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Robert J. Kuligowski, Yaping Li, and Yu Zhang

Abstract

Data from the Tropical Rainfall Measuring Mission (TRMM) have made great contributions to hydrometeorology from both a science and an operations standpoint. However, direct application of TRMM data to short-fuse hydrologic forecasting has been challenging because of the data refresh and latency issues inherent in an instrument in low Earth orbit (LEO). To evaluate their potential impact on low-latency satellite rainfall estimates, rain rates from both the TRMM Microwave Imager (TMI) and precipitation radar (PR) were ingested into a multisensor framework that calibrates high-refresh, low-latency IR brightness temperature data from geostationary platforms against the more accurate but low-refresh, higher-latency rainfall rates available from microwave (MW) instruments on board LEO platforms. The TRMM data were used in two ways: to bias adjust the other MW data sources to match the distribution of the TMI rain rates, and directly alongside the MW rain rates in the calibration dataset. The results showed a significant reduction in false alarms and also a significant reduction in bias for those pixels for which rainfall was correctly detected. The MW bias adjustment was found to have much greater impact than the direct use of the TMI and PR rain rates in the calibration data, but this is not surprising since the latter represented perhaps only 10% of the calibration dataset.

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Jiangnan Li, Petr Chylek, and Feng Zhang

Abstract

The physical characteristics of extratropical cyclones are investigated based on nonequilibrium thermodynamics. Nonequilibrium thermodynamics, using entropy as its main tool, has been widely used in many scientific fields. The entropy balance equation contains two parts: the internal entropy production corresponds to dissipation and the external entropy production corresponds to boundary entropy supply. It is shown that dissipation is always present in a cyclone and the dissipation center is not always coincident with the low-pressure center, especially for incipient cyclones. The different components of internal entropy production correspond to different dissipation processes. Usually the thermal dissipation due to turbulent vertical diffusion and convection lags geographically the dynamic dissipation due to wind stress. At the incipient stage, the dissipation is mainly thermal in nature. A concept of temperature shear is introduced as the result of thermal dissipation. The temperature shear provides a useful diagnostic for extratropical cyclone identification. The boundary entropy supply and the entropy advection are also strongly associated with cyclones. The entropy advection is generally positive (negative) in the leading (trailing) part of a cyclone. A regional study in the western Pacific clearly demonstrates that the surface entropy flux and temperature shear are the most reliable early signals of cyclones in the cyclogenesis stage.

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Peng Lu, Hua Zhang, and Jiangnan Li

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A new scheme of water cloud optical properties is proposed for correlated k-distribution (CKD) models, in which the correlation in spectral distributions between the gaseous absorption coefficient and cloud optical properties is maintained. This is an extension of the CKD method from gas to cloud by dealing with the gas absorption coefficient and cloud optical properties in the same way.

Compared to the results of line-by-line benchmark calculations, the band-mean cloud optical property scheme can overestimate cloud solar heating rate, with a relative error over 30% in general. Also, the error in the flux at the top of the atmosphere can be up to 20 W m−2 at a solar zenith angle of 0°. However, the error is considerably reduced by applying the new proposed CKD cloud scheme. The physical explanation of the large error for the band-mean cloud scheme is the absence of a spectral correlation between the gaseous absorption coefficient and the cloud optical properties. The overestimation of the solar heating rate at the cloud-top layer could affect the moisture circulation and limit the growth of cloud. It is found that the error in the longwave cooling rate caused by the band-mean cloud scheme is very small. In the infrared, the local thermal emission strongly affects the spectral distribution of the radiative flux, which makes the correlation between the gaseous absorption coefficient and cloud optical properties very weak. Therefore, there is no obvious advantage in emphasizing the spectral correlation between gas and cloud.

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Li Liu, Renhe Zhang, and Zhiyan Zuo

Abstract

As important parameters in the land–atmosphere system, both soil moisture (SM) and vegetation play a significant role in land–atmosphere interactions. Using observational data from clay and sand stations over central eastern China, the relationship between leaf area index (LAI) and SM (LAI–SM) in different types of soil was investigated. The results show that the LAI–SM correlation is significantly positive in clay but not significant in sand. The physical causes for the discrepant LAI–SM correlations in different types of soil were explored from the perspectives of evapotranspiration (ET) and soil water retention. In clay stations, increasing LAI is associated with greater soil-water-retention capacity. Although the increasing LAI corresponds to increasing ET, the impact of ET on SM is weak because of the small particle size of soil. Consequently, the LAI–SM relationship in clay is significantly positive. In sand stations, ET is negatively correlated with SM owing to the large soil particle size, resulting in a negative LAI–SM correlation in sand. However, soil water retention is weakened by the increased LAI, which may be an important factor causing the insignificant LAI–SM correlation in sand.

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Yu Nie, Hong-Li Ren, and Yang Zhang

Abstract

Considerable progress has been made in understanding the internal eddy–mean flow feedback in the subseasonal variability of the North Atlantic Oscillation (NAO) during winter. Using daily atmospheric and oceanic reanalysis data, this study highlights the role of extratropical air–sea interaction in the NAO variability during autumn when the daily sea surface temperature (SST) variability is more active and eddy–mean flow interactions are still relevant. Our analysis shows that a horseshoe-like SST tripolar pattern in the North Atlantic Ocean, marked by a cold anomaly in the Gulf Stream and two warm anomalies to the south of the Gulf Stream and off the western coast of northern Europe, can induce a quasi-barotropic NAO-like atmospheric response through eddy-mediated processes. An initial southwest–northeast tripolar geopotential anomaly in the North Atlantic forces this horseshoe-like SST anomaly tripole. Then the SST anomalies, through surface heat flux exchange, alter the spatial patterns of the lower-tropospheric temperature and thus baroclinicity anomalies, which are manifested as the midlatitude baroclinicity shifted poleward and reduced baroclinicity poleward of 70°N. In response to such changes of the lower-level baroclinicity, anomalous synoptic eddy generation, eddy kinetic energy, and eddy momentum forcing in the midlatitudes all shift poleward. Meanwhile, the 10–30-day low-frequency anticyclonic wave activities in the high latitudes decrease significantly. We illustrate that both the latitudinal displacement of midlatitude synoptic eddy activities and intensity variation of high-latitude low-frequency wave activities contribute to inducing the NAO-like anomalies.

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Youtong Zheng, Haipeng Zhang, and Zhanqing Li

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Surface latent heat flux (LHF) has been considered as the determinant driver of the stratocumulus-to-cumulus transition (SCT). The distinct signature of the LHF in driving the SCT, however, has not been found in observations. This motivates us to ask: how determinant is the LHF to SCT? To answer it, we conduct large-eddy simulations in a Lagrangian setup in which the sea-surface temperature increases over time to mimic a low-level cold air advection. To isolate the role of LHF, we conduct a mechanism-denial experiment in which the LHF adjustment is turned off. The simulations confirm the indispensable roles of LHF in sustaining (although not initiating) the boundary layer decoupling (first stage of SCT) and driving the cloud regime transition (second stage of SCT). However, using theoretical arguments and LES results, we show that decoupling can happen without the need for LHF to increase as long as the capping inversion is weak enough to ensure high entrainment efficiency. The high entrainment efficiency alone cannot sustain the decoupled state without the help of LHF adjustment, leading to the recoupling of the boundary layer that eventually becomes cloud-free. Interestingly, the stratocumulus sheet is sustained longer without LHF adjustment. The mechanisms underlying the findings are explained from the perspectives of cloud-layer budgets of energy (first stage) and liquid water path (second stage).

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Yurong Shi, Yufeng Zhang, and Riyi Li

Abstract

The present study presents local-scale urban energy balance observations under various sky conditions in a humid subtropical region. The study site is a typical urban residential area in Guangzhou in which building density is 38%, vegetation coverage is 36%, and mean building height is 21 m. The observation was conducted at a 110-m-high tower using the eddy covariance technique from September to November 2016. The median diurnal pattern and daily maxima were achieved for all radiation components and turbulent fluxes. The Bowen ratio (β) results indicated a predominant role for sensible heat flux (Q H) in the daytime and latent heat flux (Q E) at night. The sky conditions played a significant part in the urban surface energy exchanges, showing that the median daily maxima of net radiation (Q*), Q H, storage heat flux (ΔQ S), surface albedo, and β all present a consistent order from large to small for clear, cloudy, and rainy days and a different order of rainy, clear, and cloudy days for Q E. The mean daytime Q H/Q*, Q E/Q*, ΔQ S/Q*, and β changed with urban density, while Q E/Q* and β also varied with vegetation fraction. Furthermore, the adaptability of net all-wave radiation parameterization (NARP), objective hysteresis model (OHM), and local-scale urban meteorological parameterization scheme (LUMPS) were validated, given the index of agreements of 0.998 and 0.951 for Q* and ΔQ S and the reasonable RMSEs for Q H and Q E. The present study helps to verify and improve the parameterizations of energy exchange over an urban surface in the humid subtropical region.

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Nannan Qin, Da-Lin Zhang, and Ying Li

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It is well known that hurricane intensification is often accompanied by continuous contraction of the radius of maximum wind (RMW) and eyewall size. However, a few recent studies have shown rapid and then slow contraction of the RMW/eyewall size prior to the onset and during the early stages of rapid intensification (RI) of hurricanes, respectively, but a steady state in the RMW (S-RMW) and eyewall size during the later stages of RI. In this study, a statistical analysis of S-RMWs associated with rapidly intensifying hurricanes is performed using the extended best-track dataset during 1990–2014 in order to examine how frequently, and at what intensity and size, the S-RMW structure tends to occur. Results show that about 53% of the 139 RI events of 24-h duration associated with 55 rapidly intensifying hurricanes exhibit S-RMWs, and that the percentage of the S-RMW events increases to 69% when RI events are evaluated at 12-h intervals, based on a new RI rate definition of 10 m s−1 (12 h)−1; both results satisfy the Student’s t tests with confidence levels of over 95%. In general, S-RMWs tend to appear more frequently in more intense storms and when their RMWs are contracted to less than 50 km. This work suggests a new fruitful research area in studying the RI of hurricanes with S-RMWs.

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Yueting Gong, Ying Li, and Da-Lin Zhang

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Tropical cyclones (TCs) tend to change translation direction and speed when moving across Taiwan’s Central Mountain Range (CMR), which makes forecasting of landfalling points a challenging task. This study examines the statistical characteristics of unusual TC tracks around Taiwan Island during the 66-yr period of 1949–2014. Results show that 1) about 10% more TCs were deflected to the right than to the left as they moved across the CMR, but with more occurrences of the latter on Taiwan’s eastern coast and southern strait; 2) TCs around Taiwan Island moved slower than the average speed over the western North Pacific Ocean but then exhibited anomalous acceleration along Taiwan’s eastern coast and anomalous deceleration over the southern Taiwan Strait; 3) about 33% of TCs passing the island were accompanied by terrain-induced secondary low pressure centers (SCs), more favored in the northwestern, southwestern, and southeastern quadrants, with the TC–SC separation distance varying from 33 to 643 km; 4) about 36% of landfalling TCs experienced discontinuous tracks, with an average separation distance of 141 km at the time when TC centers were replaced by SCs, and smaller Froude numbers than those associated with continuous-tracking TCs; and 5) a total of 12 TCs had looping movements near Taiwan Island, most of which were accompanied by SCs on their southern or western sides. Results also indicate that a stronger SC was likely to take place when a stronger TC approached the CMR with a shorter separation distance and that a weaker SC was likely to take place when a weaker TC approached the CMR with a longer separation distance.

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Ying Zhang, Zhanqing Li, and Andreas Macke

Abstract

This study investigates and accounts for the influence of various ice cloud parameters on the retrieval of the surface solar radiation budget (SSRB) from reflected flux at the top of the atmosphere (TOA). The optical properties of ice clouds depend on ice crystal shape, size distribution, water content, and the vertical profiles of geometric and microphysical structure. As a result, the relationship between the SSRB and TOA-reflected flux for an ice cloud atmosphere is more complex and differs from that for water cloud and cloudless atmospheres. The sensitivities of the relationship between the SSRB and TOA-reflected flux are examined with respect to various ice cloud parameters. Uncertainties in the retrieval of the SSRB due to inadequate knowledge of various ice cloud parameters are evaluated thoroughly. The uncertainty study is concerned with both pure ice clouds and multiphase clouds (ice cloud above water cloud). According to the magnitudes of errors in the SSRB retrieval caused by different input variables, parameterized correction terms were introduced. If the input variables are known accurately, errors in the retrieval of the SSRB under a wide range of ice cloud conditions are expected to diminish substantially, to less than 10 W m−2 for 91% of the simulated ice cloud cases. In comparison, the same accuracy may be attained for only 19% of the retrievals for the same ice cloud cases using the retrieval algorithm designed for non-ice-cloud conditions.

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