Surface Rain Rate Research Articles

A Wind and Rain Backscatter Model Derived From AMSR and SeaWinds Data

The SeaWinds scatterometer was originally designed to measure wind vectors over the ocean by exploiting the relationship between wind-induced surface roughening and the normalized radar backscatter cross section. Rain can degrade scatterometer wind estimation; however, the simultaneous wind/rain (SWR) algorithm was developed to enable SeaWinds to simultaneously retrieve wind and rain rate data. This algorithm is based on colocating data from the Precipitation Radar on the Tropical Rainfall Measuring Mission and SeaWinds on QuikSCAT. This paper develops a new wind and rain radar backscatter model for SWR using colocated data from the Advanced Microwave Scanning Radiometer (AMSR) and SeaWinds aboard the Advanced Earth Observing Satellite II. This paper accounts for rain height in the model in order to calculate surface rain rate from the integrated rain rate. The performance of SWR using the new wind/rain model is measured by comparison of wind vectors and rain rates to the previous SWR algorithm, AMSR rain rates, and National Center for Environmental Prediction numerical weather prediction winds. The new SWR algorithm produces more accurate rain estimates and improved winds, and detects rain with a low false alarm rate.

Diurnal phase of late-night against late-afternoon of stratiform and convective precipitation in summer southern contiguous China

Using the tropical rainfall measuring mission (TRMM) Precipitation Radar (PR) observations combined with the surface rain gauge data during 1998-2006, the robust diurnal features of summer stratiform and convective precipitation over the southern contiguous China are revealed by exploring the diurnal variations of rain rate and precipitation profile. The precipitation over the southern contiguous China exhibits two distinguishing diurnal phases: late-night (2200-0600 LST) and late-afternoon (1400-2200 LST), dependent on the location, precipitation type and duration time. Generally, the maximum rain rate and the highest profile of stratiform precipitation occur in the late-afternoon (late-night) over the southeastern (southwestern) China, while most of the stratiform short-duration rain rate tends to present late-afternoon peaks over the southern China. For convective precipitation, the maximum rain rate and the highest profile occur in the late-afternoon over most of the southern contiguous China, while the convective long-duration rain rate exhibits late-night peaks over the southwestern China. Without regional dependence, the convective precipitation exhibits much larger amplitude of diurnal variations in both near surface rain rate and vertical extension compared with stratiform precipitation and the convective rain top rises most rapidly between noon and afternoon. However, there are two distinctive sub-regions. The diurnal phases of precipitation there are very weakly dependent on precipitation type and duration time. Over the eastern periphery of the Tibetan Plateau, the maximum rain rate and the highest profile of either convective or stratiform precipitation occur in the late-night. Over the southeastern coastal regions, both the near surface rain rate and rain top of convective and stratiform precipitation peak in the late-afternoon.

Cloud microphysical and rainfall responses to zonal perturbations of sea surface temperature: A cloud-resolving modeling study

The cloud microphysical and rainfall responses to zonal perturbations of sea surface temperature (SST) are investigated by analyzing the equilibrium simulation data (from day 31–40) obtained from a series of two-dimensional cloud-resolving simulations with a zonal model domain of 768 km. Four experiments imposed by zonal SST perturbations of wavenumbers 1 (SST29Z1), 2 (SST29Z2), 4 (SST29Z4), and 8 (SST29Z8) are compared to the control experiment imposed by zonally uniform SST (SST29). The model domain mean SST is 29 °C, and the two-dimensional cloud-resolving model with a cyclic lateral boundary is also imposed by zero vertical velocity and constant zonal wind. The time and model domain mean surface rain rates in SST29Z1, SST29Z2, and SST29Z8 are about 10% larger than those in SST29, whereas the mean surface rain rates in SST29Z4 and SST29 are similar. The analysis of mean surface rainfall budgets shows that local water vapor and hydrometeor changes play important roles in determining the differences and similarities in mean surface rain rate between the perturbation experiments and the control experiment. Both convective and stratiform rain rates are larger in SST29Z1 and SST29Z2 than in SST29 due to the smaller advection of rain from convective regions into raining stratiform regions and the larger vapor condensation rates associated with the larger water vapor convergence over raining stratiform regions in SST29Z1 and SST29Z2. The convective rain rates are larger in SST29Z4 and SST29Z8 than in SST29 because of the larger condensation rates associated with the larger water vapor convergence over convective regions in SST29Z4 and SST29Z8. The stratiform rain rates in SST29Z4 and SST29Z8 are smaller than in SST29 due to the smaller vapor condensation rates and smaller collection rates of cloud water by rain over raining stratiform regions in SST29Z4 and SST29Z8.

Theoretical model of rainfall in tropical cyclones for the assessment of long‐term risk

We propose a theoretical model to evaluate the rainfall intensity field due to large‐scale horizontal wind convergence in tropical cyclones (TCs). The model is intended as one component of a methodology to assess the risk of extreme rainfall intensities from TCs. The other components are a recurrence relation for the model parameters and track and a statistical representation of the deviations of rainfall intensity from model predictions. The latter are primarily caused by rainbands and local convective activity and are the focus of an upcoming communication. The vertical flux of moisture and the associated surface rain rate are calculated using basic thermodynamics and a simple numerical model for the vertical winds inside the TC boundary layer. The tropical cyclone is characterized by the radial profile of the tangential wind speed at gradient level, the storm translation velocity, the surface drag coefficient, and the average temperature and saturation ratio inside the TC boundary layer. A parametric analysis shows the sensitivity of the symmetric and asymmetric components of the rainfall field to various storm characteristics.

Virtual Field Campaigns on Deep Tropical Convection in Climate Models

Abstract High-resolution time–height data over warm tropical oceans are examined, from three global atmosphere models [GFDL’s Atmosphere Model 2 (AM2), NCAR’s Community Atmosphere Model, version 3 (CAM3), and a NASA Global Modeling and Assimilation Office (GMAO) model], field campaign observations, and observation-driven cloud model outputs. The character of rain events is shown in data samples and summarized in lagged regressions versus surface rain rate. The CAM3 humidity and cloud exhibit little vertical coherence among three distinct layers, and its rain events have a short characteristic time, reflecting the convection scheme’s penetrative nature and its closure’s concentrated sensitivity to a thin boundary layer source level. In contrast, AM2 rain variations have much longer time scales as convection scheme plumes whose entrainment gives them tops below 500 hPa interact with humidity variations in that layer. Plumes detraining at model levels above 500 hPa are restricted by cloud work function thresholds, and upper-tropospheric humidity and cloud layers fed by these are detached from the lower levels and are somewhat sporadic. With these discrete entrainment rates and instability thresholds, AM2 also produces some synthetic-looking noise (sharp features in height and time) on top of its slow rain variations. A distinctive feature of the NASA model is a separate anvil scheme, distinct from the main large-scale cloud scheme, fed by relaxed Arakawa–Schubert (RAS) plume ensemble convection (a different implementation than in AM2). Its variability is rich and vertically coherent, and involves a very strong vertical dipole component to its tropospheric heating variations, of both signs (limited-depth convective heating and top-heavy heating in strong deep events with significant nonconvective rain). Grid-scale saturation events occur in all three models, often without nonconvective surface rain, causing relatively rare episodes of large negative top-of-atmosphere cloud forcing. Overall, cloud forcing regressions show a mild net positive forcing by rain-correlated clouds in CAM3 and mild net cooling in the other models, as the residual of large canceling shortwave and longwave contributions.

Simulation of the diurnal variation of the tropical convection

The diurnal variation of the tropical convection is investigated using hourly outputs from a two-dimensional cloud-resolving model simulation. The features of cloud microphysics in tropical convections are proposed by the contrastive analysis of the simulation. The model is forced by the observational data obtained from tropical ocean global atmosphere coupled ocean-atmosphere response experiment as the initial and boundary fields. The diurnal composites are carried out in weak diurnal SST variations （case W） and strong diurnal SST signals （case S） according to the amplitude of diurnal SST variations. The simulation results show that the ice water path in case W is larger than the liquid water path in case S, and more water cloud and less ice cloud exist in case W than in case S. The results also show that the surface rain rates reach their peaks in the early afternoon in case S, while the surface rain rates reach their peaks in the night in case W. Further comparison of cloud microphysics budgets shows that the coagulation of precipitation ice （sum of snow and graupel） is less than the coagulation of liquid water （sum of cloud water and precipitation water） and ice cloud can be formed in case W.

Quantitative Precipitation Estimation over Ocean Using Bayesian Approach from Microwave Observations during the Typhoon Season

We have developed a new Bayesian approach to retrieve oceanic rain rate from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), with an emphasis on typhoon cases in the West Pacific. Retrieved rain rates are validated with measurements of rain gauges located on Japanese islands. To demonstrate improvement, retrievals are also compared with those from the TRMM/Precipitation Radar (PR), the Goddard Profiling Algorithm (GPROF), and a multi-channel linear regression statistical method (MLRS). We have found that qualitatively, all methods retrieved similar horizontal distributions in terms of locations of eyes and rain bands of typhoons. Quantitatively, our new Bayesian retrievals have the best linearity and the smallest root mean square (RMS) error against rain gauge data for 16 typhoon overpasses in 2004. The correlation coefficient and RMS of our retrievals are 0.95 and ~2 mm hr-1, respectively. In particular, at heavy rain rates, our Bayesian retrievals outperform those retrieved from GPROF and MLRS. Overall, the new Bayesian approach accurately retrieves surface rain rate for typhoon cases. Accurate rain rate estimates from this method can be assimilated in models to improve forecast and prevent potential damages in Taiwan during typhoon seasons.

Development of Nonuniform Beamfilling Correction Method in Rainfall Retrievals for Passive Microwave Radiometers over Ocean Using TRMM Observations

Nonuniform beamfilling (NUBF) is a major error source in physical retrieval algorithms for estimating rain rates using satellite-borne passive microwave radiometers. The NUBF effects over the ocean within the beam effective field of view in the 10 GHz channel (FOV10) for the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) are investigated from simultaneous measurements made by the TMI and the Precipitation Radar (PR) aboard the TRMM satellite. They are investigated with respect to variability of a non-uniform parameter defined by the lognormal assumption from January to December 2000. The parameter computed using surface rain rates estimated from PR data tends to be small (large) for stratiform (convective) rainfall in the FOV10. The parameter computed using surface rain rates estimated from brightness temperature (Tb) at 85 GHz is systematically lower in stratiform rainfall than the reference derived by the PR. Systematic differences are not found for the convective cases. To evaluate the effects on rain retrievals due to the systematic differences of the non-uniform parameters, a simple adjustment is applied, and rain retrievals from the observed TMI Tb at 10 GHz vertical polarization are performed over the ocean. Relatively large increases in rain rates retrieved using corrected parameters are found in the tropics, while the increases of the corrected parameters are similar for all latitudes. Effects of the non-uniform parameter differences on the rain retrievals are nonlinear and could be closely related to high background values of the non-uniform parameter in the tropics and more frequent high rainfall intensities.

Diurnal responses of tropical convective and stratiform rainfall to diurnally varying sea surface temperature

Two experiments were carried out using a two-dimensional cloud-resolving model to study the effects of diurnally varying sea surface temperature (SST) on diurnal variations of tropical convective and stratiform rainfall. Experiment SST29 is imposed by a constant SST of 29°C, whereas experiment SST29D is imposed by a diurnally varying SST with a time-mean of 29°C and a diurnal difference of 1°C. Both experiments are also zonally uniformly imposed by a zero vertical velocity and a constant zonal wind, and are integrated for 40 days to reach quasi-equilibrium states. The model domain mean surface rain rate is larger in SST29D than in SST29 in the late afternoon, when the ocean surface is warmer in SST29D. Convective-stratiform rainfall partitioning analysis reveals that the late-afternoon convective rainfall is larger in SST29D than in SST29, whereas the stratiform rainfalls are similar in both experiments. Further analysis of surface rainfall and cloud microphysical budgets over convective regions shows that, in the late afternoon, the larger amount of water vapor is pumped into the non-raining region through the larger surface evaporation associated with the warmer SST. This water vapor is then transported into convective regions to produce more vapor condensation and greater collection of cloud water by raindrops and larger convective rainfall in SST29D than in SST29.

A C-Band Scatterometer Simultaneous Wind/Rain Retrieval Method

Using collocated ERS scatterometer (ESCAT), Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR), and European Centre for Medium-Range Weather Forecasts (ECMWF) data, the effects of rain on ESCAT wind-only retrieval are evaluated. Additional scattering from rain causes estimated wind speeds to appear higher than expected. Selected directions of the rain-corrupted wind vectors are biased toward along-track directions under conditions of heavy rain, which is regardless of the true wind direction. Rain becomes more significant for data acquired at a high incidence angle. To compensate for rain-induced backscatter, a simultaneous wind/rain retrieval (SWRR) method, which simultaneously retrieves wind velocity and surface rain rate from ESCAT measurements with an incidence angle >40deg, is developed by using a C-band wind/rain backscatter model. The performance of SWRR under typical wind/rain conditions is evaluated through simulation and validation with collocated TRMM PR and ECMWF data sets. SWRR is shown to significantly improve wind velocity estimates, and the SWRR estimated surface rain rate has relatively high accuracy in moderate to heavy rain cases. Although SWRR-retrieved rain is somewhat biased, it can be corrected. We note that for ESCAT, about 1.5% of all the collocated measurements is affected by significant rain.

Sensitivity of convective and stratiform rainfall to sea surface temperature

The sensitivity of surface rainfall and associated cloud microphysical processes to sea surface temperature (SST) and its diurnal variation is investigated with a series of two-dimensional cloud-resolving model experiments. Experiments with time-invariant SST of 27 °C (SST27) and 31 °C (SST31) and experiments with diurnal SST amplitudes of 1 °C (SST29D1) and 2 °C (SST29D2) and time-mean SST of 29 °C are, respectively, compared with the control experiment with time-invariant SST of 29 °C (SST29). The model with a cyclic lateral boundary is imposed by zero vertical velocity and a constant zonal wind and is integrated for 40 days to reach quasi-equilibrium states for all experiments. Although the time and model domain mean surface evaporation rate is smaller in SST27 than in SST29, the addition of mean local atmospheric drying rate in SST27 leads similar mean surface rain rates in the two experiments. The smaller surface evaporation associated with the colder SST causes the larger local atmospheric drying rate over clear sky regions and the smaller local atmospheric moistening rate in SST27 than in SST29. The larger surface evaporation over clear sky regions associated with the warmer SST causes the larger water vapor transport to convective regions that leads to the larger convective rain rate and thus the larger mean surface rain rate in SST 31 than in SST29. The mean local atmospheric drying and decrease of mean hydrometeor concentration in SST29D1 and increase of mean hydrometeor concentration in SST29 cause the larger mean surface rain rates in SST29D1 than in SST29. The slight increase of mean surface rain rate with increasing diurnal SST amplitude is simply caused by increase of mean surface evaporation rate.

Responses of tropical deep convective precipitation systems and their associated convective and stratiform regions to the large‐scale forcing

AbstractResponses of convective and stratiform rainfall to large‐scale forcing in the tropical deep convective regime are investigated by analysing data from a two‐dimensional cloud‐resolving model simulation. The model is forced by large‐scale vertical velocity, zonal wind, horizontal advection, and sea‐surface temperature observed and derived from TOGA COARE. The imposed large‐scale vertical velocity shows that the ascending motion averaged from 22–27 December 1992 is significantly stronger than that averaged from 3–8 January 1993. Thus, the periods of 22–27 December 1992 and 3–8 January 1993 are, respectively, identified as strong‐forcing (SF) and weak‐forcing (WF) phases. Although the convective rain rates are similar in the two phases, stratiform rain rate is larger in the SF phase than in the WF phase, which leads to larger time and spatial mean surface rain rate in the SF phase. Similar rates of water vapour convergence and vapour condensation and collection of cloud water by rain over water‐hydrometeor‐dominated convective regions are responsible for similar convective rain rates in the two phases. Water vapour divergence is larger in the WF phase than in the SF phase, which causes smaller vapour condensation and deposition rates in the WF phase. As a result, the collection rate of cloud water by rain and the melting rate of graupel to rain are smaller in the WF phase than in the SF phase, which accounts for the smaller stratiform rain rate in the WF phase. Copyright © 2008 Royal Meteorological Society

Mitigating the impact of RADCAL beacon contamination on F15 SSM/I ocean retrievals

Data from six Special Sensor Microwave/Imagers (SSM/I) have been used to provide a 22‐year climate data record of surface wind speed, columnar water vapor, columnar cloud water, and surface rain rate. On 14 August 2006, a radar calibration (RADCAL) beacon was activated on F15, one of the SSM/I. Interference from the beacon caused retrieval biases of roughly −30% for wind and rain and 40% for vapor and cloud. We have developed a simple correction for RADCAL interference, which brings the retrieval biases down to a few percent for cloud and rain, and less than 1% for wind and vapor. The corrected F15 data are of suitable quality for weather research purposes, but we discourage use of F15 retrievals after 14 August 2006 for climate research purposes at present because it appears that RADCAL beacon interference may not be stable in time, but may depend upon the thermal environment of F15.

Torrential rainfall processes associated with a landfall of severe tropical storm Bilis (2006): A two-dimensional cloud-resolving modeling study

Torrential rainfall processes associated with a landfall of Typhoon Bilis (2006) are investigated using a two-dimensional cloud-resolving model simulation. The model is integrated for 6 days with imposed zonally-uniform vertical velocity, zonal wind, horizontal temperature and vapor advection from NCEP/GDAS data. The simulation is validated with observations in terms of surface rain rate and reflectivity. The simulated stratiform clouds cover 89% of the simulation domain that leads to dominant stratiform rainfall on 15 July 2006. The convective clouds develop to cover 29% of the simulation domain that yields dominant convective rainfall whereas the stratiform clouds shrink to 46% on 16 July 2006. The domain-mean simulation shows that the increase of domain-mean surface rain rate from 15 to 16 July 2006 mainly results from local atmospheric moistening on 15 July and local atmospheric drying on 16 July although water vapor convergence rates in the two days are similar.

A phase analysis of vorticity vectors associated with tropical convection

Three new vorticity vectors have been proposed by Gao et al to study the two-dimensional tropical convection. In the present paper, phase relations between surface rain rate and the vorticity vectors are analysed with the calculations of lag correlation coefficients based on hourly zonally-averaged mass-integrated cloud-resolving simulation data. The cloud-resolving model is integrated with the vertical velocity, zonal wind, horizontal thermal and moisture advections, and sea surface temperature observed and derived from tropical ocean global atmosphere – coupled ocean atmosphere response experiment (TOGA-COARE) for 10 days. Maximum local increase of the vertical component of the convective vorticity vector leads maximum surface rain rate by 2 hours mainly due to the interaction between vorticity and zonal gradient of ice heating. While maximum local increase of the vertical component of the moist vorticity vector leads maximum surface rain rate by 2 hours mainly because of the interaction between zonal specific humidity gradient and zonal buoyancy gradient. And the maximum local decrease of the zonal component of the dynamic vorticity vector leads maximum surface rain rate by 2 hours mainly due to the interactions between vorticity and vertical pressure gradient as well as vorticity and buoyancy.

Intercalibrated Passive Microwave Rain Products from the Unified Microwave Ocean Retrieval Algorithm (UMORA)

AbstractThe Unified Microwave Ocean Retrieval Algorithm (UMORA) simultaneously retrieves sea surface temperature, surface wind speed, columnar water vapor, columnar cloud water, and surface rain rate from a variety of passive microwave radiometers including the Special Sensor Microwave Imager (SSM/I), the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), and the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). The rain component of UMORA explicitly parameterizes the three physical processes governing passive microwave rain retrievals: the beamfilling effect, cloud and rainwater partitioning, and effective rain layer thickness. Rain retrievals from the previous version of UMORA disagreed among different sensors and were too high in the tropics. These issues have been fixed with more realistic rain column heights and proper modeling of saturation and footprint-resolution effects in the beamfilling correction. The purpose of this paper is to describe the rain algorithm and its recent improvements and to compare UMORA retrievals with Goddard Profiling Algorithm (GPROF) and Global Precipitation Climatology Project (GPCP) rain rates. On average, TMI retrievals from UMORA agree well with GPROF; however, large differences become apparent when the instantaneous retrievals are compared on a pixel-to-pixel basis. The differences are due to fundamental algorithm differences. For example, UMORA generally retrieves higher total liquid water, but GPROF retrieves a higher surface rain rate for a given amount of total liquid water because of differences in microphysical assumptions. Comparison of UMORA SSM/I retrievals with GPCP shows similar spatial patterns, but GPCP has higher global averages because of greater amounts of precipitation in the extratropics. UMORA and GPCP have similar linear trends over the period 1988–2005 with similar spatial patterns.

A cloud‐resolving modeling study of cloud radiative effects on tropical equilibrium states

The radiative effects of water and ice clouds on tropical equilibrium states are investigated based on four two‐dimensional cloud‐resolving simulations imposed by a zero vertical velocity and constant zonal wind and sea surface temperature (SST). The model is integrated for 40 d to reach equilibrium states. Experiment CRFWI includes radiative effects of both water and ice clouds whereas experiment CRF00 excludes them. Experiments CRF0I and CRFW0 are identical to CRFWI except the radiative effects of water and ice clouds are excluded, respectively. CRF00 and CRFW0 produce similar cold and dry equilibrium states whereas CRFWI and CRF0I generate similar warm and humid equilibrium states. This indicates that water clouds may not have radiative effects on equilibrium states. The experiments without ice radiative effects (CRFW0 and CRF00) emit more IR radiation into space than the experiments with ice radiative effects (CRFWI and CRF0I) do. Colder air by more IR emission strengthens air‐sea flux exchanges and lowers air capacity to hold water vapor, resulting in larger surface evaporation and drier atmosphere. An increased surface evaporation is balanced with a stronger surface rain rate in the equilibrium domain mean water balance.

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

Diurnal 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.

Use of High-Resolution Satellite Observations to Evaluate Cloud and Precipitation Statistics from Cloud-Resolving Model Simulations. Part I: South China Sea Monsoon Experiment

Abstract Cloud and precipitation simulated using the three-dimensional (3D) Goddard Cumulus Ensemble (GCE) model are compared to Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Precipitation Radar (PR) rainfall measurements and Clouds and the Earth’s Radiant Energy System (CERES) single scanner footprint (SSF) radiation and cloud retrievals. Both the model simulation and retrieved parameters are based upon observations made during the South China Sea Monsoon Experiment (SCSMEX) field campaign. The model-simulated cloud and rain systems are evaluated by systematically examining important parameters such as the surface rain rate, convective/stratiform percentage, rain profiles, cloud properties, and precipitation efficiency. It is demonstrated that the GCE model is capable of simulating major convective system development and reproduces the total surface rainfall amount as compared to rainfall estimated from the SCSMEX sounding network. The model yields a slightly higher total convective rain/stratiform rain ratio than the TMI and PR observations. The GCE rainfall spectrum exhibits a greater contribution from heavy rains than those estimated from PR or TMI observations. In addition, the GCE simulation produces much greater amounts of snow and graupel than the TRMM retrievals. The model’s precipitation efficiency of convective rain is close to the observations, but the precipitation efficiency of stratiform rain is much lower than the observations because of large amounts of slowly falling simulated snow and graupel. Compared to observations, the GCE produces more compact areas of intense convection and less anvil cloud, which are consistent with a smaller total cloud fraction and larger domain-averaged outgoing longwave radiation.

A cloud‐resolving modeling study of a torrential rainfall event over China

A torrential rainfall event that occurred in Hunan, China in June 2004 is investigated using a two‐dimensional cloud‐resolving simulation. The model is integrated for 6 days with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature, and vapor advection from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data. The comparison in surface rain rate and reflectivity between the simulations and observations shows good agreement. The simulation data during the torrential rainfall event (a 37‐hour period) are further analyzed to explore the physical causes in the surface rainfall processes. The time‐ and domain‐mean analysis shows that although their rates in the onset and mature stages are similar, vapor convergence mainly supports local vapor gain in the onset stage and surface rainfall in the mature stage. As a result, the surface rain rate is significantly smaller in the onset stage than in the mature stage. The weakening of vapor convergence finally suppresses the surface rainfall during the decay stage.