DOMINANT PHYSICAL PROCESSES ASSOCIATED WITH PHASE DIFFERENCES BETWEEN SURFACE RAINFALL AND CONVECTIVE AVAILABLE POTENTIAL ENERGY

A lag correlation analysis is conducted with a 21-day TOGA COARE cloud-resolving model simulation data to identify the phase relation between surface rainfall and convective available potential energy (CAPE) and associated physical processes. The analysis shows that the maximum negative lag correlations between the model domain mean CAPE and rainfall occurs around lag hour 6. The minimum mean CAPE lags mean and convective rainfall through the vapor condensation and depositions,water vapor convergence,and heat divergence whereas it lags stratiform rainfall via the transport of hydrometeor concentration from convective regions to raining stratiform regions,vapor condensation and depositions,water vapor storage,and heat divergence over raining stratiform regions.

Assessment of Several Moist Adiabatic Processes Associated with Convective Energy Calculation

Several methods dealing with the moist adiabatic process are described in this paper. They are based on static energy conservation, pseudo-equivalent potential temperature conservation, the strict pseudo- adiabatic equation, and the reversible moist adiabatic process, respectively. Convective energy parame- ters, which are closely related to the moist adiabatic process and which re?ect the gravitational e?ects of condensed liquid water, are reintroduced or de?ned, including MCAPE [Modi?ed-CAPE (convective avail- able potential energy)], DCAPE (Downdraft-CAPE), and MDCAPE (Modi?ed-Downdraft-CAPE). Two real case analyses with special attention given to condensed liquid water show that the selection of moist adiabatic process does a?ect the calculated results of CAPE and the gravitational e?ects of condensed liq- uid water are not negligible in severe storms. Intercomparisons of these methods show that static energy conservation is consistent with pseudo-equivalent potential temperature conservation not only in physical properties but also in calculated results, and both are good approximations to the strict pseudo-adiabatic equation. The lapse rate linked with the reversible moist adiabatic process is relatively smaller than that linked with other moist adiabatic processes, especially when considering solidi?cation of liquid water in the reversible adiabatic process.

Analysis and Comparison of Mesoscale Convective Systems over the Qinghai-Xizang (Tibetan) Plateau

A series of mesoscale convective systems (MCSs) occurred daily over the Qinghai-Xizang Plateau during 25–28 July 1995. In this paper, their physical characteristics and evolutions based on infrared satellite imagery, their largescale meteorological conditions, and convective available potential energy (CAPE) are analyzed. It is found that similar diurnal evolution is present in all these MCSs. Their initial convective activities became active at noon LST by solar heating, and then built up rapidly. They formed and reached a peak in the early evening hours around 1800 LST and then abated gradually. Among them, the strongest and largest is the MCS on 26 July, which developed under the conditions of the great upper-level nearly-circular Qinghai-Xizang anticyclonic high and driven by the strong low-level thermal forcing and conditional instability. All these conditions are intimately linked with the thermal effects of the plateau itself. So its development was mainly associated with the relatively pure thermal effects peculiar to the Qinghai-Xizang Plateau. The next strongest one is the MCS on 28 July, which was affected notably by the baroclinic zone linked with the westerly trough. There are different features and development mechanisms between these two strongest MCSs.

Precipitation efficiency and its relationship to physical factors

The precipitation efficiency and its relationship to physical factors are examined by analyzing a two-dimensional cloud-resolving model simulation during TOGA COARE in this study. The basic physical factors include convective avail- able potential energy, water-vapor convergence, vertical wind shear, cloud ratio, sea surface temperature, air temperature, and precipitable water. Precipitation efficiencies do not show a close relationship to air temperature nor to sea surface tem- perature nor to precipitable water. The precipitation efficiency increases as the water-vapor convergence rate increases and vertical wind shear weakens, whereas it decreases as the convective available potential energy dissipates and anvil clouds develop.

The Effect of Surface Friction on the Development of Tropical Cyclones

When tropical cyclones （hereafter referred as TCs） are over the ocean, surface friction plays a dual role in the development of TCs. Prom the viewpoint of water vapor supply, frictional convergence and Ekman pumping provide a source of moisture for organized cumulus convection and is propitious to the spin-up of TCs. On the other hand, surface friction leads to a dissipation of kinetic energy that impedes the intensification of TCs. Which role is dominant in the developing stage of TCs is a controversial issue. In the present work, the influence of surface friction on the growth of TCs is re-examined in detail by conducting two sets of numerical experiments initialized with different cyclonic disturbances. Results indicate that, because of the inherent complexities of TCs, the impact of surface friction on the evolution of TCs can not be simply boiled down to being positive or negative. In the case that a TC starts from a low-level vortex with a warm core, surface friction and the resultant vertical motion makes an important contribution to the convection in the early developing stage of the TC by accelerating the build-up of convective available potential energy （CAPE） and ensuring moisture supply and the lifting of air parcels. This effect is so prominent that it dominates the friction-induced dissipation and makes surface friction a facilitative factor in the spin-up of the TC. However, for a TC formed from a mesoscale convective vortex （MCV） spawned in a long-lasting mesoscale convective system （MCS）, the initial fields, and especially the low-level humidity and cold core, enable the prerequisites of convection （i.e., conditional instability, moisture, and lifting）, to be easily achieved even without the help of boundary-layer pumping induced by surface friction. Accordingly, the reliance of the development of TCs on surface friction is not as heavy as that derived from a lowlevel vortex. The positive effect of surface friction on the development of TCs realized through facilitating favorable conditions for convection is nearly cancelled out by the friction-induced dissipation. However, as SST is enhanced in the latter case, the situation may be changed, and different development speeds may emerge between model TCs with and without surface friction considered. In short, owing to the fact that TC development is a complicated process affected by many factors such as initial perturbations, SST, etc., the importance of surface friction to the intensification of TCs may vary enormously from case to case.

Construction of the Forecast System of Classified Severe Convection Weather in Qinghai Province Based on Ingredients-based Method

Based on the data of the cases of severe convection weather such as hail,thunderstorm(thunderstorm gale)and short-time heavy precipitation in recent 10 years,the spatial and temporal distribution characteristics of different types of severe convection weather were analyzed.The results show that the frequency of severe convection weather tended to increase,of which short-time heavy precipitation and thunderstorm weather rose,and hail and thunderstorm gale weather decreased.Severe convection weather began to extend in late spring and early autumn.Typical cases were selected to analyze the evolution mechanism,and the conceptual models of severe convective weather caused by cold advection forcing,warm advection forcing and baroclinic frontogenesis were obtained.The key predictors for the potential prediction of severe convection weather were proposed,such as CAPE(convective available potential energy)for hail weather,UH index(maximum ascending helicity)for thunderstorm gale and PWV(precipitable water vapor)for short-time heavy precipitation.ERA5 data were used to get the forecast threshold of the key factor of classified severe convection weather,and it was verified that the threshold was available.Meanwhile,the causes of the error of failure cases were analyzed.For instance,the larger deviation of CAPE was caused by the 2 m deviation of temperature.Supplementary correction method and threshold were given to provide a reference for the objective forecast and early warning of severe convection weather.

Environmental Influences on the Intensity Change of Tropical Cyclones in the Western North Pacific

The atmospheric and oceanic conditions are examined during different stages of the lifecycle of western North Pacific tropical cyclones （TCs）, with the intention to understand how the environment affects the intensity change of TCs in this area. It is found that the intensification usually occurs when the underlying sea surface temperature （SST） is higher than 26℃. TCs usually experience a rapid intensification when the SST is higher than 27.5℃ while lower than 29.5℃. However, TCs decay or only maintain its intensity when the SST is lower than 26℃. The intensifying TCs usually experience a low-to-moderate vertical wind shear （2-10 m s-l）. The larger the vertical wind shear, the slower the TCs strengthen. In addition, the convective available potential energy （CAPE） is much smaller in the developing stage than in the formation stage of TCs. For the rapidly intensifying TCs, the changes of SST, CAPE, and vertical wind shear are usually small, indicating that the rapid intensification of TCs occurs when the evolution of the environment is relatively slow.

Cold Air Activities in July 2004 and Its Impact on Intense Rainfalls over Southwest China

The severe rainfall events in the mid-summer of July 2004 and the roles of cold air in the formation of heavy precipitation are investigated by using daily observational precipitation data of China and NCEP/NCAR reanalysis. The results show that the severe rainfalls in Southwest China are closely related to the cold air activities from the mid-high latitudes, and the events take place under the cooperative effects of mid-high latitude circulation and low latitude synoptic regimes. It is the merging of a cold vortex over mid-latitudes with the northward landing typhoon and eastward Southwest China Vortex, as well as the abrupt transformation from a transversal trough into an upright one that causes three large alterations of mid-high atmospheric circulation respectively in the early and middle ten days of this month. Then, the amplitude of long waves soon magnifies, leading to the unusual intrusion of cold air to low-latitude areas in the mid-summer. Meanwhile, the warm and humid southwest summer monsoon is quite active. The strong interactions of cold air and summer monsoon over Southwest China result in the large-scale convective rainfalls on the southern side of cold air. With regard to the activities of cold air, it can influence rainfalls in three prominent ways. Firstly, the incursion of upper-level cold air is often accompanied by partial southerly upper-level jet. The ascending branch of the corresponding secondary circulation, which is on the left front side of the jet center, provides the favorite dynamic upward motion for the rainfalls. Secondly, the southward movement of cold air contributes to the establishment of atmospheric baroclinic structure, which would lead to baroclinic disturbances. The atmospheric disturbances associated with the intrusion of cold air can destroy the potential instability stratification, release the convective available potential energy （CAPE） and finally cause convective activities. In addition, the advection processes of dry and cold air at the upper level along with the advection of humid and warm air at the lower level are rather significant for the reestablishment of potential instability in the precipitation area, which is one of the crucial factors contributing to persistent rainfalls.

Sensitivity of Lake-Effect Convection to the Lake Surface Temperature over Poyang Lake in China

In this study, high-resolution weather research and forecasting(WRF) simulations are used to explore the sensitivity of lake-effect convection over Poyang Lake(PL) to the change of lake surface temperature(LST). A control experiment(CTR) with climate mean LST(303 K) is compared with six sensitivity experiments(CTR-1/2/3K and CTR+1/2/3K) in which the LSTs are set based on the mean LST difference of 6 K between the maximum and minimum. The results show that the CTR experiment reasonably reproduces the lake-effect convection, and the lake-effect convection in sensitivity experiments is significantly influenced by the LST. With the increase of LST, the initiation time of the lake-effect convection is advanced gradually, while the initiation location moves PL from its shore.The lake-effect convection strengthens(weakens) in the increase-temperature CTR+1/2/3K(decrease-temperature CTR-1/2/3K) experiments, but the lake-effect convection does not monotonically strengthen with the LST, for the strongest one occurring in the CTR+1K experiment. The corresponding diagnostic analysis shows that the upward sensible heat flux and latent heat flux over PL increase with the LST, resulting in the enhancement of the lake-land breeze and the enlargement of the convective available potential energy(CAPE). This is the main reason for the changes in the initiation time and location, as well as the intensity of lake-effect convection in different experiments.In addition, the non-monotonous variation of the level of free convection, which is mainly induced by the non-monotonous variation of the lifting condensation level, is responsible for the non-monotonous variation of the lake-effect convection intensity with the LST.