2023 Vol. 37 No. 1
Cover Note
Abstract：Atmospheric reanalysis reproduces the past atmospheric conditions through assimilation of historical meteorologi-cal observations with fixed version of a numerical weather prediction (NWP) model and data assimilation (DA) system. It is widely used in weather, climate, and even business-related research and applications. This paper reports the development of CMA’s first-generation global atmospheric reanalysis (RA) covering 1979–2018 (CRA-40; CRA refers to CMA-RA). CRA-40 is produced by using the Global Spectral Model (GSM)/Gridpoint Statistical Interpolation (GSI) at a 6-h time interval and a TL574 spectral (34-km) resolution with the model top at 0.27 hPa. A large number of reprocessed satellite data and widely collected conventional observations were assimilated during the reanalyzing process, including the reprocessed atmospheric motion vector (AMV) products from FY-2C/D/E/G satellites, dense conventional observations (at about 120 radiosonde and 2400 synoptic stations) over China, as well as MWHS-2 and GNSS-RO observations from FY-3C. The reanalysis fitting to observations is improved over time, especially for surface pressure with root-mean-square error reduced from 1.05 hPa in 1979 to 0.8 hPa, and for upper air temperature from 1.65 K in 1979 to 1.35 K, in 2018. The patterns of global analysis increments for temperature, specific humidity, and zonal wind are consistent with the changes in the observing system. Near surface temperature from the model’s 6-h forecast reflects the global warming trend reasonably. The CRA-40 precipitation pattern matches well with those of GPCP and other reanalyses. CRA-40 also successfully captures the QBO and its vertical and temporal development, hemispherical atmospheric circulation change, and moisture transport by the East Asian summer monsoon. CRA is now operationally running in near real time as a climate data assimilation system in CMA.More+
Abstract：This study investigates the effects of the assumption on the types of air-mass conservation prescribed in numerical models. First, predictions of the July 2021 (“21.7”) Henan extreme rainfall event from the Integrated Forecast System (IFS) at ECMWF were compared with those from the Yin-He Global Spectral Model (YHGSM), which is a global spectral model with total air-mass conservation (TMC) and dry air-mass conservation (DMC) options. Then, a sensitivity test between simulations from the YHGSM adopting TMC and DMC was conducted. The results show that both the IFS and YHGSM predicted relatively well the 24-h rainfall amount less than 100 mm day−1 on 20 and 21 July 2021 at lead times of 84, 60, and 36 h. For heavy precipitation exceeding 100 mm day−1, however, both models obviously underestimated the daily rainfall amount on 20 July 2021, but the YHGSM produced more precise and stable precipitation forecasts on these two days than the IFS, especially the maximum 24-h precipitation amount, with better consistency at lead times of 84, 60, and 36 h. These differences are further examined in the sensitivity test. Predictions from the YHGSM with DMC show rainfall distributions and daily rainfall amounts closer to the observations at longer lead times. It is inferred that considering sources or sinks of total water in dynamical cores with DMC may have positive feedback for the precise prediction of condensates. For extreme rainfall events, the high local loss of total water may have caused a loss of the atmospheric mass, leading to an additional decrease in surface pressure. Subsequently, the unbalanced pressure gradient force enhances the cyclonic rotation of surface wind and strengthens convergence in the lower troposphere, which in turn further strengthens the vertical velocity, circularly contributing to the enhanced precipitation if the water vapor condition is favorable.More+
Abstract：The effect of soil moisture (SM) on the onset of East Asian subtropical summer monsoon (EASSM) is investigated based on multiple sets of reanalysis data in the period of 1981–2010. It is found that the EASSM is characterized by persistent 2-m s−1 southerly winds for about 3 months in spring at 850 hPa over the subtropical region of East Asia. Considering this feature of the meridional winds, we define the EASSM onset date, and obtain that the climatologi-cal onset date is pentad 17.7, around 26 March. On the interannual timescale, the onset date of EASSM exhibits stati-stically significant correlation with the SM in southeastern China in the month preceding the onset, with wetter (drier) conditions being associated with later (earlier) onset. The physical process by which the preceding SM affects the EASSM onset is further explored by examining the surface energy balance as well as its impacts. Positive (negative) SM anomalies in southeastern China in the month before onset may induce negative (positive) surface temperature anomalies. The decreased (increased) surface temperature in southeastern China before the EASSM onset weakens (strengthens) the zonal sea–land thermal contrast in the surface and low-level atmosphere in the subtropical East Asia. The zonal sea–land thermal contrast in wetter (drier) years induces anomalous northerly (southerly) winds over southeastern China, which tends to delay (advance) the zonal thermal seasonal transition in spring and is conducive to a later (earlier) onset of EASSM. These results are helpful for understanding and prediction of the variability of EASSM and the EASSM onset.More+
2023, 37(1): 58-74.
DOI:
10.1007/s13351-023-2049-2
Abstract：Accurate quantification of soil moisture is essential to understand the land surface processes. Soil hydraulic properties influence water transport in soil and thus affect the estimation of soil moisture. However, some soil hydraulic properties are only observable at a few field sites. In this study, the effects of soil hydraulic properties on soil moisture estimation are investigated by using the one-dimensional (1-D) Richards equation at ELBARA, which is part of the Maqu monitoring network over the Tibetan Plateau (TP), China. Soil moisture assimilation experiments are then conducted with the unscented weighted ensemble Kalman filter (UWEnKF). The results show that the soil hydraulic properties significantly affect soil moisture simulation. Saturated soil hydraulic conductivity (Ksat) is optimized based on its observations in each soil layer with a genetic algorithm (GA, a widely used optimization method in hydrology), and the 1-D Richards equation performs well using the optimized values. If the range of Ksat for a complete soil profile is known for a particular soil texture (rather than for arbitrary layers within the horizon), optimized Ksat for each soil layer can be obtained by increasing the number of generations in GA, although this increases the computational cost of optimization. UWEnKF performs well with optimized Ksat, and improves the accuracy of soil moisture simulation more than that with calculated Ksat. Sometimes, better soil moisture estimation can be obtained by using opti-mized saturated volumetric soil moisture content Ksat. In summary, an accurate soil profile can be obtained by using soil moisture assimilation with optimized soil hydraulic properties.More+
Abstract：A record-breaking extreme heavy snowfall (EHS) event hit northern China during 6–8 November 2021, with two maximum snowfall centers in North China (NC) and Northeast China (NEC), which inflicted severe socioeconomic impacts. This paper compares the differences in the synoptic processes and moisture supply associated with the EHS event in NC and NEC, as well as the atmospheric circulation anomalies before the event, to provide a reference for better prediction and forecasting of EHS in northern China. Synoptic analyses show that a positively tilted, inverted 500-hPa trough channeled cold-air outbreaks into NC, while dynamic updrafts along the front below the trough promoted moisture convergence over this region. In NEC, the dynamic updraft south of the frontogenesis region firstly triggered a low-level Yellow–Bohai Sea cyclone, which then converged with the 500-hPa trough to ultimately form an NEC cold vortex. Calculation of the vorticity tendency indicates that absolute vorticity advection was a better indicator than absolute vorticity divergence for the movement of the trough/ridge at the synoptic scale. Moreover, NOAA’s HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model results reveal that the moisture for the EHS over NC mainly originated from the mid-to-low levels over the Asian–African region and the Eurasian mid-to-high latitudes, accounting for 32% and 31%, respectively. In contrast, the source of water vapor for the EHS over NEC was mainly the Eurasian mid-to-high latitudes and East Asia, with contributions of 38% and 28%, respec-tively. The findings of this study shed some fresh light on the distinctive contributions of different moisture sources to local precipitation. Further analyses of the atmospheric circulation anomalies in October reveal that a phase shift in the Arctic Oscillation related to the weakening of the polar vortex could have served as a useful indicator for the cold-air outbreaks in this EHS event.More+
Abstract：Mêdog County, with its mountains and valleys, is located in the southeastern Tibetan Plateau (TP) and at the lower reaches of the Yarlung Zangbo River. This area has the highest annual rainfall amount over the TP, and in situ measurements are very scarce due to frequent debris flows and transportation difficulties. A monitoring campaign focused on cloud and precipitation observations was established in Mêdog in 2019 as a part of the Second Tibetan Plateau Scientific Expedition and Research Program. This paper evaluates the accuracy of micro rain radar (MRR) measurements and investigates the variations in precipitation vertical structure in Mêdog using observations collected from the MRR, disdrometer, and rain gauges in summer 2021. The measurements from the three instruments show a strong consistency, with correlation coefficients exceeding 0.93. Although the profiles of integral rain parameters for different rain rate categories in Mêdog are similar to those in other regions, the vertical evolution of raindrop size distributions shows significant differences. For lightest rain, the evaporation of small raindrops and breakup of large raindrops are clear during their descent. For the rainfall rate category of 0.2–2.0 mm h−1 (2.0–20.0 mm h−1), concentrations of small and medium (large) drops show almost uniform vertical structures, while the large (medium) drop number displays a positive (negative) gradient. A disturbance at height of 1.5–2.0 km above ground level (AGL) is observed in the heavy rainfall due to strong updrafts. In general, the MRR measurements in Mêdog are robust. The raindrop breakup process is more apparent in Mêdog than in other regions, resulting in high concentration of size-limited raindrops. In addition, it is found that the interaction between steep terrain and Mêdog convective rain causes the strong updrafts between 1.5 and 2.0 km AGL.More+
Abstract：The thermal wind balance in tropical cyclone (TC) eyewalls has been controversial for decades. This study reveals the relationship between the acceleration and curvature on the TC secondary circulation streamline, providing a way to judge thermal wind balance or imbalance in TCs from a simple but clear perspective. According to the relationship between the curvature and acceleration on the streamline, the vertical and radial components of the acceleration cannot be zero simultaneously on the streamline curve, implying that the thermal wind imbalance corresponds to the curvature of the streamline. On the regular scales of TCs, we discuss the conditions of the thermal wind balance approximation and find that the conditions become more stringent with increasing altitudes. In the TC secondary circulation, as an indication of thermal wind imbalance, gradient wind imbalance can be found in the low-level eyewall since there is usually a large curvature when the inflow in the low-level eyewall turns into updrafts sharply. Additionally, gradient wind imbalance also appears at the top level of TC eyewalls because the stringent conditions are too easily broken there.More+
Abstract：The monsoon low-level jet (MLLJ) originates at Mascarene high and after traveling thousands of kilometers enters India from the western boundary causing deep convection, cloudiness, and rainfall. Although its core lies at 850 hPa, it has a large vertical extent; therefore, different meteorological parameters at different levels have a large influence on the Indian summer monsoon rainfall. This study aims to examine the upper-air climatology of 9 stations on the west coast and central Peninsular India and to find out the effects of various parameters at different standard pressure levels on the Indian summer monsoon rainfall variability. We used the 34-yr (1971–2004) actual upper-air radiosonde/radio wind and standard synoptic surface observations data from these 9 stations and reported some new aspects of the MLLJ. The NCEP/NCAR and ECMWF reanalysis wind data have also been used to holistically study the features of MLLJ over sea and land areas. This study, as opposed to some recent studies, confirms the splitting of MLLJ into two branches, which can be seen on a few days during the monsoon season. Further analyses show that the change in geopotential height between 800 and 900 hPa has a strong bearing on the strength of MLLJ. The change in the upper-air pressure gradient force over the Indian landmass can cause a change in the wind speed of MLLJ during the monsoon season.More+
Abstract：The Xiaoxing’an Mountains, located in the temperate monsoon climate zone in Northeast China, have the largest and most complete virgin Korean pine forest in Asia, which has great potential for carbon sequestration. Based on the observational data of the eddy-covariance system at Wuying National Climate Observatory in January 2015–November 2017, the CO2 flux in the forest ecosystem around the observatory was quantitatively studied and the distribution characteristics of the flux source area were analyzed by the Kljun model and the Agroscope Reckenholz–Tänikon footprint tool, providing references for assessing the carbon source/sink potential of the unique forest area. The results showed that the annual total carbon flux around the observatory in 2015, 2016, and 2017 was −756.84, −834.73, and −629.37 gC m−2, respectively, higher than that of other forest ecosystems. The forest of the study area in the Xiaoxing’an Mountains was a strong carbon sink, with the strongest carbon fixation capacity in June and weakest in October, and the carbon flux of each month was less than zero. The flux source area under stable atmospheric conditions was larger than that under unstable conditions, and the source area was larger in the nongrowing season than in the growing season. The size of the source area was largest in winter, followed by spring, autumn, and summer. The maximum length of the source area was 1614.12 m (5639.33 m) under unstable (stable) conditions when the flux contribution rate was 90%. The peak flux contribution was located near the sensor (i.e., within 200 m) in all seasons. The contribution of the source area from the coniferous and broadleaved mixed forest on the west side of the observatory was greater than (3.4 times) that from the Korean pine forest on the east side.More+
Vol. 37, No. 1, Feb 2023
Section
- All
- SPECIAL COLLECTION ON CHINA’S FIRST GENERATION GLOBAL ATMOSPHERE AND LAND REANALYSIS (CRA)
- RECORD DELUGE IN HENAN, CHINA IN JULY 2021: CHALLENGES IN MECHANISMS, FORECASTS, AND SERVICES
- ORIGINAL PAPER
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