Abstract

Cloudiness derived from surface observations and the Geostationary Operational Environmental Satellite VISSR (Visible–Infrared Spin Scan Radiometer) Atmospheric Sounder (VAS) are compared with thermodynamic properties derived from upper-air soundings over the Gulf Stream locale during a developing winter storm. The Gulf Stream locale covers the United States mid-Atlantic coastal states, the Gulf Stream, and portions of the Sargasso Sea. Cloudiness is found quite frequently in this region. Cloud-top pressures are derived from VAS using the CO₂ slicing technique and a simple threshold procedure. Cloud-base heights and cloud fractions are obtained from National Weather Service hourly reporting stations. The saturation pressure differences, defined as the difference between air parcel pressure and saturation-level pressure (lifted condensation level), are derived from upper-air soundings. Collocated comparisons with VAS and surface observations are also made. Results indicate that cloudiness is observed nearly all of the time during the 6-day period, well above the 8-yr mean. High, middle, and low opaque cloudiness are found approximately equally. Furthermore, of the high- and midlevel cloudiness observed, a considerable amount is determined to be semitransparent to terrestrial radiation. Comparisons of satellite-inferred cloudiness with surface observations indicate that the satellite can complement surface observations of cloud cover, particularly above 700 mb.

Surface-observed cloudiness is segregated according to a composite cloud fraction and compared to the mean saturation pressure difference for a 1000–600-mb layer. The analysis suggests that this conserved variable may be a good indicator for estimating cloud fraction. Large negative values of saturation pressure difference correlate highly with clear skies, while those approaching zero correlate with overcast conditions. Scattered and broken cloud fractions are associated with increasing values of the saturation pressure difference. Furthermore, cloud fractions observed in this study are considerably higher than those reported in similar studies and by other cloud fraction formulations.

Abstract

Fields of cloudiness derived from the Geostationary Operational Environmental Satellite VISSR (Visible–Infrared Spin Scan Radiometer) Atmospheric Sounder are analyzed over the Gulf Stream locale (GSL) to investigate seasonal and geographical variations. The GSL in this study is defined as the region bounded from 31° to 38°N and 82° to 66°W. This region covers an area that includes the United States mid-Atlantic coast states, the Gulf Stream, and portions of the Sargasso Sea. Clouds over the GSL are found approximately three-quarters of the time between 1985 and 1993. However, large seasonal variations in the frequency of cloudiness exist. These seasonal variations show a distinct relationship to gradients in sea surface temperature (SST). For example, during winter when large SST gradients are present, large gradients in cloudiness are found. Clouds are observed least often during summer over the ocean portion of the GSL. This minimum coincides with an increase in atmospheric stability due to large-scale subsidence. Cloudiness is also found over the GSL in response to mesoscale convergence areas induced by sea surface temperature gradients. Geographical variations in cloudiness are found to be related to the meteorology of the region. During periods of cold-air advection, which are found most frequently in winter, clouds are found less often between the coastline and the core of the Gulf Stream and more often over the Sargasso Sea. During cyclogenesis, large cloud shields often develop and cover the entire domain.

Satellite estimates of cloudiness are found to be least reliable over land at night during the cold months. In these situations, the cloud retrieval algorithm often mistakes clear sky for low clouds. Satellite-derived cloudiness over land is compared with daytime surface observations of cloudiness. Results indicate that retrieved cloudiness agrees well with surface observations. Relative humidity fields taken from global analyses are compared with satellite cloud heights at three levels in the atmosphere. Cloudiness observed at these levels is found at relative humidities in the 75%–100% range but is also observed at humidities as low as 26%.

Abstract

A three-dimensional mesoscale planetary boundary layer (PBL) numerical model is used to investigate mesoscale circulations over the Carolina coastal and Gulf Stream baroclinic zones. Idealized ambient onshore and offshore flows are investigated, which represent the synoptic conditions during the Intensive Observation Period-2 (IOP-2) of the 1986 Genesis of Atlantic Lows Experiment (GALE). For the easterly onshore flow, a confluence zone appears west of the Gulf Stream in response to the effect of the oceanic baroclinicity. The confluence zone is nearly parallel to the coastline and the SST isotherms, with northeasterly (southwesterly) flow to the west (east). A shallow coastal front forms below 2 km as the cyclonic shear of the ageostrophic flow becomes strong. Quasi-stationary rainbands are produced by cumulus convection along the coastal front. The northern part of the front and the rainbands later encroach inland as the cold air intensity over ground weakens due to onshore warm air advection. The modeled coastal circulation is in agreement with the observations, suggesting that differential boundary-layer modification may be the main mechanism for the formation of the coastal front. The existence of an onshore ambient flow appears to be a necessary condition for the presence of the Coastal front. For the northerly offshore ambient flow, the rainband therefore appears along the eastern edge of the Gulf Stream, which then moves slowly downstream in response to the generated atmospheric baroclinicity. For both flows, the development of the rainbands is sensitive to variations in eddy Prandtl number, and their growth rate can be explained in terms of conditional symmetric instability.

Abstract

This paper documents evidence of a diurnal variability in cloudiness over the Gulf Stream locale. The Gulf Stream locale (GSL) is defined as the region covering 31°–38°N, 82°–71°W. The Gulf Stream, which occupies a portion of the GSL, is a warm current of water that flows south to north along the east coast of the United States and provides conditions conducive for the development of cloudiness. Cloud heights derived from the GOES VISSR (Visible-infrared Spin Scan Radiometer) Atmospheric Sounder (VAS) are obtained and used to produce a 7-yr climatology of the diurnal variation in the frequency of low-, middle-,and high-level cloudiness. The climatology is segregated into summer and winter seasons.

Diurnal variations are found during the summer and winter. Satellite observations over land indicate a maximum in the frequency of low cloudiness during daytime and a minimum at night. In addition, high cloudiness is found to increase significantly late in the afternoon and evening. Over the Gulf Stream region, high cloudiness is found most frequently in the mid- to late morning hours. A midafternoon maximum in low cloudiness is found along the coastline of Georgia and South Carolina and north of the Gulf Stream east of Virginia. Nocturnal minimums in low cloudiness are reported in these regions. Results suggest that summertime low and high cloudiness over the GSL are related to prevalent convective activity. An analysis of the diurnally oscillating pattern of boundary layer convergence, derived from analyses from the National Meteorological Center's step coordinate model, indicates a strong relationship to the presence of high cloudiness. The strong correspondence between the timing of these two parameters suggests that atmosphere dynamics play a significant role in the diurnal cycle in high cloudiness.

In winter, when convective activity is suppressed there is less detectable response of the atmosphere to the 24-h solar cycle manifest in the diurnal variations of clouds. Nevertheless low- and midlevel cloudiness are found most frequently in the predawn hours, except over the Gulf Stream where low clouds exhibit an afternoon maximum and a nocturnal minimum. Surface observations of cloudiness support the diurnal variations reported by VAS.

Abstract

Numerical experiments were conducted to assess the impact of Omega dropwindsonde (ODW) data and Special Sensor Microwave/Imager (SSM/I) rain rates in the analysis and prediction of Hurricane Florence (1988). The ODW data were used to enhance the initial analysis that was based on the National Meteorological Center/Regional Analysis and Forecast System (NMC/RAFS) 2.5° analysis at 0000 UTC 9 September 1988. The SSM/I rain rates at 0000 and 1200 UTC 9 September 1988 were assimilated into the Naval Research Laboratory's limited-area model during model integration.

Results show that the numerical prediction with the ODW-enhanced initial analysis was superior to the control without ODW data. The 24-h intensity forecast error is reduced by about 75%, landfall location by about 95% (reduced from 294 to 15 km), and landfall time by about 5 h (from 9 to 4 h) when the ODW data were included. Results also reveal that the assimilation of SSM/I-retrieved rain rates reduce the critical landfall location forecast error by about 43% (from 294 to 169 km) and the landfall time forecast error by about 7 h (from 9 to 2 h) when the NMC/RAFS 2.5° initial analysis was not enhanced by the ODW data. The assimilation of SSM/I rain rates further improved the forecast error of the landfall time by 4 h (from 4 to 0 h) when the ODW data were used. This study concludes that numerical predictions of tropical cyclone can benefit from assimilations of ODW data and SSM/I-retrieved rain rates.

Abstract

Numerical simulations of tropical cyclones in an axisymmetric model with the Betts convective adjustment scheme and the 1974 Kuo cumulus parameterization are compared. It is shown that the storm with the Betts scheme has a slightly more intense mature stage than the storm with the Kuo scheme. For both schemes, the parameterized heating is dominant initially, while the grid-scale heating is dominant at the mature stage. The storms begin to intensify rapidly when the grid-scale heating extends through a deep layer. The Betts scheme is more effective at removing water vapor and delays the onset of grid-scale heating. This results in later development of the storm with the Betts scheme. The storm evolution with both the Betts and Kuo schemes is sensitive to the treatment of the evaporation of liquid water in the grid-scale condensation scheme. This suggests that a prognostic equation for liquid water should be used when simulating tropical cyclones with a model resolution fine enough for grid-scale heating to be important.

Abstract

Stomatal resistance (R_s) forms a pivotal component of the surface energy budget and of the terrestrial biosphere–atmosphere interactions. Using a statistical–graphical technique, the R_s-related interactions between different atmospheric and physiological variables are resolved explicitly from observations made during the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE). A similar analysis was undertaken for the R_s parameterization schemes, as used in the present models. Three physiological schemes (the Ball–Woodrow–Berry, Kim and Verma, and Jacobs) and one operational Jarvis-type scheme were evaluated in terms of their ability to replicate the terrestrial biosphere–atmosphere interactions.

It was found that all of the R_s parameterization schemes have similar qualitative behavior for routine meteorological applications (without carbon assimilation). Compared to the observations, there was no significant difference found in employing either the relative humidity or the vapor pressure deficit as the humidity descriptor in the analysis. Overall, the relative humidity–based interactions were more linear than the vapor pressure deficit and hence could be considered more convenient in the scaling exercises. It was found that with high photosynthesis rates, all of the schemes had similar behavior. It was found with low assimilation rates, however, that the discrepancies and nonlinearity in the interactions, as well as the uncertainties, were exaggerated.

Introduction of CO₂ into the analysis created a different dimension to the problem. It was found that for CO₂-based studies, the outcome had high uncertainty, as the interactions were nonlinear and the schemes could not converge onto a single interpretive scenario. This study highlights the secondary or indirect effects, and the interactions are crucial prior to evaluation of the climate and terrestrial biosphere–related changes even in the boundary layer perspective. Overall, it was found that direct and indirect effects could lead the system convergence toward different scenarios and have to be explicitly considered for atmospheric applications at all scales.

Abstract

A new convective parameterization scheme proposed by Betts is tested in a tropical cyclone model. The convective adjustment scheme adjusts the local temperature and moisture structures towards the observed quasi-equilibrium thermodynamic state and includes nonprecipitating shallow convection as well as deep convection. The numerical model used for this study is an axisymmetric, primitive equation, hydrostatic, finite difference model with 15 vertical levels and a horizontal resolution of 20 km. The spectral radiation boundary condition, which uses a different gravity wave speed for each vertical mode, is implemented in the model.

It is shown that the convective scheme is capable of simulating the developing, rapidly intensifying, and mature stages of a tropical cyclone from a weak vortex. At the mature stage, the minimum surface pressure and maximum low level tangential wind speed are around 923 mb and 58 m s⁻¹. During the early developing stage, the latent heat release is from the convective parameterization, but at the mature stage the latent heat release is mainly due to the grid-scale phase change.

For comparison, an experiment is conducted with the parameterized convection excluded, leaving only the grid-scale condensation and evaporation. The results show that the development of a tropical cyclone can be modeled with crude grid-scale condensation and evaporation processes for the 20 km horizontal resolution, similar to other studies. However, the storm with the explicit convective latent heat release is considerably less intense than that with the parameterized convective latent heat release.

Abstract

A statistical–dynamical study was performed on the role of hydrometeorological interactions in the midlatitudes and the semiarid Tropics. For this, observations from two field experiments, the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) and the Hydrological Atmospheric Pilot Experiment (HAPEX)–Sahel, representative of the midlatitudes and the semiarid tropical conditions, and simulated results from a land surface model, Simplified Simple Biosphere (SSiB) model were statistically analyzed for direct and interaction effects. The study objectives were to test the hypothesis that there are significant differences in the land surface processes in the semiarid tropical and midlatitudinal regimes and to identify the nature of the differences in the evapotranspiration exchanges for the two biogeographical domains. Results suggest there are similarities in the direct responses but the interactions or the indirect feedback pathways could be very different. The arid tropical regimes are dominated through vegetative pathways (via variables such leaf area index, stomatal resistance, and vegetal cover); the midlatitudes show soil wetness (moisture)–related feedback. In addition, for the midlatitudinal case, the vegetation and the soil surface acted in unison, leading to more interactive exchanges between the vegetation and the soil surface. The water-stressed semiarid tropical surface, on the other hand, showed response either directly between the vegetation and the atmosphere or between the soil and the atmosphere with very little interaction between the vegetation and the soil variables. Thus, the semiarid Tropics would require explicit bare ground and vegetation fluxes consideration, whereas the effective (combined vegetation and soil fluxes) surface representation used in various models may be more valid for the midlatitudinal case. This result also implied that with higher resource (water) availability the surface invested more in the surrounding environment. On the other hand, with poor resource availability (such as water stress in the tropical site), the surface components retain individual resources without sharing.

Abstract

Two mesoscale circulations, the Sandhills circulation and the sea breeze, influence the initiation of deep convection over the Sandhills and the coast in the Carolinas during the summer months. The interaction of these two circulations causes additional convection in this coastal region. Accurate representation of mesoscale convection is difficult as numerical models have problems with the prediction of the timing, amount, and location of precipitation. To address this issue, the authors have incorporated modifications to the Kain–Fritsch (KF) convective parameterization scheme and evaluated these mesoscale interactions using a high-resolution numerical model. The modifications include changes to the subgrid-scale cloud formulation, the convective turnover time scale, and the formulation of the updraft entrainment rates. The use of a grid-scaling adjustment parameter modulates the impact of the KF scheme as a function of the horizontal grid spacing used in a simulation. Results indicate that the impact of this modified cumulus parameterization scheme is more effective on domains with coarser grid sizes. Other results include a decrease in surface and near-surface temperatures in areas of deep convection (due to the inclusion of the effects of subgrid-scale clouds on the radiation), improvement in the timing of convection, and an increase in the strength of deep convection.