Browsing by Author "Zhang, Z."
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Item A framework for quantifying the impacts of sub-pixel reflectance variance and covariance on cloud optical thickness and effective radius retrievals based on the bispectral method(American Institute of Physics (AIP), 2017-02-22) Zhang, Z.; Werner, F.; Wind, G.; Platnick, S.; Ackerman, A. S.; Girolamo, L. Di; Marshak, A.; Meyer, Kerry; Cho, H. M.The so-called bi-spectral method retrieves cloud optical thickness (τ) and cloud droplet effective radius (rₑ) simultaneously from a pair of cloud reflectance observations, one in a visible or near infrared (VIS/NIR) band and the other in a shortwave-infrared (SWIR) band. A cloudy pixel is usually assumed to be horizontally homogeneous in the retrieval. Ignoring sub-pixel variations of cloud reflectances can lead to a significant bias in the retrieved τ and rₑ. In this study, we use the Taylor expansion of a two-variable function to understand and quantify the impacts of sub-pixel variances of VIS/NIR and SWIR cloud reflectances and their covariance on the τ and rₑ retrievals. This framework takes into account the fact that the retrievals are determined by both VIS/NIR and SWIR band observations in a mutually dependent way. In comparison with previous studies, it provides a more comprehensive understanding of how sub-pixel cloud reflectance variations impact the τ and rₑ retrievals based on the bi-spectral method. In particular, our framework provides a mathematical explanation of how the sub-pixel variation in VIS/NIR band influences the rₑ retrieval and why it can sometimes outweigh the influence of variations in the SWIR band and dominate the error in rₑ retrievals, leading to a potential contribution of positive bias to the rₑ retrieval.Item GRB 140102A: Insight into Prompt Spectral Evolution and Early Optical Afterglow Emission(2021-05-27) Gupta, Rahul; Oates, S. R.; Pandey, S. B.; Castro-Tirado, A. J.; Joshi, Jagdish C.; Hu, Y.-D.; Valeev, A. F.; Zhang, B. B.; Zhang, Z.; Kumar, Amit; Aryan, A.; Lien, A.; Kumar, B.; Cui, Ch.; Wang, Ch.; Dimple; Bhattacharya, D.; Sonbas, E.; Bai, J.; Tello, J. C.; Gorosabel, J.; Castro Cerón, J. M.; Porto, J. R. F.; Misra, K.; De Pasquale, M.; Caballero-García, M. D.; Jelínek, M.; Kubánek, P.; Minaev, P. Yu.; Cunniffe, R.; Sánchez-Ramírez, R.; Guziy, S.; Jeong, S.; Tiwari, S. N.; Razzaque, S.; Bhalerao, V.; Pintado, V. C.; Sokolov, V. V.; Zhao, X.; Fan, Y.; Xin, Y.We present and perform a detailed analysis of multi-wavelength observations of \thisgrb, an optical bright GRB with an observed reverse shock (RS) signature. Observations of this GRB were acquired with the BOOTES-4 robotic telescope, the \fermi, and the \swift missions. Time-resolved spectroscopy of the prompt emission shows that changes to the peak energy (\Ep) tracks intensity and the low-energy spectral index seems to follow the intensity for the first episode, whereas this tracking behavior is less clear during the second episode. The fit to the afterglow light curves shows that the early optical afterglow can be described with RS emission and is consistent with the thin shell scenario of the constant ambient medium. The late time afterglow decay is also consistent with the prediction of the external forward shock (FS) model. We determine the properties of the shocks, Lorentz factor, magnetization parameters, and ambient density of \thisgrb, and compare these parameters with another 12 GRBs, consistent with having RS produced by thin shells in an ISM-like medium. The value of the magnetization parameter (RB≈18) indicates a moderately magnetized baryonic dominant jet composition for \thisgrb. We also report the host galaxy photometric observations of \thisgrb obtained with 10.4m GTC, 3.5m CAHA, and 3.6m DOT telescopes and find the host (photo z = 2.8⁺⁰˙⁷₋₀.₉) to be a high mass, star-forming galaxy with a star formation rate of $20 \pm 10 \msun$ yr⁻¹.Item Improving cloud optical property retrievals for partly cloudy pixels using coincident higher‐resolution single band measurements: A feasibility study using ASTER observations(American Geophysical Union, 2018-10-09) Werner, F.; Zhang, Z.; Wind, G.; Miller, D.J.; Platnick, S.; Girolamo, L. DiClear‐sky contamination is a challenging and long‐lasting problem for cloud optical thickness (τ) and effective droplet radius (rₑ𝒻𝒻) retrievals using passive satellite sensors. This study explores the feasibility of improving both _ and rₑ𝒻𝒻retrievals for partly cloudy (PCL) pixels by using available subpixel samples in a visible to near‐infrared (VNIR) band, which many satellite sensors offer. Data is provided by high‐resolution reflectance (R) observations and cloud property retrievals by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) at horizontal resolutions between 30‐960m. For partly cloudy 960‐m observations, the clear‐sky component of the pixels induces significant underestimations of up to 58% for τ, while overestimations in rₑ𝒻𝒻 can exceed 41%. This yields underestimations in the derived liquid water path and cloud droplet number concentration of up to 68% and 72%, respectively. By means of three different assumptions it is shown that subpixel R observations in the VNIR can be used to estimate higher‐resolution R for the second band in the retrieval scheme, as well as the subpixel cloud cover. The estimated values compare well to actually observed ASTER results and are used to retrieve cloud properties, which are unbiased by the clear‐sky component of PCL pixels. While the presented retrieval approach is only evaluated for marine boundary layer clouds, it is computationally efficient and can be easily applied to observations from different imagers. As an example, the PCL retrieval scheme is applied to data by the Moderate Resolution Imaging Spectroradiometer (MODIS), where similar biases for PCL pixels are observed.Item Quantifying the Impacts of Subpixel Reflectance Variability on Cloud Optical Thickness and Effective Radius Retrievals Based On High-Resolution ASTER Observations(American Geophysical Union, 2018-04-26) Werner, F.; Zhang, Z.; Wind, G.; Miller, D. J.; Platnick, S.Recently, Zhang et al. (2016, https://doi.org/10.1002/2016JD024837) presented a mathematical framework based on a second-order Taylor series expansion in order to quantify the plane-parallel homogeneous bias (PPHB) in cloud optical thickness (τ) and effective droplet radius (r ₑ𝒻𝒻) retrieved from the bispectral solar reflective method. This study provides observational validation of the aforementioned framework, using high-resolution reflectance observations from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) over 48 marine boundary layer cloud scenes. ASTER reflectances at a horizontal resolution of 30 m are aggregated up to a scale of 1,920 m, providing retrievals of τ and r ₑ𝒻𝒻 at different spatial resolutions. A comparison between the PPHB derived from these retrievals and the predicted PPHB from the mathematical framework reveals a good agreement with correlation coefficients of r > 0.97 (for Δτ) and r > 0.79 (for Δr ₑ𝒻𝒻 ). To test the feasibility of PPHB predictions for present and future satellite missions, a scale analysis with varying horizontal resolutions of the subpixel and pixel-level observations is performed, followed by tests of corrections with only limited observational high-resolution data. It is shown that for reasonably thick clouds with a mean subpixel τ larger than 5, correlations between observed and predicted PPHB remain high, even if the number of available subpixels decreases or just a single band provides the information about subpixel reflectance variability. Only for thin clouds the predicted Δr ₑ𝒻𝒻 become less reliable, which can be attributed primarily to an increased retrieval uncertainty for r ₑ𝒻𝒻 .Item Soft X-ray prompt emission from a high-redshift gamma-ray burst EP240315a(2024-04-25) Liu, Y.; Sun, H.; Xu, D.; Svinkin, D. S.; Delaunay, J.; Tanvir, N. R.; Gao, H.; Zhang, C.; Chen, Y.; Wu, X.-F.; Zhang, B.; Yuan, W.; An, J.; Bruni, G.; Frederiks, D. D.; Ghirlanda, G.; Hu, J.-W.; Li, A.; Li, C.-K.; Li, J.-D.; Malesani, D. B.; Piro, L.; Raman, G.; Ricci, R.; Troja, E.; Vergani, S. D.; Wu, Q.-Y.; Yang, J.; Zhang, B.-B.; Zhu, Z.-P.; Postigo, A. de Ugarte; Demin, A. G.; Dobie, D.; Fan, Z.; Fu, S.-Y.; Fynbo, J. P. U.; Geng, J.-J.; Gianfagna, G.; Hu, Y.-D.; Huang, Y.-F.; Jiang, S.-Q.; Jonker, P. G.; Julakanti, Y.; Kennea, J. A.; Kokomov, A. A.; Kuulkers, E.; Lei, W.-H.; Leung, J. K.; Levan, A. J.; Li, D.-Y.; Li, Y.; Littlefair, S. P.; Liu, X.; Lysenko, A. L.; Ma, Y.-N.; Martin-Carrillo, A.; O'Brien, P.; Parsotan, Tyler; Quirola-Vasquez, J.; Ridnaia, A. V.; Ronchini, S.; Rossi, A.; Mata-Sanchez, D.; Schneider, B.; Shen, R.-F.; Thakur, A. L.; Tohuvavohu, A.; Torres, M. A. P.; Tsvetkova, A. E.; Ulanov, M. V.; Wei, J.-J.; Xiao, D.; Yin, Y.-H. I.; Bai, M.; Burwitz, V.; Cai, Z.-M.; Chen, F.-S.; Chen, H.-L.; Chen, T.-X.; Chen, W.; Chen, Y.-F.; Chen, Y.-H.; Cheng, H.-Q.; Cui, C.-Z.; Cui, W.-W.; Dai, Y.-F.; Dai, Z.-G.; Eder, J.; Fan, D.-W.; Feldman, C.; Feng, H.; Feng, Z.; Friedrich, P.; Gao, X.; Guan, J.; Han, D.-W.; Han, J.; Hou, D.-J.; Hu, H.-B.; Hu, T.; Huang, M.-H.; Huo, J.; Hutchinson, I.; Ji, Z.; Jia, S.-M.; Jia, Z.-Q.; Jiang, B.-W.; Jin, C.-C.; Jin, G.; Jin, J.-J.; Keereman, A.; Lerman, H.; Li, J.-F.; Li, L.-H.; Li, M.-S.; Li, W.; Li, Z.-D.; Lian, T.-Y.; Liang, E.-W.; Ling, Z.-X.; Liu, C.-Z.; Liu, H.-Y.; Liu, H.-Q.; Liu, M.-J.; Liu, Y.-R.; Lu, F.-J.; LU, H.-J.; Luo, L.-D.; Ma, F. L.; Ma, J.; Mao, J.-R.; Mao, X.; McHugh, M.; Meidinger, N.; Nandra, K.; Osborne, J. P.; Pan, H.-W.; Pan, X.; Ravasio, M. E.; Rau, A.; Rea, N.; Rehman, U.; Sanders, J.; Santovincenzo, A.; Song, L.-M.; Su, J.; Sun, L.-J.; Sun, S.-L.; Sun, X.-J.; Tan, Y.-Y.; Tang, Q.-J.; Tao, Y.-H.; Tong, J.-Z.; Wang, H.; Wang, J.; Wang, L.; Wang, W.-X.; Wang, X.-F.; Wang, X.-Y.; Wang, Y.-L.; Wang, Y.-S.; Wei, D.-M.; Willingale, R.; Xiong, S.-L.; Xu, H.-T.; Xu, J.-J.; Xu, X.-P.; Xu, Y.-F.; Xu, Z.; Xue, C.-B.; Xue, Y.-L.; Yan, A.-L.; Yang, F.; Yang, H.-N.; Yang, X.-T.; Yang, Y.-J.; Yu, Y.-W.; Zhang, J.; Zhang, M.; Zhang, S.-N.; Zhang, W.-D.; Zhang, W.-J.; Zhang, Y.-H.; Zhang, Z.; Zhang, Z.; Zhang, Z.-L.; Zhao, D.-H.; Zhao, H.-S.; Zhao, X.-F.; Zhao, Z.-J.; Zhou, L.-X.; Zhou, Y.-L.; Zhu, Y.-X.; Zhu, Z.-C.; Zuo, X.-X.Long gamma-ray bursts (GRBs) are believed to originate from core collapse of massive stars. High-redshift GRBs can probe the star formation and reionization history of the early universe, but their detection remains rare. Here we report the detection of a GRB triggered in the 0.5--4 keV band by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated as EP240315a, whose bright peak was also detected by the Swift Burst Alert Telescope and Konus-Wind through off-line analyses. At a redshift of z=4.859, EP240315a showed a much longer and more complicated light curve in the soft X-ray band than in gamma-rays. Benefiting from a large field-of-view (∼3600 deg²) and a high sensitivity, EP-WXT captured the earlier engine activation and extended late engine activity through a continuous detection. With a peak X-ray flux at the faint end of previously known high-z GRBs, the detection of EP240315a demonstrates the great potential for EP to study the early universe via GRBs.