxes modeled from high spatial resolution remotely sensed data may be estimated only approximately. Comparisons between modeled versus observed fluxes averaged over regional scales (≈225 km2), on the other hand, exhibit excellent agreement. ...

Journal ArticleDOI
TL;DR: In this article, a conditional sampling flux measurement technique was evaluated for four scalars (temperature, water vapor, ozone, and carbon dioxide) by comparison with direct eddy correlation measurements at two sites.
Abstract: The conditional sampling flux measurement technique was evaluated for four scalars (temperature, water vapor, ozone, and carbon dioxide) by comparison with direct eddy correlation measurements at two sites. The empirical constant β relating the turbulent flux to the accumulated concentration difference between updrafts and downdrafts was computed from 10-Hz turbulence measurements. Comparison between the simulated relaxed eddy accumulation flux formulation and the eddy correlation measurements allowed the direct determination of β for all four scalars. The β models previously proposed overpredicted the measured β by about 8%–10%. It was found that a mean β = 0.58 reproduced the eddy correlation measurements independent of the scalar type being analyzed, roughness and atmospheric stability conditions, in agreement with previous studies. The role of energy-containing eddy motion in the deviations between the measured and predicted β was considered using orthonormal wavelet expansion in conjunction ...

Journal ArticleDOI
TL;DR: In this article, the authors studied the electrification of Florida thunderstorms in greater depth using the National Aeronautics and Space Administration/United States Air Force electric field mill network; a small fleet of aircraft; the dual-frequency, dual-polarization CP-2 radar of the National Center for Atmospheric Research (NCAR), and other radars including the NCAR Doppler CP-3 and CP-4.
Abstract: A prime objective of the Convection and Precipitation/Electrification experiment was to Study the electrification of Florida thunderstorms in greater depth using the National Aeronautics and Space Administration/United States Air Force electric field mill network; a small fleet of aircraft; the dual-frequency, dual-polarization CP-2 radar of the National Center for Atmospheric Research (NCAR), and other radars including the NCAR Doppler CP-3 and CP-4. Analyses of three small, isolated storms on 19 July 1991 suggest that the onset of electrification coincides with the appearance of significant volumes of differential reflectivity, indicative of liquid raindrops larger than 2-mm diameter, above the −7°C level accompanied by the nearly simultaneous appearance of significant depolarization, initially associated with the freezing of these drops. Although the relationship of the onset of electrification to various aspects of the radar reflectivity factor Z are more ambiguous, a rapid increase in the al...

Journal ArticleDOI
TL;DR: In this article, a numerical scheme was developed to couple a multilayer canopy radiation model, a photosynthesis model for C3 species, and a leaf stomatal conductance model with a single-leaf energy balance equation.
Abstract: A numerical scheme was developed to couple a multilayer canopy radiation model, a photosynthesis model for C3 species, and a leaf stomatal conductance model with a single-leaf energy balance equation. This coupled leaf and canopy model was used to simulate the responses of a horizontally uniform forest canopy to its ambient microenvironment, using micrometeorological data collected from field measurements in a forest canopy. Emphasis was placed on issues associated with modeling the transient responses of plant leaves. For example, thermal storage was found to be important in modeling transience of leaf temperature, which in turn influenced the transient heating or cooling of the atmosphere by plant leaves. In addition, modification of the Ball-Berry leaf stomatal conductance model, using an exponential formula with an empirical time lag, yielded more realistic transient leaf stomatal conductances, which were important for estimating evaporation and CO2 assimilation.

Journal ArticleDOI
TL;DR: In this article, a parametric form for the raindrop size distribution (DSD) that is an appropriate model for tropical rainfall, and involves statistically independent parameters is derived, and the coefficients of the new relations are directly related to the shape parameters of the particular DSD that one starts with.
Abstract: This paper addresses the problem of finding a parametric form for the raindrop size distribution (DSD) that(1) is an appropriate model for tropical rainfall, and (2) involves statistically independent parameters. Such a parameterization is derived in this paper. One of the resulting three "canonical" parameters turns out to vary relatively little, thus making the parameterization particularly useful for remote sensing applications. In fact, a new set of r drop-size-distribution-based Z-R and k-R relations is obtained. Only slightly more complex than power laws, they are very good approximations to the exact radar relations one would obtain using Mie scattering. The coefficients of the new relations are directly related to the shape parameters of the particular DSD that one starts with. Perhaps most important, since the coefficients are independent of the rain rate itself, the relations are ideally suited for rain retrieval algorithms.

Journal ArticleDOI
TL;DR: In this article, a methodology for obtaining estimates of the spatial distribution of fog water volume collected by a tree canopy in complex terrain is described, including assumptions about the shape and spacing of the trees, their fog water collection efficiency, the fog frequency, and the vertical rate of change of the liquid water content (LWC) within ground-based clouds.
Abstract: A methodology for obtaining estimates of the spatial distribution of fog water volume collected by a tree canopy in complex terrain is described. The method includes assumptions about the shape and spacing of the trees, their fog water collection efficiency, the fog frequency, and the vertical rate of change of the liquid water content (LWC) within ground-based clouds. The method was applied to a 655-km2 area surrounding Roundtop Mountain, Quebec, Canada, during a carefully selected sample period from the summer of 1993. Field measurements of fog water volume were used to estimate the cloud-base height and the rate of change of the LWC with height. Topographic data were used both as a forcing function in the wind flow model and as a means of defining the three-dimensional geometry for deposition calculations. The goal is the development of a simple model that can be used over large geographic areas. Results of the application are presented over various domains ranging from 2 to 164 Km2 in size. S...

Journal ArticleDOI
TL;DR: In this article, the size spectra of oscillating raindrops were determined from photographic measurements in Illinois showers at night using strobe lights, and the oscillations were detected from modulations in the fall streaks produced by backscattered light near the primary rainbow.
Abstract: The size spectra of oscillating raindrops were determined from photographic measurements in Illinois showers at night. The oscillations were detected from modulations in the fall streaks produced by backscattered light near the primary rainbow. Drop sizes were determined from the fall speed using strobe lights. A continuous record of raindrop size distributions was obtained from a disdrometer located beneath the camera sample volume. Results show that oscillations begin near 1-mm diameter, at the onset size for vortex shedding. This finding is consistent with the authors' recent laboratory and field studies for small raindrops. The size spectra show that all raindrops above 1 mm are oscillating, out to the largest size measured of 4.2-mm diameter. Extrinsic sources of drop oscillations were evaluated using a collision model with viscous decay and using the pressure forcing from turbulence and wind shear in the surface layer. Based on the disdrometer size distributions, the number of oscillating d...

Journal ArticleDOI
TL;DR: In this article, the interaction of the atmospheric boundary layer with the heterogeneous pine forest in HAPEX-MOBILHY on a scale of order 10 km is studied and a coupled atmosphere-soil-vegetation model is used and is run for 16 June 1986 in a stand-alone mode using prescribed dynamics.
Abstract: The interaction of the atmospheric boundary layer with the heterogeneous pine forest in HAPEX-MOBILHY on a scale of order 10 km is studied. A state-of-the-art, coupled atmosphere-soil-vegetation model is used and is run for 16 June 1986 in a stand-alone mode using prescribed dynamics. Published values for the effective roughness lengths of heat and momentum from different origins are used to show the impact on the surface fluxes and the boundary layer development. The modal simulations indicate that the coupled atmosphere-vegetation system is rather sensitive to the value for the roughness length of heat. This affects in particular the sensible heat flux and, as a consequence, the boundary layer height and profiles of mean quantities in the boundary layer. The model results are compared with observations made at a forest town, with radiosonde profiles, and with aircraft data. The best overall agreement for the boundary layer quantities is obtained by using a roughness length of heat that is three...

Journal ArticleDOI
TL;DR: In this article, a wintertime small-scale sea breeze associated with high levels of air pollution is described, in which the urban heat island plays an important role, and the sea breeze frequently penetrates into the heavily urbanized area, extending about 20 km from the bayshore.
Abstract: A wintertime small-scale sea breeze associated with high levels of air pollution is described, in which the urban heat island plays an important role. Over a major portion of the Kanto plain, the winter surface air temperature varies diurnally, ranging from 0° to 12°C on average. The water temperature in the innermost part of Tokyo Bay, surrounded by the plain, is maintained at 8°–10°C. The land–sea temperature contrast that generates sea breezes, however, is intensified by two processes: (i) the heat island associated with the Tokyo metropolitan area on the northwestern shore of the bay increases the land air temperature, and (ii) the nocturnal outflow of cool land air, the result of radiative cooling, covers the bay, causing the sea air temperature to be lower than the water temperature during the morning hours. As a result, the sea breeze frequently penetrates into the heavily urbanized area, extending about 20 km from the bayshore. Since the sea breeze is coupled with the heat island, it does...

Journal ArticleDOI
TL;DR: In this article, the mesoscale atmospheric circulation over the southwestern Ross Sea sector during winter is examined by prescribing lead fractions in the range of the observed values (i.e., between 0% and 30%).
Abstract: In this study the mesoscale atmospheric circulation over the southwestern Ross Sea sector during winter is examined. The hydrostatic meso-gamma-scale atmospheric model MAR (Modele Atmospherique Regional) is used. Polar night is assumed, and an idealized large-scale situation is prescribed, with zero geostrophic forcing. The impact of a partial sea-ice cover on the atmospheric circulation is assessed by prescribing lead fractions in the range of the observed values (i.e., between 0% and 30%). Simulations show that the propagation of katabatic airstreams over Terra Nova Bay is facilitated by the presence of leads because the identity of cold, dense, katabatic air is better marked in warmer environmental maritime conditions. Boundary layer fronts and mesocyclone activity are associated with the katabatic airstreams. They are enhanced by the presence of leads. In particular, when the lead fraction is prescribed to be between 20% and 30%, the model simulates mesocyclone intensities comparable to those observed. Taking into account that such a lead fraction is situated in the upper range of the observed values in the central Ross Sea during winter, these results suggest that winter Ross Sea mesocyclones could nor always result from a pure mesoscale forcing. In contrast to the summer situation, no snow precipitation occurs for the simulated winter case, probably because of the too-low absolute humidity content of the air. Such model behavior is in agreement with the observations, which reveal a summer precipitation maximum at McMurdo Station on Ross Island. It is also found that the position of the simulated mesocyclone over Terra Nova Bay is nor sensitive to the lead fraction. Furthermore, this meteorological situation favors the advection of relatively mild and moist maritime air over a long distance in the ice-sheet interior. This process, which is referred to as a moist-air intrusion, could affect the Antarctic ice-sheet mass balance.

Journal ArticleDOI
TL;DR: The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed in this article, where the growth environment was an 1100m-thick stratiform cloud with cloud-base and cloud-top temperatures of −7.5° and −12°C, respectively.
Abstract: The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed. The growth environment was an 1100-m-thick stratiform cloud with cloud-base and cloud-top temperatures of −7.5° and −12°C, respectively. The cloud was characterized by a low droplet concentration of 21 cm−3 and a large droplet median volume diameter of 29 µm, with a concentration of interstitial aerosol particles of less than 15 cm−3 (larger than 0. 13 µm in diameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 µm observed at cloud base. The air mass was sufficiently clean to ensure only a small number of active cloud condensation nuclei. Consequently, small concentrations of cloud droplets led to concentrations of over 300 L−1 for droplets larger than 40 µm, which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0...

Journal ArticleDOI
A. Huggel1, W. Schmid1, A. Waldvogel1
TL;DR: The relationship between raindrop size distribution, measured with a disdrometer, and a radar parameter of the melting-layer bright band is investigated in this article, where a good correlation (about −0.7) between the strength of the bright band (ΔZe) and the intercept and slope parameters of the Marshall-Palmer drop size distribution (N0, ∇) is observed for horizontally uniform precipitation of moderate intensity.
Abstract: The relationship between raindrop size distribution, measured with a disdrometer, and a radar parameter of the melting-layer bright band is investigated. The data, obtained in July 1993 in Switzerland, cover 120 h of precipitation. A good correlation (about −0.7) between the strength of the bright band (ΔZe) and the intercept and slope parameters of the Marshall-Palmer drop size distribution (N0, ∇) is observed for horizontally uniform precipitation of moderate intensity (one-third of total rainfall). Steep spectrum with many small drops and no large drops are associated with small values for ΔZe, whereas flat spectra with relatively few small drops and with large drops exhibit a well-defined bright band with a large ΔZe. Considering ΔZe allows a significantly better derivation of the rainfall rate from radar measurements than is possible with standard Z-R relationships. The rms errors of the 10-min averages of rainfall intensity can be reduced by 20%–40%.