Abstract

The Advanced Geostationary Radiation Imager (AGRI) on board the Fengyun-4A (FY-4A) satellite provides visible radiances that contain critical information on clouds and precipitation. In this study, the impact of assimilating FY-4A/AGRI all-sky visible radiances on the simulation of a convective system was evaluated with an observing system simulation experiment (OSSE) using a localized particle filter (PF). The localized PF was implemented into the Data Assimilation Research Testbed (DART) coupled with the Weather Research and Forecasting (WRF) Model. The results of a 2-day data assimilation (DA) experiment generated encouraging outcome at a synoptic scale. Assimilating FY-4A/AGRI visible radiances with the localized PF significantly improved the analysis and forecast of cloud water path (CWP), cloud coverage, rain rate, and rainfall areas. In addition, some positive impacts were produced on the temperature and water vapor mixing ratio in the vicinity of cloudy regions. Sensitivity studies indicated that the best results were achieved by the localized PF configured with a localization distance that is equivalent to the model grid spacing (20 km) and with an adequately short cycling interval (30 min). However, the localized PF could not improve cloud vertical structures and cloud phases due to a lack of related information in the visible radiances. Moreover, the localized PF was compared with the ensemble adjustment Kalman filter (EAKF) and it was indicated that the localized PF outperformed EAKF even when the number of ensemble members was doubled for the latter, indicating a great potential of the localized PF in assimilating visible radiances.

Abstract

The Weather Research and Forecasting (WRF) model was employed to provide precipitation forecasts during the 2008/09 and 2009/10 winters (wet season) for Israel and the surrounding region where complex terrain dominates. The WRF precipitation prediction has been coupled with the Hydrological Model for Karst Environment (HYMKE) to forecast the upper Jordan River streamflow. The daily WRF precipitation forecasts were verified against the measurements from a dense network of rain gauges in northern and central Israel, and the simulation results using the high-resolution WRF indicated good agreement with the actual measurements. The daily precipitation amount calculated by WRF at rain gauges located in the upper parts of the Jordan River basin showed good agreement with the actual measurements. Numerical experiments were carried out to test the impact of the WRF model resolution and WRF microphysical schemes, to determine an optimal model configuration for this application. Because of orographic forcing in the region, it is necessary to run WRF with a 4–1.3-km grid increment and with sophisticated microphysical schemes that consider liquid water, ice, snow, and graupel to produce quality precipitation predictions. The hydrological modeling system that ingests the high-resolution WRF forecast precipitation produced good results and improved upon the operational streamflow forecast method for the Jordan River that is now in use. The modeling tools presented in this study are used to support the water-resource-assessment process and studies of seasonal hydroclimatic forecasting in this region.

Abstract

The National Center for Atmospheric Research (NCAR) and the U.S. Army Test and Evaluation Command have developed a multiscale, rapid-cycling, real-time, four-dimensional data-assimilation and forecasting system that has been in operational use at five Army test ranges since 2001. This system was employed to provide operational modeling support for the Joint Urban 2003 (JU2003) Dispersion Experiment, conducted in Oklahoma City, Oklahoma, during July 2003. To better support this mission, modifications were made to the nonlocal boundary layer (BL) parameterization (known as the Medium Range Forecast scheme) of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model, in order to improve BL forecasts. The NCEP–Oregon State University–Air Force–Hydrologic Research Laboratory land surface model was also improved to better represent urban forcing. Verification of the operational model runs and retrospectively simulated cases show 1) a significantly reduced low bias in the forecast surface wind speed and 2) more realistic daytime BL heights. During JU2003, the forecast urban heat island, urban dry bubble, and urban BL height agree reasonably well with observations and conceptual models. An analysis of three-dimensional atmospheric structures, based on model analyses for eight clear-sky days during the field program, reveals some interesting features of the Oklahoma City urban BL, including complex thermally induced circulations and associated convergence/divergence zones, a nocturnal thermal shadow downwind of the urban area, and the reduction of low-level jet wind speeds by more vigorous nocturnal mixing over the city.

ABSTRACT

The changing characteristics of lake ice phenology over the Tibetan Plateau (TP) are investigated using historical satellite retrieved datasets during 2002–15 in this study. The results indicate that the freezing process mainly starts in December, and the ice melting process generally occurs in April for most lakes. However, the changes in lake ice phenology have varied depending on the location in recent years, with delayed break-up dates and prolonged ice durations in the southern TP, but no consistent changes have occurred in the lakes in the northern TP. Further analysis presents a close connection between the variation in the lake ice break-up date/ice duration over the southern TP and the winter North Atlantic Oscillation (NAO). The positive NAO generally excites an anomalous wave activity that propagates southward from the North Atlantic to North Africa and, in turn, strengthens the African–Asian jet stream at its entrance. Because of the blocking effect of the TP, the enhanced westerly jet can be divided into two branches and the south branch flow can deepen the India–Myanmar trough, which further strengthens the anomalous cyclonic circulation and water vapor transport. Therefore, the increased water vapor transport from the northern Indian Ocean to the southern region of the TP can increase the snowfall over this region. The increased snow cover over the lake acts as an insulating layer and lowers the lake surface temperature in the following spring by means of snow–ice feedback activity, resulting in a delayed ice break-up date and the increased ice duration of the lakes over the southern TP in recent years.

Abstract

Despite considerable progress in understanding the hurricane vortex using balanced models, the validity of gradient wind balance in the eyewall remains controversial in observational studies. In this paper, the structure and development of unbalanced forces and flows in hurricanes are examined, through the analyses of the radial momentum and absolute angular momentum (AAM) budgets, using a high-resolution (i.e., Δx = 6 km), fully explicit simulation of Hurricane Andrew (1992).

It is found from the radial momentum budgets that supergradient flows and accelerations, even after temporal and azimuthal averaging, are well organized from the bottom of the eye center to the upper outflow layer in the eyewall. The agradient accelerations are on average twice greater than the local Coriolis force, and caused mainly by the excess of the centrifugal force over the pressure gradient force. It is shown by the AAM budgets that supergradient flows could occur not only in the inflow region as a result of the inward AAM transport, but also in the outflow region through the upward transport of AAM. The eyewall is dominated by radial outflow in which the upward transport of AAM overcompensates the spindown effect of the outflow during the deepening stage. The intense upper outflow layer is generated as a consequence of the continuous outward acceleration of airflows in the eyewall updrafts. In spite of the pronounced agradient tendencies, results presented here suggest that the azimuthally averaged tangential winds above the boundary layer satisfy the gradient wind balance within an error of 10%.

The analyses of instantaneous fields show pronounced asymmetries and well-organized wavenumber-2 structures of the agradient flows and forces in the form of azimuthally propagating vortex–Rossby waves in the eyewall. These waves propagate cyclonically downstream with a speed half the tangential winds near the top of the boundary layer and vertically upward. Agradient flows/forces and AAM transport in the eye are also discussed.

Abstract

Two new postprocessing methods are proposed to reduce numerical weather prediction’s systematic and random errors. The first method consists of running a postprocessing algorithm inspired by the Kalman filter (KF) through an ordered set of analog forecasts rather than a sequence of forecasts in time (ANKF). The analog of a forecast for a given location and time is defined as a past prediction that matches selected features of the current forecast. The second method is the weighted average of the observations that verified when the 10 best analogs were valid (AN). ANKF and AN are tested for 10-m wind speed predictions from the Weather Research and Forecasting (WRF) model, with observations from 400 surface stations over the western United States for a 6-month period. Both AN and ANKF predict drastic changes in forecast error (e.g., associated with rapid weather regime changes), a feature lacking in KF and a 7-day running-mean correction (7-Day). The AN almost eliminates the bias of the raw prediction (Raw), while ANKF drastically reduces it with values slightly worse than KF. Both analog-based methods are also able to reduce random errors, therefore improving the predictive skill of Raw. The AN is consistently the best, with average improvements of 10%, 20%, 25%, and 35% with respect to ANKF, KF, 7-Day, and Raw, as measured by centered root-mean-square error, and of 5%, 20%, 25%, and 40%, as measured by rank correlation. Moreover, being a prediction based solely on observations, AN results in an efficient downscaling procedure that eliminates representativeness discrepancies between observations and predictions.

Abstract

In this study, the inner-core structures of Hurricane Andrew (1992) are explicitly simulated using an improved version of the Penn State–NCAR nonhydrostatic, two-way interactive, movable, triply nested grid mesoscale model (MM5). A modified Betts–Miller cumulus parameterization scheme and an explicit microphysics scheme were used simultaneously to simulate the evolution of the larger-scale flows over the coarser-mesh domains. The intense storm itself is explicitly resolved over the finest-mesh domain using a grid size of 6 km and an explicit microphysics package containing prognostic equations for cloud water, ice, rainwater, snow, and graupel. The model is initialized with the National Centers for Environmental Prediction analysis enhanced by a modified moisture field. A model-generated tropical-storm-like vortex was also incorporated. A 72-h integration was made, which covers the stages from the storm’s initial deepening to a near–category 5 hurricane intensity and the landfall over Florida.

As verified against various observations and the best analysis, the model captures reasonably well the evolution and inner-core structures of the storm. In particular, the model reproduces the track, the explosive deepening rate (>1.5 hPa h⁻¹), the minimum surface pressure of 919 hPa preceding landfall, the strong surface wind (>65 m s⁻¹) near the shoreline, as well as the ring of maximum winds, the eye, the eyewall, the spiral rainbands, and other cloud features. Of particular significance is that many simulated kinematics, thermodynamics, and precipitation structures in the core regions compare favorably to previous observations of hurricanes.

The results suggest that it may be possible to predict reasonably the track, intensity, and inner-core structures of hurricanes from the tropical synoptic conditions if high grid resolution, realistic model physics, and proper initial vortices (depth, size, and intensity) in relation to their larger-scale conditions (e.g., SST, moisture content, and vertical shear in the lower troposphere) are incorporated.

Abstract

No abstract available.

Abstract

Despite considerable research, understanding of the temporal evolution of the inner-core structures of hurricanes is very limited owing to the lack of continuous high-resolution observational data of a storm. In this study, the results of a 72-h explicit simulation of Hurricane Andrew (1992) with a grid size of 6 km are examined to explore the inner-core axisymmetric and asymmetric structures of the storm during its rapid deepening stage. Based on the simulation, a conceptual model of the axisymmetric structures of the storm is proposed. Most of the proposed structures confirm previous observations. The main ingredients include a main inflow (outflow) in the boundary layer (upper troposphere) with little radial flow in between, a divergent slantwise ascent in the eyewall, a penetrative dry downdraft at the inner edge of the eyewall, and a general weak subsiding motion in the eye with typical warming/drying above an inversion located near an altitude of about 2–3 km. The storm deepens as the axes of these features contract.

It is found that the inversion divides the eye of the hurricane vertically into two parts, with a deep layer of warm/dry air above and a shallow pool of warm/moist air below. The air aloft descends at an average rate of 5 cm s⁻¹ and has a residency time of several days. In contrast, the warm/moist pool consists of air from the main inflow and penetrative downdrafts, offset somewhat by the air streaming in a returning outflow into the eyewall in the lowest 2 km; it is subject to the influence of the upward heat and moisture fluxes over the underlying warm ocean. The warm/moist pool appears to play an important role in supplying high-θ _e air for deep convective development in the eyewall. The penetrative downdraft is dry and originates from the return inflow in the upper troposphere, and it is driven by sublimative/evaporative cooling under the influence of the (asymmetric) radial inflow of dry/cold air in the midtroposphere. It penetrates to the bottom of the eye (azimuthally downshear with a width often greater than 100 km) in a radially narrow zone along the slantwise inner edge of the eyewall.

It is further shown that all the meteorological fields are highly asymmetric. Whereas the storm-scale flow features a source–sink couplet in the boundary layer and dual gyres aloft, the inner-core structures exhibit alternative radial inflow and outflow and a series of inhomogeneous updrafts and downdrafts. All the fields tilt more or less with height radially outward and azimuthally downshear. Furthermore, pronounced fluctuations of air motion are found in both the eye and the eyewall. Sometimes, a deep layer of upward motion appears at the center of the eye. All these features contribute to the trochoidal oscillation of the storm track and movement. The main steering appears to be located at the midtroposphere (∼4.5 km) and the deep-layer mean winds represent well the movement of the hurricane.

Abstract

Although considerable progress has been made in understanding the development of hurricanes, our knowledge of their three-dimensional structures of latent heat release and inner-core thermodynamics remains limited. In this study, the inner-core budgets of potential temperature (θ), moisture (q), and equivalent potential temperature (θe) are examined using a high-resolution (Δx = 6 km), nonhydrostatic, fully explicit simulation of Hurricane Andrew (1992) during its mature or intensifying stage.

It is found that the heat energy is dominated by latent heat release in the eyewall, sublimative–evaporative cooling near the eye–eyewall interface, and the upward surface fluxes of sensible and latent heat from the underlying warm ocean. The latent heating (θ) rates in the eyewall range from less than 10°C h^–1 to greater than 100°C h^–1, depending upon whether latent heat is released in radial inflow or outflow regions. The latent heating rates decrease inward in the inflow regions and become negative near the eye–eyewall interface. It is shown that the radial θ advective cooling in the inflow regions accounts for the initiation and maintenance of the penetrative downdrafts at the eye–eyewall interface that are enhanced by the sublimative-evaporative cooling. It is also shown that the vertical θ advection overcompensates the horizontal θ advection for the generation of the warm-cored eye, and the sum of latent heating and radial advective warming for the development of intense cooling in the eyewall. The moisture budgets show the dominant upward transport of moisture in the eyewall updrafts (and spiral rainbands), partly by the low-level outflow jet from the bottom eye regions, so that the eyewall remains nearly saturated.

The θe budgets reveal that θe could be considered as an approximately conserved variable in the eyewall above the boundary layer even in the presence of deposition–sublimation and freezing–melting. The development of higher-θe surfaces at the eye–eyewall interface is discussed in the context of deep convection, the θe gradient and the mass recycling across the eyewall. It is concluded that the simulated hurricane is thermodynamically maintained by the upward surface flux of higher-θe air from the underlying warm ocean, the descent of higher-θe air in the upper troposphere along the eye–eyewall interface, and the recycling of some warmed-eye air at the eye–eyewall interface.