Abstract

The evolution of El Niño–Southern Oscillation (ENSO) is an essential predictor for global climate anomalies, but ENSO’s effect is regulated by the considerable diversity of post-ENSO evolution. This study identified that there coexist a decaying mode and a developing mode of ENSO in the ENSO decaying periods (from September to August) through empirical orthogonal function (EOF) and extended EOF analyses. The diversity of ENSO evolution during the decaying periods is actually determined by the ENSO developing mode, which is unsteady and susceptible to other signals, whereas the ENSO decaying mode is quite steady. Using multiple datasets, segmented datasets, and sea surface temperature anomalies (SSTAs)-residual EOF, the key signals related to the diversity of ENSO evolution are identified and verified in the South Atlantic and equatorial South Pacific. Based on the decaying and developing modes, a prediction model was constructed to predict the boreal summer Niño-3 index. The ENSO developing mode, explaining very small variance, plays a decisive role in determining the prediction of the summer SSTAs in the Pacific. The forecasting skill of summer ENSO can also be greatly improved when continuous SSTAs and interbasin interactions in the whole tropics are considered. This prediction result indicates that identified precursory disturbance signals and interbasin interactions for ENSO development are crucial for boreal summer ENSO prediction.

Abstract

Atmospheric particles exhibit various sizes and nonspherical shapes, which are factors that primarily determine the physical–optical properties of particles. The “sizes” of nonspherical particles can be specified based on various size descriptors, such as those defined with respect to a volume-equivalent spherical radius, projected-area-equivalent spherical radius, geometric radius, or effective radius. Microphysical and radiative transfer simulations as well as remote sensing implementations often require the conversions of particle size distributions (PSDs) in terms of the number concentration, projected area, and volume. The various size descriptors cause ambiguity in the PSD interconversion, and thereby result in potentially misleading quantification of the physical–optical properties of atmospheric nonspherical particles. The present study aims to provide a generalized formula for interconversions of PSDs in terms of physical variables and size descriptors for arbitrary nonspherical particles with lognormal and gamma distributions. In contrast to previous studies, no empirical parameters are included, allowing intrinsic understanding of the nonspherical particle effects on the PSD interconversion. In addition, we investigate the impact of different size descriptors on the single-scattering properties of nonspherical particles. Consistent single-scattering properties among different nonspherical particles with the same size parameter are found when the size descriptor is the effective radius, whereby their mechanisms are suggested based on a modified anomalous diffraction theory. The overarching goal of this work is to eliminate the ambiguity associated with a choice of the size descriptor of nonspherical particles for Earth-atmosphere system models, cloud–aerosol remote sensing, and analyses of in situ measured atmospheric particles.

Abstract

This study reports the development of tangent-linear and adjoint models for a vector radiative transfer model called TAMU-VRTM. This vector radiative transfer model is further validated in the case of the atmosphere–ocean coupled system, although previous validation was conducted for single and multiple layers. The TAMU-VRTM and tangent-linear and adjoint models can be applied to remote sensing and data assimilation based on spaceborne and airborne polarimetric observations. The tangent-linear and adjoint models accurately and efficiently compute the derivatives of output Stokes parameters with respect to input variables of the TAMU-VRTM. An inversion algorithm can straightforwardly compute the Jacobian matrix from the derivatives of Stokes parameters using the chain rule. We validate the tangent-linear and adjoint models by comparing them with the finite-difference method, and show that the finite-difference results converge to the tangent-linear and adjoint results. Furthermore, the adjoint model can efficiently compute the derivatives of observables with respect to the scattering phase matrix elements. This capability can be used to evaluate the scattering phase matrix assumed in an inversion algorithm and has potential for applications to inferring scattering phase matrix elements of cloud, aerosol, and hydrosol particles.

Abstract

A database (TAMUoic2019) of the scattering, absorption, and polarization properties of horizontally oriented hexagonal plates (HOPs) and horizontally oriented hexagonal columns (HOCs) at three wavelengths (355, 532, and 1064 nm) is developed for applications to radiative transfer simulations and remote sensing implementations involving oriented ice crystals. The maximum dimension of oriented ice crystals ranges from 50 to 10 000 μm in 165 discrete size bins. The database accounts for 94 incident directions. The single-scattering properties of oriented ice crystals are computed with the physical-geometric optics method (PGOM), which is consistent with the invariant-imbedding T-matrix method for particles with size parameters larger than approximately 100–150. Note that the accuracy of PGOM increases as the size parameter increases. PGOM computes the two-dimensional phase matrix as a function of scattering polar and azimuth angles, and the phase matrix significantly varies with the incident direction. To derive the bulk optical properties of ice clouds for practical radiative transfer applications, the optical properties of individual HOPs and HOCs are averaged over the probability distribution of the tilting angle of oriented ice crystals based on the use of the TAMUoic2019 database. Simulations of lidar signals associated with ice clouds based on the bulk optical properties indicate the importance of the fraction of oriented ice crystals and the probability distribution of the tilting angle. Simulations of optical phenomena caused by oriented ice crystals demonstrate that the computed single-scattering properties of oriented ice crystals are physically rational.

Abstract

Island thermal effects on the trail cloud band over the central North Pacific are investigated for the lee of Hawaii using satellite observations and a regional atmospheric model. The trail cloud band develops around noon and peaks in cloudiness in the early afternoon. The analysis of numerical simulations of the Kauai wake suggests that a dynamically induced convergence zone forms in the lee of Kauai and Oahu (maximum elevation at 1.5 and 1.2 km, respectively) under the trade wind flow. The island thermal effect significantly modulates the island wake and creates a diurnal cycle of development and decay in the lee cloud band. As solar radiation heats up the island from morning to afternoon, warm air moves downstream (warm advection) from the island in the wake zone, increasing the air temperature, decreasing the air pressure, and enhancing low-level wind convergence in favor of the formation of the trail clouds. Conversely the cold advection during night suppresses cloud formation in the wake. The warm advection and the convergence in the wake increase with the upstream trade wind strength, consistent with satellite observations that the cloudiness increases in the wake under strong wind conditions in the afternoon.

The similarity in the trail cloud and its diurnal cycle between Kauai and Oahu suggests that the thermal wake effect is quite common. The conditions for such a thermal wake are discussed.

Abstract

Using precipitation data from rain gauge stations over China, the authors examine the long-term variation of the durations of persistent rainfall over eastern China for the past 40 years. The variation in the regional rainfall was related to a change in the global-mean surface temperature from the relatively cold period of the 1960s–70s to the relatively warm period of the 1980s–90s. Compared to the cold period, the persistent rainfall in the warm period began earlier and ended later over southern China, lengthening the rainy season by 23 days, but it began later and ended earlier over northern China, shortening the rainy season by 14 days. This change in the durations of persistent rainfall contributed to the pattern of the long-term change in rainfall: southern floods and northern droughts. The earlier beginning of the rainy season over southern China was associated with a more westward subtropical high over the western North Pacific and a stronger low-level low near the eastern Tibetan Plateau during spring. On the other hand, the later ending of the rainy season over southern China and the shorter rainy season over northern China were related to a more westward subtropical high over the western Pacific and a weaker trough near the eastern Tibetan Plateau during summer.

The snow cover over the Tibetan Plateau exhibited a positive trend in winter and spring, which increased the local soil moisture content and cooled the overlying atmosphere during spring and summer. The sea surface temperature over the tropical Indian Ocean and the western North Pacific also displayed a positive trend. The cooling over land and the warming over oceans reduced the thermal contrast between East Asia and the adjacent oceans. Moreover, the low-level low pressure system over East Asia weakened during summer. Under such circumstances, the East Asian summer monsoon circulation weakened, with anomalous northerly winds over eastern China. Correspondingly, the mei-yu front stagnated over the Yangtze River valley, and the associated pattern of vertical motions increased the rainfall over the valley and decreased the rainfall over northern China.

Abstract

An hourly dataset of automatic weather stations over Beijing Municipality in China is developed and is employed to analyze the spatial and temporal characteristics of urban heat island intensity (UHII) over the built-up areas. A total of 56 stations that are located in the built-up areas [inside the 6th Ring Road (RR)] are considered to be urban sites, and 8 stations in the suburban belts surrounding the built-up areas are taken as reference sites. The reference stations are selected by using a remote sensing method. The urban sites are further divided into three areas on the basis of the city RRs. It is found that the largest UHII generally takes place inside the 4th RR and that the smallest ones occur in the outer belts of the built-up areas, between the 5th RR and the 6th RR, with the areas near the northern and southern 6th RR experiencing the weakest UHI phenomena. On a seasonal basis, the strongest UHII generally occurs in winter and weak UHII is dominantly observed in summer and spring. The UHII diurnal variations for each of the urban areas are characterized by a steadily strong UHII stage from 2100 local solar time (LST) to 0600 LST and a steadily weak UHII stage from 1100 to 1600 LST, with the periods 0600–1100 LST and 1600–2100 LST experiencing a swift decline and rise, respectively. UHII diurnal variation is seen throughout the year, but the steadily strong UHII stage at night is longer (shorter) and the steadily weak UHII stage during the day is shorter (longer) during winter and autumn (summer and spring).

Abstract

Hourly datasets obtained by automatic weather stations in Beijing, China, are developed and employed to analyze the spatial and temporal characteristics of relative humidity (RH) and urban dryness island intensity (UDII) over built-up areas. A total of 36 stations inside the sixth ring road are considered as urban sites, while six stations in suburban belts surrounding the built-up areas are taken as reference sites. Results show that the RH is obviously smaller in urban areas than in suburban areas, indicating the effect of urbanization on near-surface atmospheric moisture and RH. A further analysis of relations between RH and temperature on varied time scales shows that the variations in RH in the urban areas are not due solely to changes in temperature. The annual and seasonal mean UDII are high in central urban areas, with the strongest UDII values occurring in autumn and the weakest values occurring in spring. The diurnal UDII variations are characterized by a steadily strong UDII stage from 2000 to 0800 LT and a minimum at 1500 or 1600 LT. The rapid shifts of UDII from high (low) to low (high) occur during the periods 0800–1600 LT (1600–2000 LT). The occurrence time of the peaks varies among different seasons: the peaks appear at 0700, 2100, 2000, and 0800 LT for spring, summer, autumn, and winter, respectively. Further analysis shows that large UDII values appear in the evenings and early nights in late summer and early to midautumn and that low UDII values mainly occur in the afternoon hours of spring, winter, and late autumn.

Abstract

A fast and flexible model is developed to simulate the transfer of thermal infrared radiation at wavenumbers from 700 to 1300 cm⁻¹ with a spectral resolution of 0.1 cm⁻¹ for scattering–absorbing atmospheres. In a single run and at multiple user-defined levels, the present model simulates radiances at different viewing angles and fluxes. Furthermore, the model takes into account complicated and realistic scenes in which ice cloud, water cloud, and mineral dust layers may coexist within an atmospheric column. The present model is compared to a rigorous reference model, the 32-stream Discrete Ordinate Radiative Transfer model (DISORT) code. For an atmosphere with three scattering layers (water, ice, and mineral dust), the root-mean-square error of the simulated brightness temperatures at the top of the atmosphere is approximately 0.05 K, and the relative flux errors at the boundary and internal levels are much smaller than 1%. Within the same computing environment, the fast model runs more than 10 000, 6000, and 4000 times faster than DISORT under single-layer, two-layer, and three-layer cloud–aerosol conditions, respectively. With its computational efficiency and accuracy, the present model may optimally facilitate the forward radiative transfer simulations involved in remote sensing implementations based on high-spectral-resolution and narrowband infrared measurements and in the data assimilation applications of the weather forecasting system. The selected 0.1-cm⁻¹ spectral resolution is an obstacle to extending the present model to strongly absorptive bands (e.g., 600–700 cm⁻¹). However, the present clear-sky module can be substituted by a more accurate model for specific applications involving spectral bands with strong absorption.

Abstract

Correlations of the urban heat island intensity (UHII) and key surface variables with the short-duration intense rainfall (SDIR) events are examined for the Beijing urban areas by applying hourly data of a high-density automatic weather station (AWS) network. Higher frequencies (amounts) of the SDIR events are found in or near the central urban area, and most of the SDIR events begin to appear in late evening and nighttime, but tend to end in late night and early morning. Correlations of the UHII with the SDIR frequency (amount) are all highly significant for more than 3 h ahead of the beginning of the SDIR events. Although the UHII at immediate hours (<3 h) before the SDIR occurrence is more indicative of SDIR events, their occurrence more depends on the magnitude of the UHII at earlier hours. The UHII before the beginning of the SDIR events also shows high-value centers in the central urban area, which is generally consistent with the distribution of the SDIR events. The spatial and temporal patterns of regional SDIR events exhibit similar characteristics to the site-based SDIR events and also show a good relationship with the UHII in the urban areas. In addition to the UHII over the urban areas, surface air temperature, surface air pressure, relative humidity, and near-surface wind directions at the Beijing station experience large changes before and after the beginning time of regional SDIR events, and have the potential to indicate the occurrence of SDIR events in the studied area.