Warm-sector Rainfall Research Articles

Could Developing Frontal Rainfall Influence Warm‐Sector Rainfall?

AbstractForecasting warm‐sector rainfall (WR) remains a major challenge, primarily due to weak synoptic forcing. Through cloud‐permitting numerical simulations, in addition to direct triggering mechanism from low‐level jets, we identify the important role of gravity waves in a heavy WR event in South China via convective preconditioning. The preconditioning manifests as mid‐level moistening and destabilization with wave‐like variations. This process is driven by fast‐propagating (∼24 m s−1) n = 2 waves, associated with lower‐tropospheric ascents and upper‐tropospheric descents. Waves are generated during the evolution of northern frontal rainfall (FR). As FR intensifies, surges in low‐level diabatic cooling mainly resulting from microphysical processes, trigger n = 2 waves, which further precondition the environment along their path. In contrast, a sensitivity experiment involving stably developing FR fails to reproduce the preconditioning process by waves and the subsequent occurrence of WR. Overall, our study illuminates a new pathway through which FR significantly influences WR via gravity waves.

Sensitivity of Nocturnal Warm Sector Rainfall Simulation to the Configuration of Initial and Lateral Boundary Conditions: A Case Study in Southern China Based on the Operational TRAMS Model

AbstractA heavy nocturnal warm sector rainfall occurred over the western coastal region of South China in June 2020, of which the precipitation magnitude was seriously underestimated by the operational TRAMS (Tropical Regional Atmosphere Model System) model. In this study, by improving the configuration of initial conditions (ICs) and lateral boundary conditions (LBCs) in offline nesting, the simulation of the convection initiation (CI) process can be well improved. The CI simulation is found to be sensitive to the vertical resolution in ICs in this case. When the low‐level vertical resolution of ICs is increased, the nocturnal near‐surface cold layer caused by inland mountains can be resolved better, which is necessary to form the convergence line along the coastline. Another important impact of increasing the vertical resolution of ICs is the successful simulation of horizontal convective rolls (HCRs) over the northern South China Sea. The HCRs apparently increase the depth of the warm‐moist marine boundary layer jet (MBLJ), and finally lead to the CI along the coastline. The increased vertical resolutions of LBCs can reduce the discontinuity of simulated moist tongue across the southern lateral boundary of the TRAMS model, which provides more moisture for the warm sector rainfall through MBLJ. LBCs with Higher temporal frequency help to alleviate the western position bias of rain belt by reducing the temporal interpolation error. Our study highlights the importance of finer ICs and LBCs in offline nesting for regional operational NWP systems in the South China region.

Convection-permitting ensemble forecasts of a double-rainbelt event in South China during the pre-summer rainy season

Heavy rainfall occurred at the inland frontal zone and coastal warm sector in South China on 12–13 June 2019. Three convection-permitting ensemble forecast (CPEF) experiments with perturbed initial conditions (ICs) and lateral boundary conditions (BCs) as well as model physical schemes have been conducted to predict the double-rainbelt process. This study evaluates the predictability of different CPEF experiments and analyzes the causes of precipitation forecast errors by identifying the sensitive factors for frontal rainfall (FR) and warm-sector rainfall (WR). In this double-rainbelt event, the forecast skill of FR is significantly better than that of WR. The experiment with perturbed ICs/BCs combining suitable model physical schemes performs best. Based on the optimal ensemble experiment, the ensemble sensitivity analysis has been conducted for the two rainbelts. FR is sensitive to the synoptic forcing and its sensitive area mainly locates near the frontal system, while the coastal boundary layer jet and water vapor from the South China Sea play an important role in the WR process. The ICs and BCs greatly influence the location of FR through varying the movement direction and velocity of the fronts. The physical schemes have a great impact on convection triggering along the coasts, thus affecting the occurrence of the whole WR process, of which the choice of microphysical scheme is critical.

Contrasting Mesoscale Convective System Features of Two Successive Warm-Sector Rainfall Episodes in Southeastern China: A Satellite Perspective

Based on Himawari-8 satellite observations, the mesoscale convective system (MCS) behaviors of two successive but distinct warm-sector rainfall episodes (EP1 and EP2) on 6–7 May 2018 over southeastern China were compared, with the latter episode being a record-breaking rainfall event. Results showed that MCSs played a dominant role in EP2, but not in EP1, by contributing over 80% of the extreme rainfall total and all the 10-min rainfalls over 20 mm. MCS occurrences were more frequent in EP2 than EP1, especially in the coastal rainfall hotspots, along with more frequent merging processes. Overall, the MCS samples in EP2 were larger in size, more intense, and moved slower and more in parallel to their orientation, which facilitated local rainfall accumulation. Two new indices are proposed—the overlap index (OLI) and merging potential index (MPI)—to evaluate two MCS processes vital for rainfall production: the repeated passage of an individual MCS over given areas and the merging between MCSs, respectively. Both OLI and MPI in EP2 were significantly larger than in EP1, which tended to produce larger maximum rainfall amount and stronger 10-min rain rates in the following hour. These results demonstrate the potential value of satellite-based MCS information for heavy rainfall nowcasting, which is particularly significant for warm-sector rainfall with its limited predictability.

Multiscale Perspectives on an Extreme Warm-Sector Rainfall Event over Coastal South China

On 22 June 2017, an extreme warm-sector rainfall event hit the western coastal area of South China, with maximum hourly and 12-h rainfall accumulations of 189.4 and 464.8 mm, respectively, which broke local historical records. Multisource observations were used to reveal multiscale processes contributing to the extreme rainfall. The results showed that a marine boundary layer jet (BLJ) coupled with a synoptic low-level jet (LLJ) inland played an important role in the formation of an extremely humid environment with a very low lifting condensation level of near-surface air. Under the favorable pre-convective conditions, convection was initialized at a mesoscale convergence line, aided by topographic lifting in the evening. During the nocturnal hours, the rainstorm developed and was maintained by a quasi-stationary mesoscale outflow boundary, which continuously lifted warm, moist air transported by the enhanced BLJ. When producing the extreme rainfall rates, the storm possessed relatively weak convection, with the 40 dBZ echo top hardly reaching 6 km. The extreme rainfall was produced mainly by the warm rain microphysical processes, mainly because the humid environment and the deep warm cloud layer facilitated the clouds’ condensational growth and collision–coalescence, and also reduced rain evaporation. As the storm evolved, the raindrop concentration increased rapidly from its initial stage and remained high until its weakening stage, but the mean raindrop size changed little. The extreme rain was characterized by the highest concentration of raindrops during the storm’s lifetime with a mean size of raindrops slightly larger than the maritime regime.

Organized Warm-Sector Rainfall in the Coastal Region of South China in an Anticyclone Synoptic Situation: Observational Analysis

The formation and regional difference of the organized warm-sector rainfall (OWSR) near the coast of South China in an anticyclone synoptic situation were investigated, by analyzing relevant events in recent years using high-resolution observational and reanalysis data. The results show that the anticyclone synoptic situation produces marked northerly boundary-layer winds inland, and obvious northeasterly, easterly/southwesterly and southeasterly boundary-layer winds near the coast of eastern and western Guangdong Province and Guangxi Province, respectively. Compared with the northeasterly boundary-layer winds, the southwesterly, southeasterly or easterly boundary-layer winds promote favorable environmental conditions and stronger convergence with the northerly boundary-layer winds for convection initiation. Consequently, OWSR is prone to occur near the coast of western Guangdong Province and Guangxi Province. The southeasterly boundary-layer winds tend to converge beside the mountains near the boundary between Guangxi and Guangdong provinces, promoting the formation of a stable convective line along the mountains. The convective line persists with the assistance of southwesterly upper-level winds, which encourage convective cells to propagate along the convective line, producing heavy OWSR along the mountains near the boundary between Guangxi and Guangdong provinces. In contrast, a west–east convective line tends to form and maintain near the coast of Yangjiang, owing to stable convergence between the easterly (or southwesterly) and northerly boundary-layer winds reinforced by the mountains near Yangjiang. Furthermore, the coupling of westerly upper-level winds with the easterly (southwesterly) boundary-layer winds facilitates an expansion (eastward propagation) of the convective line, causing west–east-oriented heavy OWSR near the coast of Yangjiang.

Observational Analysis of the Characteristics of the Synoptic Situation and Evolution of the Organized Warm-Sector Rainfall in the Coastal Region of South China in the Pre-Summer Rainy Season

The characteristics of the synoptic situation and the evolution of the organized warm-sector rainfalls (OWSRs) in the coastal region of South China in the pre-summer rainy season were investigated, using a period (2011–2016) of high-resolution observational data and European Centre for Medium-Range Weather Forecasts Re-Analysis Interim (ERA-Interim) data. The results show that a strong southwesterly low-level jet (LLJ) ahead of a trough over southwestern China with a marked boundary-layer jet (BLJ) over the northern South China Sea (synoptic situation SWLLJ) or a prominent, low-level anticyclone over the Yangtze River Basin (synoptic situation ACR) is present when the OWSRs occur in the coastal region of South China. The OWSRs are prone to initiate on the windward side of the coastal mountains, owing to the convergence enhanced by the colliding of the BLJ with the mountains and the coupling of double LLJs near the coast (for SWLLJ), or due to the convergence between northerly and southeasterly winds near the coastal mountains (for ACR). The OWSRs present a long extension when the LLJ axis is nearby. The translation of the LLJ itself also promotes the long extension of the OWSRs. In contrast, the OWSRs show a short extension when the LLJ axis is farther away or ACR occurs. Meanwhile, the OWSRs are directed northeastward in Guangxi Province and more eastward in Guangdong Province, probably owing to the orientation difference of the LLJ in these two provinces. The rainfall systems in the ACR situation tend to move eastward, whereas those in the SWLLJ situation are prone to move eastward when equivalently strong or much-stronger upper-level winds overlay the LLJ, but move northeastward when much weaker upper-level winds couple with the LLJ.

Contrasting frontal and warm-sector heavy rainfalls over South China during the early-summer rainy season

Heavy rainfalls occur frequently during early summer (April–June) over South China, causing severe floods. Using 12 years of hourly rain-gauge data, we identify a large number of regional heavy rainfall events and categorize them into two major types based on the strength of synoptic forcing. It is shown that frontal heavy rainfalls associated with fronts or shear lines mainly occurs over inland regions, whereas warm-sector heavy rainfall under weakly forced synoptic environment is observed in coastal areas. The rainfall maxima of both types tend to form over the low-lying plains or sea surfaces adjacent to windward mountains. Frontal heavy rainfall usually propagates southwards with a cold front, while warm-sector events move relatively slowly and produce coherent patterns of rainfall. The occurrence of warm-sector rainfall increases markedly from April to June in a close association with the onset of summer monsoon, in contrast to frontal rainfall with less monthly variation. Warm-sector rainfall also exhibits pronounced diurnal variation, with a peak in the early morning, as a result of intensified convergence between nocturnal low-level jets in southerly monsoon and land breezes. In contrast, frontal rainfall has an afternoon peak, due to the arrival of eastward-propagating rain systems and daytime heating on land. As frontal (warm-sector) heavy rainfall has relatively high (low) predictability in numerical models, understanding their occurrence and formation is of benefit to operational forecasting and decision-making for disaster prevention.

Evaluation of latest GPM-Era high-resolution satellite precipitation products during the May 2017 Guangdong extreme rainfall event

This study evaluates the performance of latest version 5B (V5B) Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) Final Run products during a 60-year return extreme precipitation storm on 7 May 2017 over southern China with gauge observations as the reference dataset. Version 4 (V4) Global Satellite Mapping of Precipitation (GSMaP) products and quantitative precipitation estimates derived from a local ground-based S-band dual polarization weather radar (radar, hereafter) were used for parallel comparisons. The satellite-only products (IMERGUncal and GSMaP_MVK) and gauge-corrected products (IMERGCal, GSMaP_Gauge) were selected for this study. The results showed that: 1) GSMaP_MVK, IMERGUncal, GSMaP_Gauge and IMERGCal generally capture the spatio-temporal patterns of storm-accumulated rainfall with correlation coefficient (CC) values about 0.76, 0.70, 0.68 and 0.72, respectively, while radar was well correlated with gauge measurement (CC about 0.94); 2) The GSMaP_Gauge (−19.38%), IMERGCal (−40.23%), GSMaP_MVK (−57.12%), IMERGUncal (−58.77%) satellite-based precipitation products all underestimated the storm-accumulated precipitation, while ground radar overestimated by 27.48%; 3) Both IMERGCal and IMERGUncal outperformed their GSMaP counterparts in capturing the time-series with much higher CC (0.50 vs. -0.21, 0.51 vs. 0.17); 4) Among the satellite-based QPE products, when the rainfall rates are <5 mm/h, IMERGCal performs best with highest CSI. The V4 GSMaP standard products and latest V5B IMERG final run products still had resolution and accuracy limitations in estimating extreme precipitation for this type of warm sector rainfall storm. These findings will help developers of rainfall retrieval products to understand the errors and also help improve modeling of hydrological extremes.

Classification of persistent heavy rainfall events over South China and associated moisture source analysis

Persistent heavy rainfall events (PHREs) over South China during 1981–2014 were selected and classified by an objective method, based on the daily precipitation data at 752 stations in China. The circulation characteristics, as well as the dry-cold air and moisture sources of each type of PHREs were examined. The main results are as follows. A total of 32 non-typhoon influenced PHREs in South China were identified over the study period. By correlation analysis, the PHREs are divided into three types: SC-A type, with its main rainbelt located in the coastal areas and the northeast of Guangdong Province; SC-B type, with its main rainbelt between Guangdong Province and Guangxi Region; and SC-C type, with its main rainbelt located in the north of Guangxi Region. For the SC-A events, dry-cold air flew to South China under the steering effect of troughs in the middle troposphere which originated from the Ural Mountains and West Siberia Plain; whereas, the SC-C events were not influenced by the cold air from high latitudes. There were three water vapor pathways from low-latitude areas for both the SC-A and SC-C PHREs. The tropical Indian Ocean was the main water vapor source for these two PHRE types, while the South China Sea also contributed to the SC-C PHREs. In addition, the SC-A events were also influenced by moist and cold air originating from the Yellow Sea. Generally, the SC-C PHREs belonged to a warm-sector rainfall type, whose precipitation areas were dominated by southwesterly wind, and the convergence in wind speed was the main reason for precipitation.

The Impact of a Prominent Rain Shadow on Flooding in California's Santa Cruz Mountains: A CALJET Case Study and Sensitivity to the ENSO Cycle

Abstract Data from the California Land-Falling Jets Experiment (CALJET) are used to explore the causes of variations in flood severity in adjacent coastal watersheds within the Santa Cruz Mountains on 2–3 February 1998. While Pescadero Creek (rural) experienced its flood of record, the adjacent San Lorenzo Creek (heavily populated), attained only its fourth-highest flow. This difference resulted from conditions present while the warm sector of the storm, with its associated low-level jet, high moisture content, and weak static stability, was overhead. Rainfall in the warm sector was dominated by orographic forcing. While the wind speed strongly modulated rain rates on windward slopes, the wind direction positioned the edge of a rain shadow cast by the Santa Lucia Mountains partially over the San Lorenzo basin, thus protecting the city of Santa Cruz from a more severe flood. Roughly 26% ± 9% of the streamflow at flood peak on Pescadero Creek resulted from the warm-sector rainfall. Without this rainfall, th...

A simulating study on resolvable—Scale microphysical parameterization in a mesoscale model

The Penn State/ NCAR Mesoscale Model (MM5) is used to simulate the precipitation event that occurred during 1–2 May 1994 to the south of the Yangtze River. In five experiments the Kain–Fritsch scheme is made use of for the subgrid–scale convective precipitation, but five different resolvable–scale microphysical parameterization schemes are employed. They are the simple super-saturation removal scheme, the warm rain scheme of Hsie et al. (1984), the simple ice scheme of Dudhia (1989), the complex mixed–phase scheme developed by Reisner et al. (1993), and the GSFC microphysical scheme with graupel. Our interest is how the various resolvable-scale schemes affect the domain-averaged precipitation, the precipitation distribution, the sea level pressure, the cloud water and the cloud ice. Through a series of experiments about a warm sector rainfall case, results show that although the different resolvable-scale scheme is used, the differences of the precipitation characteristics among all five runs are not very obvious. However, the precipitation is over-predicted and the strong mesoscale low is produced by the simple super-saturation removal scheme. The warm rain scheme with the inclusion of condensation and evaporation under-predicts the precipitation and allows the cloud water to reach the 300 hPa level. The scheme of the addition of graupel increases the resolvable-scale precipitation by about 20%-30%. The inclusion of supercooled liquid water in the grid-scale scheme does not affect significantly the results.

Structure and mechanism of precipitation and the effect of orography in a wintertime warm sector

A case study is presented showing the three-dimensional structure and evolution of precipitation upwind, over, and downwind of the south Wales hills during the passage of a wintertime warm sector that gave rather heavy and prolonged rainfall. The precipitation structure was synthesized from a network of weather radars and autographic raingauges; it is interpreted within a dynamical framework derived from routine upper air soundings supplemented by serial ascents upwind and downwind of the hills. The warm sector was characterized by a fast-moving airstream with potential instability (PI) not only at low levels due to its passage over a warm sea but also in the middle troposphere. The middle-level PI occurred extensively and played a significant role in determining the distribution and amount of precipitation. It has not received much attention until now, mainly because the limitations of the humidity element in conventional radiosondes tend to cause its magnitude to be underestimated. The middle-level PI was due to differential thermal advection in an intense and nearly vertical baroclinic zone that extended ahead of the surface cold front at middle levels. Large-scale ascent was slight in the warm sector but, nevertheless, middle-level PI was realized even over the sea in scattered ‘Mesoscale Precipitation Areas’ (MPAs) which travelled rapidly at the speed of the winds in the middle troposphere (about 120 km hr−1). Once the airstream experienced orographic uplift, fresh outbreaks of middle-level convection occurred extensively between existing MPAs. The fresh outbreaks were observed first as middle-level precipitation echoes over the sea up to 100 km upwind of the hills, indicating that the orographic ascent aloft began far upwind of the hills. Thus some of the precipitation generated by orographic ascent in the middle levels reached low levels over the hills despite the drift of the precipitation in the strong winds; there it seeded low-level orographic cloud which gave heavy rain over the hills. Some of the convective precipitation generated aloft as a result of the middle-level PI also reached the ground downwind of the hills, thereby displacing to some extent the commonly-observed rain shadow. The importance of seeding from aloft for releasing heavy orographic rainfall has long been recognized. Sometimes the seeding is achieved by precipitation generated aloft by large scale ascent; however, the present study suggests that in certain circumstances the hills may be able to generate their own seeds when they are not being generated by large scale ascent. In the case study the occurrence of heavy warm-sector rainfall over the hills appears to have been favoured by the existence of middle-level PI which, although not being realized generally by large scale ascent, required only a small amount of local orographic ascent to realize it. Thus forecasting techniques that predict rainfall on the basis of the forecast large-scale vertical ascent may fail to identify some important situations of orographic warm sector rain unless the interaction of PI and orography is realistically taken into account.