Pre-summer Heavy Rainfall Research Articles

Roles of Synoptic to Quasi-Monthly Disturbances in Generating Two Pre-Summer Heavy Rainfall Episodes over South China

In this study, power spectral analysis and bandpass filtering of daily meteorological fields are performed to explore the roles of synoptic to quasi-monthly disturbances in influencing the generation of pre-summer heavy rainfall over South China. Two heavy rainfall episodes are selected during the months of April−June 2008−15, which represent the collaboration between the synoptic and quasi-biweekly disturbances and the synoptic and quasi-monthly disturbances, respectively. Results show that the first heavy rainfall episode takes place in a southwesterly anomalous flow associated with an anticyclonic anomaly over the South China Sea (SCS) at the quasi-biweekly scale with 15.1% variance contributions, and at the synoptic scale in a convergence zone between southwesterly and northeasterly anomalous flows associated with a southeastward-moving anticyclonic anomaly on the leeside of the Yungui Plateau and an eastward-propagating anticyclonic anomaly from higher latitudes with 35.2% variance contribution. In contrast, the second heavy rainfall episode takes place in southwest-to-westerly anomalies converging with northwest-to-westerly anomalies at the quasi-monthly scale with 23.2% variance contributions to the total rainfall variance, which are associated with an anticyclonic anomaly over the SCS and an eastward-propagating cyclonic anomaly over North China, respectively. At the synoptic scale, it occurs in south-to-southwesterly anomalies converging with a cyclonic anomaly on the downstream of the Yungui Plateau with 49.3% variance contributions. In both cases, the lower-tropospheric mean south-to-southwesterly flows provide ample moisture supply and potentially unstable conditions; it is the above synoptic, quasi-biweekly or quasi-monthly disturbances that determine the general period and distribution of persistent heavy rainfall over South China.

Effects of doubled carbon dioxide on rainfall responses to radiative processes of water clouds

The effects of doubled carbon dioxide on rainfall responses to radiative processes of water clouds are investigated in this study. Two groups of two-dimensional cloud-resolving model sensitivity experiments with regard to pre-summer heavy rainfall around the summer solstice and tropical rainfall around the winter solstice are conducted and their five-day averages over the model domain are analyzed. In the presence of radiative effects of ice clouds, doubled carbon dioxide changes pre-summer rainfall from the decrease associated with the enhanced atmospheric cooling to the increase associated with the enhanced infrared cooling as a result of the exclusion of radiative effects of water clouds. Doubled carbon dioxide leads to the reduction in tropical rainfall, caused by the removal of radiative effects of water clouds through the suppressed infrared cooling. In the absence of radiative effects of ice clouds, doubled carbon dioxide changes pre-summer rainfall from the increase associated with the strengthened atmospheric warming to the decrease associated with the weakened release of latent heat caused by the elimination of radiative effects of water clouds. The exclusion of radiative effects of water clouds increases tropical rainfall through the strengthened infrared cooling, which is insensitive to the change in carbon dioxide.

Effects of doubled carbon dioxide on rainfall responses to large-scale forcing: A two-dimensional cloud-resolving modeling study

Rainfall responses to doubled atmospheric carbon dioxide concentration were investigated through the analysis of two pairs of two-dimensional cloud-resolving model sensitivity experiments. One pair of experiments simulated pre-summer heavy rainfall over southern China around the summer solstice, whereas the other pair of experiments simulated tropical rainfall around the winter solstice. The analysis of the time and model domain mean heat budget revealed that the enhanced local atmospheric warming was associated with doubled carbon dioxide through the weakened infrared radiative cooling during the summer solstice. The weakened mean pre-summer rainfall corresponded to the weakened mean infrared radiative cooling. Doubled carbon dioxide increased the mean tropical atmospheric warming via the enhanced mean latent heat in correspondence with the strengthened mean infrared radiative cooling during the winter solstice. The enhanced mean tropical rainfall was associated with the increased mean latent heat.

Radiative Effects of Water Clouds on Heat, Cloud Microphysical and Surface Rainfall Budgets Associated with Pre-Summer Torrential Rainfall

This study investigates thermal, cloud microphysical and surface-rainfall responses to the radiative effects of water clouds by analyzing two pairs of two-dimensional cloud-resolving model sensitivity experiments of a pre-summer heavy rainfall event. In the presence of the radiative effects of ice clouds, exclusion of the radiative effects of water clouds reduces the model domain mean rain rate through the mean hydrometeor increase, which is associated with the decreases in the melting of graupel and cloud ice caused by enhanced local atmospheric cooling. In the absence of the radiative effects of ice clouds, removal of the radiative effects of water clouds increases model domain mean rain rate via the enhancements in the mean net condensation and the mean hydrometeor loss. The enhanced mean net condensation and increased mean latent heat are related to the strengthened mean infrared radiative cooling in the lower troposphere. The increased mean hydrometeor loss associated with the reduction in the melting of graupel is caused by the enhanced local atmospheric cooling.

A modified scheme that parameterizes depositional growth of ice crystal: A modeling study of pre-summer torrential rainfall case over Southern China

Depositional growth of cloud ice is estimated and its parameterization schemes are compared through the two-dimensional cloud-resolving modeling analysis of pre-summer heavy rainfall over southern China. Hsie et al. (1980) and Krueger et al. (1995) developed parameterization schemes to calculate depositional growth of cloud ice by estimating the growth timescale under the assumption that the ice crystal concentration is independent of crystal size. A new scheme is proposed by Zeng et al. (2008) under the assumption that the ice crystal concentration is proportional to the mass of ice crystal. Hsie's and Krueger's schemes produce small amount of cloud ice similar to what Zeng's scheme with low ice crystal concentration does. When ice crystal concentration is increased to a high value in Zeng's scheme, the simulation generates anomalous depositional growth of cloud ice and thus anomalous area expansion of stratiform rainfall. Zeng's scheme is modified by changing radius of base ice crystal from 0 to 40μm in the calculation of depositional growth of cloud ice. The modified scheme with high ice crystal concentration greatly reduces growth of cloud ice and thus fractional coverage of stratiform rainfall.

Effects of Vertical Wind Shear on the Pre-Summer Heavy Rainfall Budget: A Cloud-Resolving Modeling Study

This study investigates the effects of vertical wind shear on the torrential rainfall response to the large-scale forcing using a rainfall separation analysis of a pair of two-dimensional cloud-res...

Torrential rainfall responses to ice microphysical processes during pre-summer heavy rainfall over southern China

In this study, the effects of key ice microphysical processes on the pre-summer heavy rainfall over southern China during 3–8 June 2008 were investigated. A series of two-dimensional sensitivity cloud-resolving model simulations were forced with zonally uniform vertical velocity, zonal wind, horizontal temperature, and water vapor advection data from the National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS). The effects of key ice microphysical processes on the responses of rainfall to large-scale forcing were analyzed by comparing two sensitivity experiments with a control experiment. In one sensitivity experiment, ice crystal radius, associated with depositional growth of snow from cloud ice, was reduced from 100 µm in the control experiment to 50 µm, and in the other sensitivity experiment the efficiency of the growth of graupel from the accretion of snow was reduced to 50% from 100% in the control experiment. The results show that the domain-mean rainfall responses to these ice microphysical processes are stronger during the decay phase than during the onset and mature phases. During the decay phase, the increased mean rain rate resulting from the decrease in ice crystal radius is associated with the enhanced mean local atmospheric drying, the increased mean local hydrometeor loss, and the suppressed mean water vapor divergence. The increased mean rain rate caused by the reduction in accretion efficiency is related to the reduced mean water vapor divergence and the enhanced mean local hydrometeor loss.

Effects of large-scale forcing and ice clouds on pre-summer heavy rainfall over southern China in June 2008: A partitioning analysis based on surface rainfall budget

The effects of large-scale forcing and ice clouds on rainfall are studied by analyzing a series of sensitivity experiments of a pre-summer heavy rainfall event over southern China from 3 to 8 June 2008. The analysis is conducted categorizing grid-scale rainfall simulation data into eight rainfall types based on different rainfall processes. The rainfall with water vapor divergence and local atmospheric drying and hydrometeor loss/convergence (TfM) has the largest contribution to total rainfall during the development of rainfall when the imposed large-scale forcing produces water vapor convergence. TfM and the rainfall with water vapor convergence and local atmospheric moistening and hydrometeor loss/convergence (tFM) have the largest contributors to total rainfall during the decay phase when the imposed large-scale forcing generates water vapor divergence. TfM becomes the largest contributor during the decay phase when the exclusion of ice clouds decreases tFM and increases TfM. The removal of ice clouds enhances the contribution of the rainfall with water vapor convergence to total rainfall during the life span of the rainfall event.

Effects of vertical wind shear and cloud radiative processes on responses of rainfall to the large-scale forcing during pre-summer heavy rainfall over southern China

The pre-summer heavy rainfall over southern China during 3–8 June 2008 is simulated using a two-dimensional cloud-resolving model. The model is integrated with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature and vapour advection from National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) data. The effects of vertical wind shear and cloud radiative processes on the response of rainfall to large-scale forcing are analysed through the comparison of two sensitivity experiments with the control experiment. One sensitivity experiment excludes the large-scale vertical wind shear and the other excludes the cloud radiative effects. During the decay phase of convection, the increase in model domain-mean surface rain-rate resulting from the exclusion of vertical wind shear is associated with the slowdown in the decrease of perturbation kinetic energy due to the exclusion of barotropic conversion from mean kinetic energy to perturbation kinetic energy. The increase in domain-mean rain-rate from the exclusion of cloud radiative effects is related to the enhancement of condensation and associated latent heat as a result of strengthened radiative cooling. The increase in the domain-mean surface rain-rate is mainly associated with the increase of convective rainfall, which is in turn related to the local atmospheric change from moistening to drying. During the onset and mature phases of convection, the domain-mean surface rain-rates are generally insensitive to vertical wind shear and cloud radiative effects whereas convective and stratiform rain-rates are sensitive to vertical wind shear and cloud radiative effects. The decrease in convective rain-rate and the increase in stratiform rain-rate are primarily associated with the enhanced transport of hydrometeor concentration from convective regions to raining stratiform regions. Copyright © 2011 Royal Meteorological Society

Radiative effects of water clouds on rainfall responses to the large-scale forcing during pre-summer heavy rainfall over Southern China

Water clouds are an essential ingredient of precipitation systems and their radiative effects may significantly impact rainfall processes. This study investigates radiative effects of water clouds on rainfall using a two-dimensional cloud-resolving model simulation of a pre-summer heavy rainfall event over southern China from 3 to 8 June 2008, forced with large-scale data from NCEP/GDAS. During the onset phase, the exclusion of radiative effects of water clouds increased the mean rain rate through the mean hydrometeor change from loss to gain in the presence of radiative effects of ice clouds, whereas it increased the mean rain rate through the increases in the mean net condensation and mean hydrometeor loss in the absence of radiative effects of ice clouds. During the development phase, the removal of radiative effects of water clouds enhanced the mean rainfall through the mean hydrometeor change from gain to loss. During the mature phase, the elimination of radiative effects of water clouds weakened the mean rainfall through the decreased mean net condensation. The exclusion of radiative effects of ice clouds enhanced the decrease in the mean rainfall. During the decay phase, the removal of radiative effects of water clouds increased the mean rainfall through the enhanced net condensation.

Microphysical and radiative effects of ice clouds on responses of rainfall to the large-scale forcing during pre-summer heavy rainfall over southern China

Ice clouds are an important part of precipitation systems and their microphysical and radiative effects may significantly impact rainfall simulations. This study investigates these effects using a two-dimensional cloud-resolving model simulation of a pre-summer heavy rainfall event over southern China from 3–8 June 2008, forced with NCEP/GDAS data. The microphysical and radiative effects of ice clouds on rainfall responses to the large-scale forcing during pre-summer heavy rainfall over southern China are analyzed by comparing the control experiment and two sensitivity experiments with the exclusion of ice radiative effects and with the total exclusion of ice microphysics (the exclusion of both ice radiative and microphysical effects), respectively. During the development phase, the total exclusion of ice microphysics decreased the model domain mean surface rain rate primarily by suppressing the convective rain rate through the exclusion of ice radiative effects on 4 June 2008 and by reducing the stratiform rain rate through the exclusion of ice microphysical effects on 5 June. During the peak phase on 6 June, the mean rain rate significantly increased from the previous day. The reduction in the mean rain rate by the total exclusion of ice microphysics continued from the previous day due to the continuous decrease in the stratiform rain rate caused by the exclusion of ice microphysical effects. During the decay phase on 7 June, the total exclusion of ice microphysics increased the mean rain rate by enhancing convective rainfall through the exclusion of ice microphysical effects. These results indicate that the microphysical and radiative effects of ice clouds play equally important roles in pre-summer heavy rainfall events.