Search Results

You are looking at 11 - 20 of 113 items for

  • Author or Editor: Wei Wang x
  • All content x
Clear All Modify Search
Gyula Molnar and Wei-Chyung Wang

Abstract

Cloud optical properties, in particular the optical thickness, affect the earth-atmosphere radiation budget, and their potential changes associated with climate changes may induce feedback effect. A one-dimensional radiative-forcing model was used to illustrate that the difference in the vertical distribution of the radiative forcing between C02 increase and changes of solar constant can result in a different τ feedback. Recently, Wang et al. carried out a general circulation model study of the climatic effect of atmospheric trace gases CH4, CFCS, and N2O, and the model results indicate that these trace gases provide an important radiative energy source for the present climate. Because the radiative-forcing behavior of CO2 is different from that of these other gases, the simulations also show that different radiative forcing can lead to quite different climatic effects. Consequently, increases in these trace gases may also induce different τ feedback than that due to CO2 increase. Since no study was attempted before to address this aspect, here a one-dimensional model is used to investigate the τ feedback associated with trace gases using an updated τ scheme that relates τ to cloud liquid water content through cloud layer latent heat flux. Because of the different changes in the τ vertical distribution the τ feedback is calculated to be a small negative value for a C02 increase, but much larger negative values for increases of trace gases. The strongest negative feedback is found for CFCs.

Similar experiments were also feedback conducted using a revised version of the Somerville and Remer τ scheme, which relates τ to cloud liquid water content through cloud temperature. The results indicate that the negative feedback for C02 increases for a single cloud layer becomes much smaller when multiple-layer clouds are used, mainly due to the compensating effect of changes in τ values between high and low clouds. Because this scheme assumes a strong functional dependence of the local temperature, the τ feedback is also found to be sensitive to model dimensionally. In addition, the strength and sometimes even the sign of the τ feedback calculated from both schemes depend on the vertical distribution of cloud cover for the control climate, indicating the complexity of cloud-radiation interaction Clearly, more observational and theoretical studies are needed to understand the cloud microphysics and their relation to large-scale climate variables.

Full access
Wei Wang and Eric W. Gill

Abstract

The errors in the current radial velocity measurements are examined using Bartlett beamforming and Multiple Signal Classification (MUSIC) direction-finding algorithms with a linear phased array antenna system. A variety of radar and environmental parameters are examined. Suggestions for the optimal choice of operating parameters are proposed. The MUSIC algorithm has shown promising performance in current measurement when beamforming is used to first establish the maximum current velocity. Comparisons of radar field data and current meter measurements show RMS radial velocity differences in magnitude of 7.44 and 6.64 cm s−1 for the Bartlett beamforming and MUSIC–Bartlett algorithms, respectively. The results indicate that there are advantages to using a MUSIC–Bartlett approach in operational applications.

Full access
J. A. Whitehead and Wei Wang

Abstract

A model of deep ocean circulation driven by turbulent mixing is produced in a long, rectangular laboratory tank. The salinity difference is substituted for the thermal difference between tropical and polar regions. Freshwater gently flows in at the top of one end, dense water enters at the same rate at the top of the other end, and an overflow in the middle removes the same amount of surface water as is pumped in. Mixing is provided by a rod extending from top to bottom of the tank and traveling back and forth at constant speed with Reynolds numbers >500. A stratified upper layer (“thermocline”) deepens from the mixing and spreads across the entire tank. Simultaneously, a turbulent plume (“deep ocean overflow”) from a dense-water source descends through the layer and supplies bottom water, which spreads over the entire tank floor and rises into the upper layer to arrest the upper-layer deepening. Data are taken over a wide range of parameters and compared to scaling theory, energetic considerations, and simple models of turbulently mixed fluid. There is approximate agreement with a simple theory for Reynolds number >1000 in experiments with a tank depth less than the thermocline depth. A simple argument shows that mixing and plume potential energy flux rates are equal in magnitude, and it is suggested that the same is approximately true for the ocean.

Full access
Wei Wang and Nelson L. Seaman

Abstract

A comparison study of four cumulus parameterization schemes (CPSs), the Anthes–Kuo, Betts–Miller, Grell, and Kain–Fritsch schemes, is conducted using The Pennsylvania State University–National Center for Atmospheric Research mesoscale model. Performance of these CPSs is examined using six precipitation events over the continental United States for both cold and warm seasons. Grid resolutions of 36 and 12 km are chosen to represent current mesoscale research models and future operational models. The key parameters used to evaluate skill include precipitation, sea level pressure, wind, and temperature predictions. Precipitation is evaluated statistically using conventional skill scores (such as threat and bias scores) for different threshold values based on hourly rainfall observations. Rainfall and other mesoscale features are also evaluated by careful examination of analyzed and simulated fields, which are discussed in the context of timing, evolution, intensity, and structure of the precipitation systems.

It is found that the general 6-h precipitation forecast skill for these schemes is fairly good in predicting four out of six cases examined in this study, even for higher thresholds. The forecast skill is generally higher for cold-season events than for warm-season events. There is an increase in the forecast skill in the 12-km model, and the gain is most obvious in predicting heavier rainfall amounts. The model’s precipitation forecast skill is better in rainfall volume than in either the areal coverage or the peak amount. The scheme with the convective available potential energy–based closure assumption (Kain–Fritsch scheme) appears to perform better. Some systematic behaviors associated with various schemes are also noted wherever possible.

The partition of rainfall into subgrid scale and grid scale is sensitive to the particular parameterization scheme chosen, but relatively insensitive to either the model grid sizes or the convective environments.

The prediction of mesoscale surface features in warm-season cases, such as mesoscale pressure centers, wind-shift lines (gust fronts), and temperature fields, strongly suggests that the CPSs with moist downdrafts are able to predict these surface features more accurately.

Full access
Guoxing Chen and Wei-Chyung Wang

Abstract

Recently, Chen et al. used a combination of observations and WRF simulations to illustrate that the anthropogenic aerosol–cloud microphysics–radiation interactions over the southeast Pacific can potentially reduce the excessive shortwave radiation reaching the sea surface, a common bias identified in CMIP5 models. Here, with the aid of a mixed-layer ocean, the authors further study the implications of the shortwave radiation reduction to the underlying air–sea coupling, focusing on the SST sensitivity to the changes. Results show that responses of the air–sea coupling include two negative feedbacks (a large decrease in the latent heat flux and a small decrease in the sensible heat flux, both associated with the surface cooling) and a positive feedback (an increase in the cloud cover, caused by the increase in the relative humidity within the boundary layer, especially during the daytime). The 0.1°C (W m−2)−1 SST sensitivity is about half that documented in CMIP5 models. In addition, an effective daytime cloud fraction weighted with the solar diurnal cycle is proposed to facilitate diagnosing the intensity of cloud–radiation interactions in general circulation models.

Full access
Wei-Chyung Wang and Kerang Li

Abstract

In recent years the semiarid region of northern China, which has total annual precipitation between 200 and 500 mm, has shown signs of severe desertification. Intensive theoretical and observational studies are currently underway to examine the climate changes and other contributing factors. In this study, we used the 1951–86 monthly precipitation measurements in this region to study their fluctuations and relationship with the El Niño/Southern Oscillation. Three main features are identified: 1) a 2–3 year quasi-periodic fluctuation, 2) a tendency for rainfall deficiency for the whole region during ENSO years, and 3) a significant correlation between the precipitation fluctuation in the southern part of this region and Southern Oscillation index, with the former lagging the latter by 2–5 months. These features are also evident from analysis of the proxy data during the last hundred years. Discussions on the possible link between the precipitation fluctuation, the summer monsoon, the western Pacific subtropical high, and ENSO are also presented.

Full access
Wei Wang and Thomas T. Warner

Abstract

The Penn State/NCAR mesoscale model has been used in a study of special static- and dynamic-initialization techniques that improve a very-short-range forecast of the heavy convective rainfall that occurred in Texas, Oklahoma and Kansas during 9–10 May 1979, the SESAME IV study period. In this study, the model is initialized during the precipitation event. Two types of four-dimensional data assimilation (FDDA) procedures are used in the dynamic-initialization experiments in order to incorporate data during a 12-hour preforecast period. With the first type, FDDA by Newtonian relaxation is used to incorporate sounding data during the preforecast period. With the second FDDA procedure, radar-based precipitation-rate estimates and hourly raingage data are used to define a three-dimensional latent-heating rate field that contributes to the diabatic heating term in the model's thermodynamic equation during the preforecast period. This latent-heating specification procedure is also employed in static-initialization experiments, where the observed rainfall rate and radar echo pattern near the initial time of the forecast are used to infer a latent-heating rate that is specified in the mesoscale model's thermodynamic equation during the early part of the actual forecast. The precipitation forecasts from these static- and dynamic-initialization experiments are compared with the forecast produced when only operational radiosonde data are used in a conventional static initialization.

The conventional (control) initialization procedure that used only operational radiosonde data produced a precipitation prediction for the first three to four hours of the forecast period that would have been inadequate in an operational setting. Whereas at the initial time of the forecast there was substantial convective precipitation observed in a band near the edge of an elevated mixed layer, the model did not initiate the heavy rainfall until about the fourth hour of the forecast.

The use of the experimental static initialization with prescribed latent heating during the first forecast hour produced greatly improved rainfall rates during the first three to four hours. The success of the technique was shown to be not especially sensitive to moderate variations in the duration, intensity and vertical distribution of the imposed heating. Applications of the Newtonian-relaxation procedure during the preforecast period, that relaxed the model solution toward the initial large-scale analysis, produced a better precipitation forecast than did the control, with a maximum in approximately the correct position, but the intensities were too small. Combined use of either the preforecast or in-forecast latent-heat forcing with the Newtonian relaxation produced an improved forecast of rainfall intensity compared to use of the Newtonian relaxation alone. Even though both the experimental static- and dynamic-initialization procedures produced considerably improved very-short-range precipitation forecasts, compared to the control, the experimental static-initialization procedure that used latent-heat forcing during the first forecast hour did slightly better for this case.

Full access
Wei Tan, Xin Wang, Weiqiang Wang, Chunzai Wang, and Juncheng Zuo

Abstract

This study investigates variations of sea surface temperature (SST) anomalies in the South China Sea (SCS) during developing autumn of various El Niño events. The warm SST anomalies are observed in the SCS for canonical El Niño and El Niño Modoki I, whereas the cold SST anomalies are found for El Niño Modoki II. The ocean heat budget analyses show that the latent heat flux change induced by various types of El Niño events is a major contributor to the SCS SST variations. An anomalous anticyclone resides near the Philippine Sea for canonical El Niño and El Niño Modoki I, which induces the southerly wind anomalies over the SCS and thus weakens the climatological northeasterly in boreal autumn. The weakened surface wind speed reduces heat loss from the ocean, leading to a warmer state in the SCS. However, for El Niño Modoki II, the anomalous anticyclone shifts westward to the west of the SCS, and thus the northeasterly wind anomalies appear in the SCS. The northeasterly anomalies enhance the climatological northeasterly monsoon, increase the wind speed, and increase heat loss from the ocean, thus resulting in a cooling in the SCS. The anomalous anticyclone associated with El Niño events also increases shortwave radiation. The increases of the shortwave radiation can also contribute to the SCS warming for canonical El Niño and El Niño Modoki I in addition to the warm effect from the latent heat flux. Because the cooling effect from the latent heat flux is larger than that of the shortwave radiation for El Niño Modoki II, the SCS for El Niño Modoki II tends to be cool.

Full access
Wei-Chyung Wang and Gerald A. Domoto

Abstract

A modified two-flux approximation is employed to compute the transfer of radiation in a finite, inhomogeneous, turbid atmosphere. A perturbation technique is developed to allow the treatment of non-gray gaseous absorption with multiple scattering. The perturbation method, which employs a backscatter factor as a parameter, can be used with anisotropic particle scattering as well as Rayleigh scattering.

This method is used to study the effect of aerosols on radiative solar heating and infrared cooling as well as the radiative-convective temperature distribution in the earth's atmosphere. It is found that the effect of aerosols in the infrared cannot be neglected; while in the visible, the effect can be the same order as that due to absorption by water vapor. For a high surface albedo (>0.30) heating of the earth-atmosphere system results due to the presence of aerosols. The aerosols also reduce the amount of convection needed to maintain a stable atmosphere. For the case of a dense haze a temperature inversion is found to exist close to the ground.

Full access
Sergey Sokolovskiy, Ying-Hwa Kuo, and Wei Wang

Abstract

Assimilation into numerical weather models of the refractivity, Abel-retrieved from radio occultations, as the local refractivity at ray tangent point may result in large errors in the presence of strong horizontal gradients (atmospheric fronts, strong convection). To reduce these errors, other authors suggested modeling the Abel-retrieved refractivity as a nonlocal linear function of the 3D refractivity, which can be used as a linear observation operator for assimiliation. The authors of this study introduce their approach for the nonlocal linear observation operator, which consists of modeling the excess phase path, calculated along certain trajectories below the top of an atmospheric model. In this study (not aimed at development of an observation operator for any specific atmospheric model), both approaches are validated by assessing the accuracy of both linearized observation operators by numerical simulations with the high-resolution Weather Research and Forecasting (WRF) model and comparing them to the accuracy of interpretation of the Abel-retrieved refractivity as local. Improvement of the accuracy of about an order of magnitude is found with the nonlocal refractivity and further improvement is found with the excess phase path. The effect of horizontal resolution of an atmospheric model on the accuracy of modeling local and nonlocal linear observables is also investigated, and it is demonstrated that the nonlocal linear modeling of radio occultation observables is especially important for weather prediction models with sufficiently high horizontal resolution, grid size <100 km (mesoscale models).

Full access