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

The Naval Research Laboratory’s limited-area numerical prediction system, a version of Navy Operational Regional Atmospheric Prediction System, was used to investigate the interaction between Hurricane Florence (1988) and its upper-tropospheric environment. The model was initialized with the National Meteorological Center (now the National Centers for Environmental Prediction)/Regional Analysis and Forecasting Systems 2.5° analysis at 0000 UTC 9 September 1988, enhanced by a set of Omega dropwindsonde data through a three-pass nested-grid objective analysis.

Diagnosis of the 200-mb level structure of the 12-h forecast valid for 1200 UTC 9 September 1988 showed that the outflow layer was highly asymmetric with an outflow jet originating at approximately 3° north of the storm. In agreement with the result of an idealized simulation (), there was a thermally direct, circum-jet secondary circulation in the jet entrance region and a thermally indirect one in a reversed direction in the jet exit region. In several previous studies, it was postulated that an approaching westerly jet had modulated the convection and intensity variations of Florence. In a variational numerical experiment in this study, the approaching westerly jet was flattened out by repeatedly setting the jet-level meridional wind component and zonal temperature perturbations to zero in the normal mode initialization procedure. Compared with the control experiment, the variational experiment showed that the sudden burst of Florence’s inner core convection was highly correlated with the approaching upper-tropospheric westerly jet. These experiments also suggested that the approaching upper-tropospheric westerly jet was crucial to the intensification of Florence’s inner core convection between 1000 and 1500 UTC 9 September, which occurred prior to the deepening of the minimum sea level pressure (from 997 to 987 mb) between 1200 UTC 9 September and 0000 UTC 10 September.

Many earlier studies have attempted an explanation for the effect on tropical cyclones of upper-tropospheric forcings from the eddy angular momentum approach. The result of this study provides an alternative but complementary mechanism of the interaction between an upper-level westerly trough and a tropical cyclone.

Abstract

The structure and dynamics of the outflow layer of tropical cyclones are studied using a three-dimensional numerical model. Weak and strong tropical cyclones are produced by the numerical model when starting from idealized initial vortices embedded in mean hurricane soundings. The quasi-steady state outflow layers of both the weak and strong tropical cyclones have similar characteristics 1) the circulations are mainly anticyclonic (except for a small region of cyclonic flow near the center) and highly asymmetric about the center, 2) the outflow layer is dominated by a narrow but elongated outflow jet, which contributes up to 50% of the angular momentum transport and 3) the air particles in the outflow jet mostly originate from the lower level, following “in-up-and-out” trajectories.

We found that there are secondary circulations around the outflow jet, very much like those associated with midlatitude westerly jet streaks. In the jet entrance region, the secondary circulation is thermally direct. That is, the ascending motion is located on the anticyclonic shear side of the jet, and the descending motion on the cyclonic shear side. There is a radially outward (perpendicular to the jet) flow above the jet and inflow below it. In the jet exit region, the secondary circulation is weaker and reversed in its direction (thermally indirect). The secondary circulations leave pronounced signatures on the relative humidity, potential vorticity, and tropopause height fields. The secondary circulation is more intense in the stronger tropical cyclone (with a stronger outflow jet) than in the weaker tropical cyclone.

The sensitivities to upper-tropospheric forcing of the outflow are tested in numerical experiments with prescribed forcings. It is found that the simulated tropical cyclone intensifies when its upper levels within a radius of approximately 500 km are accelerated and forced to be more divergent. Convection plays a key role in transforming the upper level divergence into low level convergence. In another experiment, additional regions of convection are initiated in the ascending branches of the circum-jet secondary circulations away from the inner region when the outflow jet between the radii of 500 and 1000 km is accelerated. These regions of convection become competitive with the inner core convection and eventually weaken the tropical cyclone. In both experiments, cumulus convection is the major link between the upper-level forcing and tropical cyclone's response.

Abstract

Episodic events of both Saharan dust outbreaks and African easterly waves (AEWs) are observed to move westward over the eastern tropical Atlantic Ocean. The relationship between the warm, dry, and dusty Saharan air layer on the nearby storms has been the subject of considerable debate. In this study, the Weather Research and Forecasting model is used to investigate the radiative effect of dust on the development of AEWs during August and September, the months of maximum tropical cyclone activity, in years 2003–07. The simulations show that dust radiative forcing enhances the convective instability of the environment. As a result, most AEWs intensify in the presence of a dust layer. The Lorenz energy cycle analysis reveals that the dust radiative forcing enhances the condensational heating, which elevates the zonal and eddy available potential energy. In turn, available potential energy is effectively converted to eddy kinetic energy, in which local convective overturning plays the primary role. The magnitude of the intensification effect depends on the initial environmental conditions, including moisture, baroclinity, and the depth of the boundary layer. The authors conclude that dust radiative forcing, albeit small, serves as a catalyst to promote local convection that facilitates AEW development.

Abstract

The mutual adjustment between upper-tropospheric troughs and the structure of western Atlantic Tropical Cyclones Florence (1988) and Irene (1981) are analyzed using satellite and in situ data. Satellite-observed tracers (e.g., cirrus clouds, water vapor, and ozone) art used to monitor the circulation within the tropical cyclones' environment. The tropical cyclones' convection is inferred from satellite flown passive microwave and infrared sensors. In addition, numerical model simulations are used to analyze and interpret these satellite observations. The study suggests that the initiation and maintenance of intense convective outbreaks in these tropical cyclones during their mature stage are related to the channeling and strengthening of their outflow by upper-tropospheric troughs. The convection can be enhanced in response to the outflow jet-induced import of eddy relative angular momentum and ascending motion associated with the thermally direct circulation. The channeling and strengthening of the outflow occurs when the upper-tropospheric troughs are located in a favorable position relative to the tropical cyclones. Both Florence and Irene intensify after the onset of these intense convective episodes. Satellite observations also suggest that the cessation in the convection of the two tropical cyclones occurs when the upper-tropospheric troughs move near or over the tropical cyclones, resulting in the weakening of their outflow and the entrainment of dry upper-tropospheric air into their inner core.

Abstract

Fields of rainfall rates, integrated water vapor (IWV), and marine surface wind speeds retrieved by the Special Sensor Microwave/Imager (SSM/I) during the intensive observational period 4 on 4 January 1989 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) were analyzed. Subjectively analyzed and model-simulated frontal structures were used to examine the spatial relationship of the SSM/I observed fields to the rapidly intensifying storm and the associated fronts. Qualitative and quantitative comparisons of SSM/I retrievals with GOES imagery, conventional observations, and results produced from the Naval Research Laboratory's (NRL) limited-area numerical model were also made.

SSM/I rainfall was found along the cold and warm fronts, with heavy precipitation within frontal bands. The spatial pattern and characteristics of SSM/I precipitation closely resembled those simulated by the model. Both the warm and the cold front were found to be located near the area of the strongest gradient in IWV. In the warm sector, areas of IWV greater than 40 mm were found, an amount supported by model simulations. Both SSM/I rain rate and IWV distribution were found to be useful in locating the cold and warm fronts. There was good agreement on the relationship of frontal locations to the precipitation patterns and IWV gradients. Most of the high-wind area near the storm center was obscured by clouds for marine surface wind retrieval. SSM/I-retrieved marine surface winds outside the cloud shield (flag 0) were compared to ship- and buoy-reported winds. It was found that the retrieved wind estimates were within 0–3 m s⁻¹ of in situ observation over areas of slow wind shifts. The errors became larger in regions of rapid wind shifts.

Abstract

Ensembles of numerical model forecasts are of interest to operational early warning forecasters as the spread of the ensemble provides an indication of the uncertainty of the alerts, and the mean value is deemed to outperform the forecasts of the individual models. This paper explores two ensembles on a severe weather episode in Spain, aiming to ascertain the relative usefulness of each one. One ensemble uses sensible choices of physical parameterizations (precipitation microphysics, land surface physics, and cumulus physics) while the other follows a perturbed initial conditions approach. The results show that, depending on the parameterizations, large differences can be expected in terms of storm location, spatial structure of the precipitation field, and rain intensity. It is also found that the spread of the perturbed initial conditions ensemble is smaller than the dispersion due to physical parameterizations. This confirms that in severe weather situations operational forecasts should address moist physics deficiencies to realize the full benefits of the ensemble approach, in addition to optimizing initial conditions. The results also provide insights into differences in simulations arising from ensembles of weather models using several combinations of different physical parameterizations.