The Unevenness of Warm‐Season Precipitation Over the Steep Terrain in North China and Its Related Environmental Conditions

Preconvective environments of severe convective winds over North China and South China

Precipitation sensitivity to soil moisture and its seasonal and diurnal changes are investigated in Bangladesh and surrounding regions using a regional climate model with a 5-km grid spacing. In the control experiment, soil moisture is calculated by a land surface scheme, and simulated accuracy of seasonal and diurnal variations in precipitation intensity and frequency is capable of assessing the soil moisture impact on precipitation. In sensitivity experiments with wetter land surfaces, daytime precipitation intensity decreases over the southern plains for both the premonsoon and mature monsoon seasons because of the weakening of surface heating and vertical mixing in the planetary boundary layer (PBL). Weakened vertical turbulent flux of moisture reduces condensation heating and upward motion in the mid- and upper troposphere, which suppresses development of convective precipitation. The simulated precipitation intensity response to soil moisture suggests that land surface wetness contributes to the seasonal contrast in observed precipitation intensity (i.e., stronger in the premonsoon than the mature monsoon seasons). Meanwhile, the precipitation frequency response to soil moisture varies with season and by region. Over the southern plains in the wet land surface experiments, daytime precipitation frequency decreases (increases) during the premonsoon (mature monsoon) season compared with the dry land surface experiments, as influenced by seasonal differences in relative humidity and the condensation process in the lower troposphere. Around the northern mountainous area, higher soil moisture increases precipitation frequency regardless of season because of additional water vapor supply from the ground and frequent orographic precipitation forced by the mountainous topography.

Global greening and its relationship with climate change remain the hot topics in recent years, and are of critical importance for understanding the interactions between the terrestrial ecosystem carbon cycle and the climate system. China, especially north China, has contributed a lot to global greening during the past few decades. As a water-limited ecosystem, human activities, not precipitation amount, were thought as the main contributor to the greening of north China. Considering the importance of precipitation event characteristics (PEC) in the altered precipitation regimes, we integrated long-term normalized difference vegetation index (NDVI) and meteorological datasets to reveal the role of precipitation regimes, especially PECs, on vegetation growth across temperate grasslands in north China. Accompanied with a significantly decreased growing season precipitation (GSP), NDVI increased significantly in the largest area of the temperate grasslands during 1982–2015, i.e., greening. We found that 28.44% of the area was explained by PECs, including more heavy or extreme precipitation events, alleviated extreme drought, and fewer light events, while only 0.92% of the area was associated with GSP. NDVI did not always increase over the 30 years and there was a decrease during 1996–2005. Taking afforestation projects in desertified lands into account, we found that precipitation, mainly PECs, explained more the increase and decline of NDVI during 1982–1995 and 1996–2005, respectively, while an equivalent explanatory power of precipitation and afforestation projects to the increase in NDVI after 2005. Our study indicates a possible higher productivity under future precipitation regime scenario (e.g., fewer but larger precipitation events) or intensive afforestation activity, implying more carbon sequestration or livestock production of temperate steppe in the future.

The global atmospheric circulation is maintained by the conversion of available potential energy (APE) into kinetic energy. At midlatitudes, this conversion occurs to a large extent in extratropical cyclones through baroclinic instability. Although kinetic energy is easily defined locally, APE is typically defined as a global integral. Therefore, local APE conversion is not well understood.Here, we investigate local APE conversion within the North Atlantic storm track using ERA5 reanalysis data. We utilize a recently introduced formulation of APE, which is exact and defined locally for individual air parcels. First, we explore APE conversion during a period of rapid cyclogenesis, which we then extend to a climatology of extratropical cyclones.Our results indicate that the synoptic upper-level flow determines the distribution of high APE values, which are primarily located in the high-latitude upper troposphere. We show that APE is converted locally into kinetic energy by descending air parcels within the ageostrophic circulation, for example, induced by a jet streak upstream of an extratropical cyclone. The local APE originates not only from advection from the polar, upper-tropospheric APE reservoir, but also from local generation by vertical motion. In fact, the net baroclinic conversion of APE to kinetic energy is the result of much larger positive and negative local contributions. Thus, the global Lorenz energy cycle is more complex on synoptic scales. In addition, we show that surface heat fluxes resulting from air-sea interactions and latent heat release act as diabatic sinks for APE. However, the effect of surface heat fluxes is small compared to the conversion of APE to kinetic energy, as little APE is located in the mid-latitude lower troposphere.In summary, the study shows that the local APE perspective allows the energetics of North Atlantic extratropical cyclones to be better understood in terms of local APE advection as well as adiabatic (ascent and descent) and diabatic effects.

Based on the data about summer precipitation in North China, tropical MJO (short for Madden-Julian Oscillation) index, and NCEP/NCAR (short for National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data, this paper analyzed the relationship between summer precipitation 2018 in North China and MJO by using multiple statistical methods. Findings indicate that, (1) Summer precipitation in North China is closely related to MJO. During the times when the MJO was in phases 5 and 6, North China had heavy precipitation. (2) The correlation mechanism is mainly: At 850 hPa, when MJO storm clouds move eastward, anticyclonic circulation is excited in the north side of the convection area, thus forming a pair of cyclones and anticyclones. Though MJO storm clouds cannot move northward to higher latitude to have direct impact on the summer precipitation in North China, the anticyclonic circulation on the north side of cyclones during the eastward movement of MJO to phases 5 and 6 can strengthen the water-vapor transfer of southerly wind (corresponding to RMM1) or that of southeast wind (corresponding to RMM2) in summer in North China, thus providing favorable water-vapor conditions for precipitation in North China. At 500 hPa, when MJO moves eastward into phases 5 and 6, the western Pacific subtropical high will move northward to the area near the Korean Peninsula and be strengthened, thus blocking weather systems that come from the west, and facilitating ascending motions in North China. At 200 hPa, MJO-associated convection will generate disturbances at upper troposphere and transmit the impacts of disturbances to downstream areas along the westerly jet waveguide, thus forming a disturbance wave train. When MJO moves eastward into phases 5 and 6, duing to the effect of wave train propagation, disturbances at the height of the 500 hPa pressure will have a positive anomaly near the Korean Peninsula, forming an obvious barotropic structure with 500 hPa subtropical high, which strengthens the blocking to weather systems that come from the west and is favorable to the precipitation in North China. (3) MJO can be used for extended-range forecast of summer precipitation over North China.

Observed variations of convective available potential energy (CAPE) in the current climate provide one useful test of the performance of cumulus parameterizations used in general circulation models (GCMs). It is found that frequency distributions of tropical Pacific CAPE, as well as the dependence of CAPE on surface wet-bulb potential temperature (Θw) simulated by the Goddard Institute for Space Studies’s GCM, agree well with that observed during the Australian Monsoon Experiment period. CAPE variability in the current climate greatly overestimates climatic changes in basinwide CAPE in the tropical Pacific in response to a 2°C increase in sea surface temperature (SST) in the GCM because of the different physics involved. In the current climate, CAPE variations in space and time are dominated by regional changes in boundary layer temperature and moisture, which in turn are controlled by SST patterns and large-scale motions. Geographical thermodynamic structure variations in the middle and upper troposphere are smaller because of the canceling effects of adiabatic cooling and subsidence warming in the rising and sinking branches of the Walker and Hadley circulations. In a forced equilibrium global climate change, temperature change is fairly well constrained by the change in the moist adiabatic lapse rate and thus the upper troposphere warms to a greater extent than the surface. For this reason, climate change in CAPE is better predicted by assuming that relative humidity remains constant and that the temperature changes according to the moist adiabatic lapse rate change of a parcel with 80% relative humidity lifted from the surface. The moist adiabatic assumption is not symmetrically applicable to a warmer and colder climate: In a warmer regime moist convection determines the tropical temperature structure, but when the climate becomes colder the effect of moist convection diminishes and the large-scale dynamics and radiative processes become relatively important. Although a prediction based on the change in moist adiabat matches the GCM simulation of climate change averaged over the tropical Pacific basin, it does not match the simulation regionally because small changes in the general circulation change the local boundary layer relative humidity by 1%–2%. Thus, the prediction of regional climate change in CAPE is also dependent on subtle changes in the dynamics.

In this second part of a two-part study, a newly developed moist nonhydrostatic formulation of the spectral energy budget of both kinetic energy (KE) and available potential energy (APE) is employed to investigate the dynamics underlying the mesoscale upper-tropospheric energy spectra in idealized moist baroclinic waves. By calculating the conservative nonlinear spectral fluxes, it is shown that the inclusion of moist processes significantly enhances downscale cascades of both horizontal KE and APE. Moist processes act not only as a source of latent heat but also as an “atmospheric dehumidifier.” The latent heating, mainly because of the depositional growth of cloud ice, has a significant positive contribution to mesoscale APE. However, the dehumidifying reduces the diabatic contribution of the latent heating by 15% at all scales. Including moist processes also changes the direction of the mesoscale conversion between APE and horizontal KE and adds a secondary conversion of APE to gravitational energy of moist species. With or without moisture, the vertically propagating inertia–gravity waves (IGWs) produced in the lower troposphere result in a significant positive contribution to the upper-tropospheric horizontal KE spectra at the large-scale end of the mesoscale. However, including moist processes generates additional sources of IGWs located in the upper troposphere; the upward propagation of the convectively generated IGWs removes much of the horizontal KE there. Because of the restriction of the anelastic approximation, the three-dimensional divergence has no significant contribution. In view of conflicting contributions of various direct forcings, finally, an explicit comparison between the net direct forcing and energy cascade is made.

Abstract. Extratropical cyclones are the predominant weather system in the midlatitudes. They intensify through baroclinic instability, a process in which available potential energy (APE) is converted into kinetic energy (KE). While the planetary-scale conversion of APE to KE is well understood as a mechanism for maintaining the general atmospheric circulation against dissipation, the synoptic-scale perspective on this conversion is less explored. In this study, we analyze the three-dimensional distribution of APE and the physical processes that consume APE for an illustrative case study and a climatology of 285 intense North Atlantic extratropical cyclones in the winters of 1979–2021 using the ERA5 reanalysis. We utilize a recently introduced local APE framework that allows for APE to be quantified at the level of individual air parcels. The geographical APE distribution is shown to be controlled by the large-scale upper-level circulation. Cyclones draw energy from the upper-tropospheric polar APE reservoir along with the development of the associated upper-level trough. This upper-level APE is converted into KE by air descending along the trough's western flank and acts as the incipient cyclone's primary source of KE. Conversely, KE is converted back into APE during the ascent ahead of the trough, reflecting the deceleration of air parcels as they exit the cyclone region. The diabatic dissipation of APE due to surface sensible heat fluxes along the Gulf Stream front is small compared to the adiabatic conversion of APE to KE, since most of the APE is concentrated and consumed in the middle to upper troposphere and cannot be exposed to surface diabatic forcing. In conclusion, by employing a local APE framework, this study provides a detailed investigation of the synoptic dynamics linking extratropical cyclones and planetary-scale energetics.

With Recent increase in deaths due to lightning activity in India we are posed with a question - are these fatalities related to increase in the lightning activity owing to climate change? The IPCC report (2013) projects global warming of 1-5 °C by the end of 21st Century. The global warming is closely related to increased concentration of greenhouse gases. Previous studies have shown that on different temporal and spatial scales small increases in surface temperature leads to increase in thunderstorm and lightning activity. The lightning induced deaths are on the rise especially in the tropical south Asia and Africa but IPCC report does not explicitly deals with lightning activity and its future projection. Studies on this aspect become all the more important as the subgrid scale phenomena such as convective clouds and hence lightning are poorly resolved and taken in to account by climate models. In order to address this issue we have analyzed trends of lightning activity, surface temperature, upper tropospheric water vapour, cloud ice, Convective Available Potential Energy (CAPE) and aerosols. We also present correlation of these parameters with lightning activity using lightning flash rate data of Lightning Imaging Sensor aboard TRMM, TRMM Level -2 Precipitation Radar data, gridded temperature data of IMD, aerosol data acquired by MODIS. The result indicates that upper tropospheric temperature rise is more than surface temperature rise. This imply stable atmosphere with fewer thunderstorms. The increased convection transports additional water vapour into upper troposphere. The water vapour acts as green house has by absorbing infrared radiation emitted by the surface of the Earth. This results in more warming in the upper troposphere than at the surface and stabilizing of the atmosphere. However, results also show that within the thunderstorm the instability measured by CAPE is positively correlated with lightning activity. This paradox of stabilization of global mean atmosphere with increase in lighting activity leads us to conclude that tough thunderstorm activity has subdued but those develop are much more explosive producing more lightning activity and perhaps lead to more fatalities.

Comparison of kinetic energy conversion characteristics of two extreme precipitation episodes during persistent unusually heavy rainfall on 17–23 July 2021 in Henan, China

Experimental study of the wind flow in a coastal region of Japan

We have shown the relationship between seasonal, annual, and large-scale variations in convective available potential energy (CAPE) and the solar cycle in terms of temperature at the 100-hPa pressure level using daily radiosonde data for the period 1980–2006 over Delhi (28.3°N, 77.1°E) and Kolkata (22.3°N, 88.2°E) and for the period 1989–2005 over Cochin (10°N, 77°E) and Trivandrum (8.5°N, 77.0°E), India. In general, there was a tendency for increases in CAPE to be associated with decreases in temperature at the 100-hPa pressure level on all time scales. Decreasing linear trends in temperature were found at Delhi and Kolkata over the period 1990–2006. Our analysis suggests that the trend towards increasing convective activity in the troposphere leads—at least partly—to the trend towards cooling in the tropopause region. High CAPEs are, in general, associated with high rainfall. The minimum annual temperatures were observed almost simultaneously with enhanced annual CAPE during the northern summer, with a larger anti-correlation (-0.62) over Delhi than at other stations. The influence of the solar cycle on the control of temperature was significant (∼4–5°C) only around 8–10°N. Temperature variations in the upper troposphere are viewed as being jointly controlled by CAPE and the solar cycle, with the respective contribution of each being location-dependent.

The connections between intrusions of stratospheric air into the upper troposphere and deep convection in the tropical eastern Pacific are examined using a combination of data analysis, potential vorticity (PV) inversion, and numerical simulations. Analysis of NCEP–NCAR reanalyses and satellite measurements of outgoing longwave radiation during intrusion events shows increased cloudiness, lower static stability, upward motion, and a buildup of convective available potential energy (CAPE) at the leading edge of the intruding tongue of high PV. Potential inversion inversion calculations show that the upper-level PV makes the dominant contribution to the changes in the quantities that characterize convection. This supports the hypothesis that upper-level PV anomalies initiate and support convection by destabilizing the lower troposphere and causing upward motion ahead on the PV tongue. The dominant role of the upper-level PV is confirmed by simulations using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Convection only occurs when the upper-level PV anomaly is present in the simulations, and the relative contribution of the upper-level PV to changes in the quantities that characterize convection is similar to that inferred from the PV inversion calculations.

Ice clouds are an important part of precipitation systems and their thermal (radiative and latent heat) and microphysical effects may impact rainfall. In this study, the thermal and microphysical effects of ice clouds on rainfall are investigated through the diagnostic analysis of rainfall and heat budgets of a torrential rainfall simulation in north China during July 2013. During evening, the maximum reduction in rainfall caused by the inclusion of the thermal effects of ice clouds is mainly associated with the inclusion of latent‐heat effects of ice clouds, which suppresses instability and upward motions. During early morning, the maximum increase in rainfall caused by the inclusion of the thermal effects of ice clouds is mainly related to the inclusion of radiative effects of ice clouds, which enhances radiative cooling in the upper troposphere and suppresses radiative cooling in the lower troposphere and thus increases instability and upward motions. The inclusion of microphysical effects of ice clouds increases rainfall directly by the inclusion of deposition and indirectly by the increase in condensation.

本文利用日降水量资料和NCEP/NCAR再分析大气环流等资料，对华北夏季降水雨型年代际转换及环流特征作综合分析，结果表明：1) 我国东部夏季降水型的转换可划为5个时段，1961~1965年华北、东北都明显多雨；1966~1980年华北多雨，东北正常偏少；1981~2000年东北多雨，华北正常偏少；2001~2010年华北、东北少雨，淮河流域明显偏多；2011~2013年华北、东北多雨，江淮流域偏少。2) 近几年华北、东北夏季雨季与1961~1965年相似，降水量明显偏多，但环流形势与1961~1965年有明显不同。在夏季，海平面气压场上，1961~1965年，蒙古低压中心位于蒙古中南部，低压显著加深；而2011~2013年，蒙古低压中心位于蒙古东部至华北、东北地区，低压明显偏弱。在500 hPa高度场上，1961~1965年，西伯利亚槽、华北槽都有所加深，贝加尔湖脊减弱，“阶梯槽”的形势造成华北、东北降水偏多；而2011~2013年，贝加尔湖脊正常，鄂霍次克海位势高度升高，副高偏北，华北槽受东部阻挡的作用加强，结果造成华北、东北夏季降水偏多。在850 hPa风场上，1961~1965年，东亚有明显偏南风异常，蒙古地区有强大气旋性环流异常，与东亚偏南气流在华北、东北西侧产生风向辐合，造成华北、东北降水异常偏多；而2011~2013年，东亚地区无南风异常，但有明显的东南风异常，风速明显小于1961~1965年偏南风，再加上蒙古地区无气旋性环流辐合带来的动力上升条件，造成华北、东北夏季降水虽然比常年偏多，但少于1961~1965年。所以，虽然近年华北和东北夏季降水同时明显增多，与1961~1965年类似，但环流特征明显不同，突出的是1961~1965年为偏南风异常，即东亚夏季风偏强，而2011~2013年为东南风异常，而东亚夏季风并无明显加强。 In this paper, a comprehensive analysis on decadal shift of precipitation rainfall pattern and cir-culation characteristics during summer in North China is made based on such data as daily preci-pitation, general atmospheric circulation reanalyzed by NCEP/NCAR. The results of the analysis indicate that: 1) The shift of precipitation rainfall pattern during summer in Eastern China can be divided into 5 periods, i.e., 1961-1965 with significantly more rainfall both in North China and Northeast China, 1966-1980 with more rainfall in North China but normally less rainfall in North-east China, 1981-2000 with more rainfall in Northeast China but normally less rainfall in North China, 2001-2010 with less rainfall in North China and Northeast China but significantly more rainfall in Huaihe River Basin, and 2011-2013 with more rainfall in North China and Northeast China but less rainfall in Yangtze-Huaihe River Basin. 2) In recent years, the precipitation in rainy season (summer) in North China and Northeast China is similar to that during 1961-1965, which is significantly high. However, the circulation pattern is significantly different from that during 1961-1965. For the sea-level pressure field in summer, the low-pressure center in Mongolia was located in South Central Mongolia during 1961-1965, with low pressure trough significantly dee-pened; while it was located in Eastern Mongolia and toward North China and Northeast China during 2011-2013, with low pressure trough significantly weakened. At 500 hPa height field, Siberia trough and North China trough were deepened but Baikal Lake Ridge was weakened during 1961-1965. Besides, the “step trough” brought more rainfall to North China and Northeast China. During 2011-2013, however, the Baikal Lake Ridge was normal, the geopotential height of Okhotsk Sea was increased, the subtropical high was located northward, and the blocking effect by the eastern part on the North China trough was strengthened, resulting in more rainfall during summer in North China and Northeast China. At 850 hPa wind field, southerly wind was obviously abnormal in East Asia and strong cyclonic circulation anomaly also occurred in Mongolian region during 1961-1965. Wind direction convergence was caused in the west of North China and Northeast China with the southerly airflow in East Asia, resulting in abnormally more rainfall in North China and Northeast China. During 2011-2013, however, south wind was normal in East Asia but southeast wind was obviously abnormal, with wind speed significantly lower than that of southerly wind during 1961-1965. Moreover, there was no dynamic condition created by cyclonic circulation convergence in Mongolian region. As a result, the rainfall in summer in North China and Northeast China is more than that under normal condition but less than that during 1961- 1965. Therefore, similar to the period of 1961-1965, the precipitation in North China and Northeast China has increased significantly in recent years, but the circulation characteristics are markedly different. A significant difference was that the southerly wind was abnormal during 1961- 1965 (namely the summer monsoon in East Asia was strong); while the southeast wind was abnormal during 2011-2013 (namely the summer monsoon in East Asia was not markedly strong).