Xiaopeng Cui

Surface rainfall processes as simulated in a cloud‐resolving model

[1]xa0Surface rain rate can be simply formulated with the sum of moisture and cloud sources/sinks. In this study the moisture sink comprises the local moisture change, moisture convergence (with an imposed vertical velocity), and surface evaporation, whereas the cloud source/sink comprises the local hydrometeor change since the cyclic boundary condition leads to zero hydrometeor convergence. The sources/sinks and their contributions to the surface rain rate are examined based on hourly zonal mean simulation data from a two-dimensional cloud-resolving model. The model is forced by the large-scale vertical velocity, zonal wind, and horizontal advections obtained from Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Although variation in the moisture sink largely accounts for much of the variation in the surface rain rate, the cloud source/sink may modify the surface rain rate significantly. The magnitude of the cloud source/sink increases when the zonal mean surface rain rate increases from 0 to 1 mm h−1, and it decreases when the rain rate increases from 1 to 2 mm h−1. The cloud source/sink is further analyzed by breaking it into ice and water hydrometeors. The ice hydrometeors may account for more contributions to the cloud variations than the water hydrometeors, and their growth may lead the surface rain rate by 1–2 hours.

Role of surface evaporation in surface rainfall processes

[1]xa0The roles of surface evaporation in tropical surface rainfall processes in rainfall regions (raining stratiform and convective regions) and rainfall-free regions (nonraining stratiform and clear-sky regions) are investigated on the basis of the data from a series of two-dimensional cloud-resolving simulations. The model is integrated for 21 days with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature, and vapor advection, as well as sea surface temperature from the Tropical Ocean–Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). The model is also integrated to equilibrium states for 40 days with imposed zero vertical velocity, constant zonal wind, and sea surface temperatures of 31°C and 29°C for two separate experiments. The time- and zonal-mean surface evaporation mainly comes from rainfall-free regions, where the surface evaporation is largely balanced by the vapor divergence associated with the subsidence. In rainfall regions the vapor convergence determines the convective rainfall while the surface evaporation plays a negligible role. Thus surface evaporation pumps water vapor into rainfall-free regions, and the divergence transports the vapor from rainfall-free regions to rainfall regions, which supports the rainfall. Imposed forcing and sea surface temperature do not change the role of surface evaporation in rainfall processes.

A modeling study of moist and dynamic vorticity vectors associated with two‐dimensional tropical convection

[1]xa0Moist ( × ∇qv/ρ, MVV) and dynamic ( × /ρ, DVV) vorticity vectors are introduced to study 2-D tropical convection with 2-D cloud-resolving simulation data. The cloud model is forced by vertical velocity, zonal wind, horizontal advection, and sea surface temperature data obtained from TOGA COARE and is integrated for a selected 10-day period. The MVV and DVV have zonal and vertical components in the 2-D x-z frame. Analysis of zonally averaged and mass-integrated quantities shows that the vertical (zonal) component of the MVV and the sum of the cloud hydrometeor mixing ratios are in phase with a correlation coefficient of 0.78 (0.32), and the vertical (zonal) component of the DVV and the sum of the mixing ratios are in (out of) phase with a correlation coefficient of 0.52 (−0.62), indicating that the vertical component of the MVV and both zonal and vertical component of the DVV are closely associated with tropical convection. The tendency equations for the MVV and DVV are derived, and the zonally averaged and mass-integrated tendency budgets are analyzed. The tendency of the vertical component of the MVV is mainly determined by the interaction between the vorticity and the zonal gradient of condensational/depositional heating. The tendency of the zonal component of the DVV is controlled by the interaction between the vorticity, buoyancy, and vertical pressure gradient, whereas the tendency of the vertical component is determined by the interaction between the vorticity and zonal pressure gradient.

A cloud-resolving modeling study of diurnal variations of tropical convective and stratiform rainfall

[1]xa0Diurnal variations of convective and stratiform rainfalls over the tropical ocean are investigated on the basis of data from a two-dimensional cloud-resolving simulation. The model is integrated for 40 days to a quasi-equilibrium state with imposed zero zonally uniform vertical velocity, a constant zonal wind, and a constant sea surface temperature of 29°C. The convective rain rate contributes more to the diurnal variation of model domain-mean surface rain rate than the stratiform rain rate does. The calculations of the surface rainfall equation show that more vapor convergence yields higher convective rain rate and that more vapor convergence and more local vapor loss cause higher stratiform rain rate in early morning than in afternoon. Analysis of cloud microphysics budgets reveals that variations in vapor condensation and associated water microphysical processes are responsible for the diurnal variations of both convective and stratiform rain rates.

Moisture Sources of Torrential Rainfall Events in the Sichuan Basin of China during Summers of 2009–13

AbstractWater vapor sources and transport paths associated with torrential rains are very important to research and forecasts. This study investigates the main moisture sources and transport paths related to torrential rainfall events in the Sichuan basin of China, which is located east of the Tibetan Plateau, using a Lagrangian flexible particle dispersion model (FLEXPART). Based on the analysis of the torrential rainfall distribution during 2009–13, four study areas are selected in the basin. Particles that have a great contribution to the torrential rainfall events within the four study areas are traced back for 10 days, and a quantitative analysis of the contributions from various moisture sources to the torrential rainfall events is also conducted. The results indicate that a large number of target particles start at the Arabian Sea and the Bay of Bengal, land on the Indo-China Peninsula, and finally reach the study areas. This is an important moisture transport path for the torrential rainfall event...

Dominant Cloud Microphysical Processes of a Torrential Rainfall Event in Sichuan, China

High-resolution numerical simulation data of a rainstorm triggering debris flow in Sichuan Province of China simulated by the Weather Research and Forecasting (WRF) Model were used to study the dominant cloud microphysical processes of the torrential rainfall. The results showed that: (1) In the strong precipitation period, particle sizes of all hydrometeors increased, and mean-mass diameters of graupel increased the most significantly, as compared with those in the weak precipitation period; (2) The terminal velocity of raindrops was the strongest among all hydrometeors, followed by graupel’s, which was much smaller than that of raindrops. Differences between various hydrometeors’ terminal velocities in the strong precipitation period were larger than those in the weak precipitation period, which favored relative motion, collection interaction and transformation between the particles. Absolute terminal velocity values of raindrops and graupel were significantly greater than those of air upward velocity, and the stronger the precipitation was, the greater the differences between them were; (3) The orders of magnitudes of the various hydrometeors’ sources and sinks in the strong precipitation period were larger than those in the weak precipitation period, causing a difference in the intensity of precipitation. Water vapor, cloud water, raindrops, graupel and their exchange processes played a major role in the production of the torrential rainfall, and there were two main processes via which raindrops were generated: abundant water vapor condensed into cloud water and, on the one hand, accretion of cloud water by rain water formed rain water, while on the other hand, accretion of cloud water by graupel formed graupel, and then the melting of graupel formed rain water.

SAFIR-3000 Lightning Statistics over the Beijing Metropolitan Region during 2005–07

AbstractIn this study, the spatiotemporal characteristics of cloud-to-ground (CG) and intracloud (IC) lightning flashes observed by Surveillance et Alerte Foudre par Interferometrie Radioelectrique (SAFIR)-3000 over the Beijing metropolitan region (BMR) during 2005–07 were investigated. The results showed the presence of 299 lightning days with 241 688 flashes, most of which were IC lightning flashes. Only 19% of the total flashes were CG lightning flashes; 14% of these CG flashes were positive. Most lightning activity occurred during the summer months (June–August), with a major diurnal peak around 1900 Beijing standard time (BST) and a secondary peak around 2300 BST. Spatial variations in flash density and lightning days both exhibited an obvious southeastwardly increasing pattern, with higher flash densities or more lightning days occurring in the southeastern plains and lower values distributed on the northwestern mountains. The Z ratio (IC/CG lightning flashes) exhibited a similar spatial pattern, bu...

Cloud microphysical differences with precipitation intensity in a torrential rainfall event in Sichuan, China

Abstract High-resolution data of a torrential rainfall event in Sichuan, China, simulated by the WRF model, were used to analyze the cloud microphysical differences with precipitation intensity. Six-hourly accumulated rainfall was classified into five bins based on rainfall intensity, and the cloud microphysical characteristics and processes in different bins were studied. The results show that: (1) Hydrometeor content differed distinctly among different bins. Mixing ratios of cloud water, rain water, and graupel enhanced significantly and monotonously with increasing rainfall intensity. With increasing precipitation intensity, the monotonous increase in cloud water number concentration was significant. Meanwhile, number concentrations of rain water and graupel increased at first and then decreased or increased slowly in larger rainfall bins. (2) With precipitation intensity increasing, cloud microphysical conversion processes closely related to the production of rainwater, directly (accretion of cloud water by rain (QCLcr) and melting of graupel (QMLgr)) or indirectly (water vapor condensation and accretion of cloud water by graupel), increased significantly. (3) As the two main sources of rainwater, QCLcr increased monotonously with increasing precipitation intensity, while QMLgr increased slowly, even tending to cease increasing in larger rainfall bins.

On the Initiation of an Isolated Heavy-Rain-Producing Storm near the Central Urban Area of Beijing Metropolitan Region

AbstractAn isolated heavy-rain-producing thunderstorm was unexpectedly initiated in the afternoon of 9 August 2011 near the central urban area of the Beijing metropolitan region (BMR), which occurred at some distance from BMR’s northwestern mountains and two preexisting mesoscale convective systems (MCSs) to the west and north, respectively. An observational analysis shows the presence of unfavorable quasigeostrophic conditions but a favorable regional environment for the convective initiation (CI) of thunderstorms. A nested-grid cloud-resolving model simulation of the case with the finest 1.333-km resolution is performed to examine the CI of the thunderstorm and its subsequent growth. Results reveal that the growth of the mixed boundary layer, enhanced by the urban heat island (UHI) effects, accounts for the formation of a thin layer of clouds at the boundary layer top at the CI site and nearby locations as well as on the upslope sides of the mountains. It takes about 36 min for the latent-heating-driven...

Kinetic Energy Budget during the Genesis Period of Tropical Cyclone Durian (2001) in the South China Sea

AbstractA set of kinetic energy (KE) budget equations associated with four horizontal flow components was derived to study the KE characteristics during the genesis of Tropical Cyclone (TC) Durian (2001) in the South China Sea using numerical simulation data. The genesis process was divided into three stages: the monsoon trough stage (stage 1), the midlevel mesoscale convective vortex (MCV) stage (stage 2), and the establishment stage of the TC vortex (stage 3). Analysis showed that the KE of the symmetric rotational flow (SRF) was the largest and kept increasing, especially in stages 2 and 3, representing the symmetrization process during TC genesis. The KE of the SRF was mainly converted from the KE of the symmetric divergent flow (SDF), largely transformed from the available potential energy (APE). It was found that vortical hot towers (VHTs) emerged abundantly, aggregated, and merged within the MCV region in stages 1 and 2. From the energy budget perspective, massive moist-convection-produced latent h...