Diagnostic Study of Apparent Heat Sources and Moisture Sinks in the South China Sea and its Adjacent Areas during the Onset of 1998 SCS Monsoon

The apparent heat sources (?Q1 ?) and moisture sinks (?Q2 ?) are calculated based on the reanalyzed data of the South China Sea Monsoon Experiment (SCSMEX) from May 1 to August 31, 1998. It is found that the formation and distribution of the atmospheric heat sources are important for the monsoon onset. The earlier onset of the SCS monsoon is the result of enduring atmospheric heating in the Indo–China Peninsula and South China areas. The atmospheric heating firstly appears in the Indo–China Peninsula area and the sensible heat is the major one. The 30–50 day periodic oscillation of atmospheric heat sources between the SCS area and the western Pacific warm pool has a reverse phase distribution before the middle of July and the low frequency oscillation of heat sources in SCS area has an obvious longitudinal propagation. The 30–50 day low frequency oscillation has vital modificatory effects on the summer monsoon evolution during 1998. Key words Apparent heat sources - Apparent moisture sinks - The South China Sea monsoon - Diagnostic Study Sponsored by the National Key Project of Fundamental Research “ SCSMEX” and the Research Fund for the Doctoral Program of Higher Education: “ Study of the Air-sea Interaction in the SCS Monsoon Region”.

Analysis of Forecast Failure of a Regional Rainstorm in Qinghai

Based on the ground observation, ERA5 and other data, the regional rainstorm that occurred in northeastern Qinghai on the night of August 28, 2020 was analyzed. The results show that this precipitation occurred in the climate background of relatively high temperature, high humidity and extreme low pressure, and the precipitation process was divided into warm-area precipitation before the front and frontal precipitation, among which the warm-area precipitation was dominant, and it was a regional warm-area rainstorm. The global models, mesoscale models and forecasters as important operational reference all failed to make effective forecasts or prompts for the warm-area precipitation before the front in advance(24 or 12 h), the predicted precipitation was obviously small, and the predicted frontal precipitation by the models were obviously large. The western low-level meso-β-scale wind direction convergence system moving eastwards encountered the high-humidity area at the front of the meso-γ-scale wind speed convergence system to trigger this regional warm-area rainstorm. From the analysis of the mesoscale convergent system based on the vorticity budget equation, it is found that different terms played different roles in the process of warm-area rainstorm. The advective term dominated before the appearance of precipitation, which was favorable for the generation of mesoscale eddies. During the precipitation period, the torsion term and the convergence term were dominant. The torsion term was beneficial to the conversion of horizontal vorticity to vertical vorticity and the enhancement of precipitation intensity. Its maximum was generated 1-2 h earlier than the heavy precipitation. In the later period of precipitation, the convergence term was dominant, which was beneficial to the maintenance of precipitation. In the early stage of precipitation, the apparent heat source was located behind the apparent water vapor sink, which was conducive to the increase in the thickness of the heating column, and the precipitation intensity gradually increased. During the occurrence of heavy precipitation, the apparent heat source and the apparent water vapor sink basically coincided, and the latent heat released by condensation strengthened the upward movement, so that precipitation intensity increased. In this process, the water vapor mainly came from the southeast of the plateau(southwest airflow), followed by the plateau slope area(southeast airflow). During this regional warm-area rain that was wrongly predicted, the extreme minimum pressure, the torsional term in the vorticity budget equation and the abnormal water vapor transport have certain indications for the warm-area rainstorm.

Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July 2021 Extreme Heavy Rainfall in Henan,Central China

In this paper,synoptic-scale analyses of frontogenesis,moisture budget,and tropospheric diabatic heating are performed to reveal the development and maintenance mechanisms for the extreme heavy rainfall in Henan Province of central China from 19 to 21 July 2021,based on station observations and the ECMWF Reanalysis version 5(ERA5)data.The results demonstrate that owing to the blocking effect of local topography,low-level wind convergence in Henan appeared underneath high-level divergence,conducive to development and maintenance of a midtropospheric low-pressure system saddled by the Asian continental high and the western Pacific subtropical high(WPSH),during the extreme heavy rainfall.In the lower troposphere,frontogenesis occurred in the θ_(se) intensive region,as a result of the divergence and horizontal deformation(which play equally important roles),generating frontal secondary circulation with strong vertical motion favorable to heavy rainfall.Moisture budget analysis reveals that 1)with the continuous strengthening of the easterly wind from the north side of Typhoon In-Fa(2106),strong wind shear and orographic uplift led to abnormally strong convergence of water vapor flux in the boundary layer in Henan;2)there occurred extremely strong net inflow of moisture in the boundary layer from the east.Horizontally,both the apparent heat source and the moisture sink coincided with the area of heavy rainfall;vertically,however,Q_(1)exhibited a single peak with the heating center in the middle and upper troposphere,while large Q_(2)values evenly resided over 850–400 hPa;and Q_(1)(Q_(2))was dominated by vertical(horizontal)transport of potential temperature(moisture).These indicate that the latent heat release from condensation of initial heavy rainfall provided a positive feedback,leading to increasingly heavy precipitation.All these synoptic settings sustained the extreme rainfall process.

NATURE OF PRECIPITATION AND ACTIVITY OF CUMULUS CONVECTION DURING THE 1991 MEIYU SEASON OF CHANGJIANG-HUAIHE RIVER BASIN

Seasonal variability regarding the nature of precipitation and the activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River Basin(Jianghuai)has been investigated by calculating apparent heat source/apparent moisture sink and analyzing TBB(cloud-top blackbody radiation temperature)data.It is found that three periods of strong ascending motion during the Meiyu season lead to three episodes of heavy rain,and the latent heat due to the precipitation is of the sole heat source of the atmosphere.The nature of precipitation shows distinct seasonal variability,from frontal precipitation of the first episode to the extremely strong convective precipitation of the third episode.TBB field of East Asia may well reflect not only the intensity of convection and rainfall,but also the movement of rain belt and convection belt.In the whole Meiyu season.convection belt mainly stays in Jianghuai.but may shift within the domain of East Asia.Its locating in Jianghuai or not determines the maintenance or break of Meiyu.In the third episode,the narrow convection belt over Jianghuai is mainly caused by southwest monsoon which takes moist and convective atmosphere from tropical ocean.

A Study on the Mesoscale Convective Systems during the Summer Monsoon Onset over the South China Sea in 1998 Part II: Effect of Mesoscale Convective Systems on Large-Scale Fields

The apparent heat sources and apparent moisture sinks, and large-scale wind,temperature as well as the surface pressure fields during the summer monsoon onset over the northernSouth China Sea (SCS) in 1998 were diagnosed. The results suggested that there was a kind ofpositive feedback mechanism between large-scale circulations and mesoscale convective systems(MCSs). Before the monsoon onset, the large-scale background provided favorable synoptic and dynamicconditions for the summer monsoon onset and the formation of mesoscale convective activities,whereas after the summer monsoon onset, occurrence of the persistent and extensive mesoscaleconvective activities produced obvious feedback effect on large-scale circulations. Because of therelease of latent heating produced by enhanced convective activities, the intense atmosphericheating appeared over the northern SCS, which resulted in: (1) the meridional temperature gradientover the SCS reversed from upper-level to low-level and then the large-scale circulations werechanged seasonally; (2) correspondingly, the surface pressure over the northern SCS deepenedcontinually and formed a broad monsoon trough and the obvious pressure-fall areas, thus making thesubtropical high move out of the SCS eventually; (3) with the development of the low pressurecirculations in the middle and low troposphere, the MCSs further enhanced and extended southward,which was conducive to the SCS monsoon onset and maintenance over the middle and southern SCS; and(4) the deepening of monsoon trough facilitated the monsoon flow and moisture transport on itssouthern side, thus the monsoon onset reaching peak period.