Diagnostic study of diabatic heating and potential vorticity during a case of cyclogenesis

On 16-17 November, 2015, north and middle regions of Saudi Arabia were hit by a case of cyclogenesisassociated with heavy rainfall. This work presents a diagnostic study of this heavy rainfallcase based on the analysis of diabatic heating and potential vorticity. The synoptic analysis investigate that the important dynamical factors that causes this case are the northward extension of Red Sea Trough, anticyclone over the Arabian Peninsula, a travailing midlatitude upper trough, moisture transport pathways and strong upward motion arising from tropospheric instability. The calculation of diabatic heating by the thermodynamic equation illustrate that the contribution of vertical temperature advection and the adiabatic term are opposite to each other during the period of study. The largest contribution of the horizontal cold advection occurs during the first two days while the largest contribution of the horizontal warm advection occurs during the maximum development days. The dynamics of the studied case are also investigated in terms of isobaric Potential Vorticity. It is found that the location of the low-level Potential Vorticity anomaly and the Potential Vorticity generation estimates coincides with the heating region, which implies that condensation supports a large enough source to explain the existence of the low-level Potential Vorticity anomaly.

Analysis of a Late-Autumn Rainstorm in the Sichuan Basin on the Eastern Side of the Tibetan Plateau

An abnormal heavy rainfall that occurred on 27 October 2014 in the Sichuan Basin (SB), China, is analyzed. An inverted trough at 850 hPa evolved into a Southwest China Vortex (SWCV), and strong upward motion caused by interaction between the low-level jet (LLJ) at 850 hPa and the upper-level jet (ULJ) at 200 hPa triggered the rainstorm process. Under a large-scale circulation system featuring a westerly trough and subtropical high, there were two cloud bands over the northeast side and south side of the Tibetan Plateau, respectively. Influenced by the eastward-moving trough, the inverted trough, LLJ, and the SWCV, a Mesoscale Convective System (MCS) was generated near the intersection of the two cloud bands, and it was the direct rainstorm system. The MCS strengthened under the situation of the 850 hPa inverted trough, but weakened when the inverted trough evolved to into the SWCV. Eventually, it formed the phenomenon known as “existing vortex without cloud.” Through analysis of the possible reasons why precipitation strengthened (weakened) under the situation of the inverted trough (SWCV), it was found that the strengthening of precipitation was due to a strong tilting updraft in the area of the ULJ and LLJ intersection. On one hand, the upward motion was related to the vorticity advection variation with height and the low-level warm advection forcing; while on the other hand, the dew-point front near the LLJ also played a lifting role in the upward flow of the lower-layer vertical circulation. Meanwhile, the LLJ “head” was a high-value area of water vapor convergence, which provided sufficient water vapor for the rainstorm. During the SWCV, the weakening of precipitation was due to the SWCV weakening gradually; plus, the ULJ was interrupted over the SB, the upper airflow presented downdrafts, and its superposition with the ascending branch of low-level vertical circulation. This airflow structure inhibited the development of strong upward motion, whilst at the same time, the LLJ retreated toward the south and the dew-point front ultimately weakened and disappeared. Subsequently, water vapor convergence weakened and no longer supported the occurrence of heavy rainfall. Therefore, the strong upward motion caused by the ULJ-LLJ intersection and the lower-level dew-point front were the key reasons for the occurrence of this late-autumn rainstorm.

Precipitation and Moisture in Four Leading CMIP5 Models: Biases across Large-Scale Circulation Regimes and Their Attribution to Dynamic and Thermodynamic Factors

As key variables in general circulation models, precipitation and moisture in four leading models from CMIP5 (phase 5 of the Coupled Model Intercomparison Project) are analyzed, with a focus on four tropical oceanic regions. It is found that precipitation in these models is overestimated in most areas. However, moisture bias has large intermodel differences. The model biases in precipitation and moisture are further examined in conjunction with large-scale circulation by regime-sorting analysis. Results show that all models consistently overestimate the frequency of occurrence of strong upward motion regimes and peak descending regimes of 500-hPa vertical velocity [Formula: see text]. In a given [Formula: see text] regime, models produce too much precipitation compared to observation and reanalysis. But for moisture, their biases differ from model to model and also from level to level. Furthermore, error causes are revealed through decomposing contribution biases into dynamic and thermodynamic components. For precipitation, the contribution errors in strong upward motion regimes are attributed to the overly frequent [Formula: see text]. In the weak upward motion regime, the biases in the dependence of precipitation on [Formula: see text] and the [Formula: see text] probability density function (PDF) make comparable contributions, but often of opposite signs. On the other hand, the biases in column-integrated water vapor contribution are mainly due to errors in the frequency of occurrence of [Formula: see text], while thermodynamic components contribute little. These findings suggest that errors in the frequency of [Formula: see text] occurrence are a significant cause of biases in the precipitation and moisture simulation.

Evolution, Properties, and Spatial Variability of MJO Convection near and off the Equator during DYNAMO

This study investigates the evolution, structure, and spatial variability of Madden–Julian oscillation (MJO) convection observed during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign. Generally, the C-band radars located in the near-equatorial Indian Ocean—Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) on Addu Atoll (Gan) and NASA TOGA on the R/V Roger Revelle (Revelle)—observed similar trends in echo-top heights, stratiform rain fraction, and precipitation feature size across the MJO life cycle. These trends are closely related to changes in mid- to upper-tropospheric moisture, sea surface temperature (SST), zonal wind, and diagnosed vertical air motions. However, the evolution of convection, moisture, and vertical air motion at the R/V Mirai (Mirai), located in the intertropical convergence zone (ITCZ) at 8°S, exhibited a pattern nearly opposite to Gan and Revelle. When the MJO was active over the equator, convection was suppressed around Mirai owing to induced subsidence by the strong upward motion to the north. SST and zonal winds near Mirai were nearly invariant across the MJO life cycle, indicating little influence from the MJO in these fields. Compared to Gan and Revelle, Mirai had a significant amount of precipitation that fell from shallow and isolated convection. There were subtle differences in the evolution and properties of the convection observed between Gan and Revelle. Deep convection occurred slightly earlier at Gan compared to Revelle, consistent with the west-to-east progression of the MJO in the central Indian Ocean. Furthermore, convective deepening was more gradual over Revelle compared to Gan, especially during the October MJO event.

Case Study of a Split Front and Associated Precipitation during the Mei-Yu Season

This study investigated the relationship between a split front and precipitation over western Japan on 10–11 July 2007 based on data from routine observations and a global objective analysis. The split front formed at the leading edge of a dry intrusion overrunning a mei-yu front in a deep layer from the north. Strong upward motion was present ahead of the frontogenesis associated with the split front. In contrast, upward motion over the mei-yu front was suppressed by downward motion behind the split front. An intense rainband developed along the split front south of the mei-yu front. In contrast, precipitation over the mei-yu front gradually disappeared with the southward advance of the split front. Rainfall as heavy as 99 mm h−1 was observed under the rainband associated with the split front. The strong upward motion that induced heavy rains was attributed to the rising branch of the frontal circulation of the split front and the intense low-level convergence facilitated by the dry intrusion behind the split front. These findings indicate that split fronts have a substantial impact on the development and distribution of precipitation during the mei-yu season.

Numerical Prediction of Thermal Effect on Flow Field around a High-Rise Building Model

Recently, CFD (Computational Fluid Dynamics) is being more and more used in the investigation of the flow and thermal around high-rise buildings. In this study, prediction of thermal effect on flow pattern around high-rise buildings model located in the surface boundary layer was carried out using k-e turbulence model. Investigation of thermal effect on flow pattern behind the building and comparison between isothermal and non-isothermal building using standard k-e showed that the surface temperature of building wall led to a strong upward motion close to heating wall and decreased the reattachment length behind it. The model sittings of validation study was accomplished by comparing the simulation of wind flow around building models with the experimental data of Architectural Institute of Japan (AIJ) that results showed in weak wind region, standard k-e has good agreement with experimental.