Use of scatterometer based surface vorticity fields in forecasting genesis of tropical cyclones over the north Indian Ocean

. Utilizing surface vorticity fields computed with the ocean surface wind speed and direction dataobtained from QuikSCAT, a study has been undertaken to investigate the increase in surface vorticity during the genesisphase of tropical cyclones over the north Indian Ocean. Six named tropical cyclones; Agni, Hibaru, Mala, Akash, Nargisand Phyan which formed over the region during 2004-2009 have been selected for this purpose. It has been found thatthere was a steep rise in scatterometer based surface vorticity before the formation of a cyclone in the cyclogenesisregion. The peak surface vorticity in the genesis region was observed on the day of intensification of the vortex to thedepression stage or a day earlier. However, the rising trend in the genesis region begins a few days before the formationof the system. Thus, the surface vorticity fields derived on the basis of scatterometer data can provide predictiveindication of the genesis of tropical cyclones over the Bay of Bengal and Arabian Sea with a lead time of 2-3 days. Usingthis technique it is possible to increase the lead time of pre-cyclone watch period over the north Indian Ocean. No relationship was found between the peak surface vorticity anomaly during the genesis phase and the surfacevorticity anomaly at the time of peak intensity of the system during its life cycle. In other words, the peak surfacevorticity anomaly during genesis phase does not provide any indication of future maximum intensity of the cyclone.

The role of the Maritime Continent SST anomalies in maintaining the Pacific–Japan pattern on decadal time scales

The decadal Pacific–Japan (PJ) pattern, the dominant decadal mode of summer vorticity anomaly over East Asia, is characterized as a meridionally arranged wave pattern with one anomalous cyclone located over Taiwan, and two anomalous anticyclones around the South China Sea (SCS) and the Bohai Sea. This pattern can cause wetter and colder conditions in Southeast China and dryer and warmer conditions in North China. Local SST–rainfall relationship reveals that the Maritime Continent (MC) SST can act as an engine to regulate and maintain the decadal PJ pattern. Driven by enhanced convection over the MC, anomalous divergent flows in the upper troposphere move northward, cross the equator and then converge and subside over the SCS. The SCS low-level divergence, maintained by this meridional overturning circulation under the Sverdrup vorticity balance, further works as a Rossby wave source and excites the decadal PJ pattern pointing straight northward. The transhemispheric impacts of the MC SST are well reproduced by both the atmospheric general circulation model and the dry linear baroclinic model, with the former emphasizing the MC’s original forcing role and the latter highlighting the SCS anticyclone’s role in relaying and amplifying those climatic impacts. Thus, our results indicate that SST variations over the MC region can be viewed as a potential source of East Asian decadal climate predictability.

Characteristics of Mesoscale Convective Complexes that Triggered Heavy Rainfall Related to Severe Flash Flood in Luwu, Sulawesi, Indonesia

A severe flash flood hit Luwu, Sulawesi, Indonesia, on 13 July 2020. This flood was preceded by persistent heavy rainfall from 11 to 13 July 2020. In this study, we explore both the physical and dynamical processes that caused the heavy rainfall using a convection-permitting model with 1-km resolution. The heavy rainfall was modulated by the development of a pair of Mesoscale Convective Complexes (MCCs) during the night. The pair of MCCs was triggered by an anti-cyclonic vorticity anomaly over the Makassar Strait and was maintained by the warm front passing between the sea and land over central Sulawesi. This front was characterized by moist-warm and cold-dry low-level air, which may have helped to extend the lifetime of the MCCs. The northwestward propagation of the MCCs was due to the interaction between predominantly southeasterly monsoon and sea surface temperature anomalies.

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.

How Does Indian Monsoon Regulate the Northern Hemisphere Stationary Wave Pattern?

The Northern Hemisphere summer climate isstrongly affected by a circumglobal stationary Rossby wave train, which can be manifested by the first EOF mode of the geopotential height at 200 hPa. Interannual variation of this Northern Hemisphere wave (NHW) pattern has a significant impact on remarkably warm surface temperature anomalies over the North Atlantic, Northeast Europe, East Asia to Central-North Pacific, and America, particularly in 2018 and 2010. The NHW pattern is likely generated by atmospheric diabatic heating and vorticity forcing: diabatic heating is mainly confined in the Indian summer monsoon (ISM) precipitation region, whereas the anti-cyclonic vorticity forcing is distributed in the globe. The ISM is a well-known diabatic heat source; however, the main source of vorticity forcing has not been established. In general, the tropical vorticity anomaly comes from diabatic heating-induced atmospheric waves and randomly generated inherent internal waves. The linear baroclinic model experiment reveals that the NHW pattern can be generated by the westward propagating tropical waves generated by the ISM diabatic heat forcing.

Indian monsoon derailed by a North Atlantic wavetrain

The forecast of Indian monsoon droughts has been predicated on the notion of a season-long rainfall deficit linked to a warm equatorial Pacific. Here we show that nearly half of all droughts over the past century differ from this paradigm in that they (i) occur when Pacific temperatures are near-neutral and (ii) are subseasonal phenomena, characterized by an abrupt decline in late-season rainfall. This severe subseasonal rainfall deficit can be associated with a Rossby wave from mid-latitudes. Specifically, we find that the interaction of upper-level winds with an episodic North Atlantic vorticity anomaly results in a wavetrain that curves toward East Asia, disrupting the monsoon. This atmospheric teleconnection offers an avenue for improved predictability of droughts, especially in the absence of telltale signatures in the Pacific.

Rapid Destruction of a Stratospheric Potential Vorticity Anomaly by Convectively Induced Inertial Instability during the Southern Wisconsin Extreme Flooding Event of 20 August 2018

The stalling and rapid destruction of a potential vorticity (PV) anomaly in the upper troposphere–lower stratosphere (UTLS) by convectively detrained inertially unstable air is described. On 20 August 2018, 10–15 in. (~0.3–0.4 m) of rain fell on western Dane County, Wisconsin, primarily during 0100–0300 UTC 21 August (1900–2100 CDT 20 August), leading to extreme local flooding. Dynamical aspects are investigated using the University of Wisconsin Nonhydrostratic Modeling System (UWNMS). Results are compared with available radiosonde, radar, total rainfall estimates, satellite infrared, and high-resolution European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Using ECMWF analyses, the formation of the UTLS PV anomaly is traced to its origin a week earlier in a PV streamer over the west coast of North America. The rainfall maximum over southern Wisconsin was associated with this PV anomaly, whereby convection forming in the warm-upglide sector rotated cyclonically into the region. The quasi-stationarity of this rainfall feature was aided by a broad northeastward surge of inertially unstable convective outflow air into southeastern Wisconsin, which coincided with stalling of the eastward progression of the PV anomaly and its diversion into southern Wisconsin, extending heavy rainfall for several hours. Cessation of rainfall coincided with dilution of the PV maximum in less than an hour (2100–2200 CDT), associated with the arrival of negative PV in the upper troposphere. The region of negative PV was created when convection over Illinois transported air with low wind speed into northeastward shear. This feature is diagnosed using the convective momentum transport hypothesis.

Static Stability Associated with Southern Hemisphere Blocking Onsets

<p>The horizontal and temporal variation of static stability prior to blocking onset is characterized through composite analysis of blocking events in the Southern Hemisphere. It is found that a local minimum of static stability in the upper troposphere and on the tropopause is achieved over the block-onset region when blocking onset takes place. From the perspective of isentropic potential vorticity, blocking onset is accompanied by extratropical tropopause elevation and a local low isentropic potential vorticity anomaly that is formed right under the elevated tropopause. This low isentropic potential vorticity anomaly is coincident with a local minimum of static stability over the block-onset region. In addition, based on static stability budget analysis, it revealed that the decrease of static stability in the upper troposphere and on the tropopuase prior to blocking onset is attributable to horizontal advection of low static stability from subtropics to midlatitude as well as the stretching effect associated with upper-level convergence, with the horizontal advection forcing being the primary contributor. On the other hand, the vertical advection of static stability tends to oppose the decreasing static stability through advecting more stable air downward such that it stabilizes the local air over the block-onset region. Furthermore, the indirect and direct effect of latent heat to the local change of static stability over the block-onset region are also discussed, respectively.</p>

Investigating Spatiotemporal Variation of Heatwave and its Association with Blocking in the Northeastern Asia

<p>Severe heatwaves in recent decades caused tremendous financial loss and even deaths. And both the occurrence and characteristics of heatwave are changing under global warming. The spatiotemporal variation and characteristics of heatwave in Northeastern Asia are investigated on both grid and event bases in this study. We find that persistent, extensive and intense heatwave has become more frequent during the last four decades. Such trend is found significantly correlated with the increase of temperature.</p><p>As most dreadful heatwaves are reported to be accompanied by blocking, we also thoroughly analyze the association between heatwave and blocking using two leading blocking indices, examining 500hpa geopotential height (TM index) and vertically averaged potential vorticity anomaly (PV index), respectively. A discrepancy between blocking climatology of TM index and PV index is exhibited, with the former displaying two high-frequency zonal bands at the south and north regions, while the latter only showing one high frequency band in the north. However, grid-based concurrence analysis using the two blocking indices agreeably suggests that blocking favors the occurrence of heatwave, especially in the north region where blocking often occurs. We further explicitly investigate their extended temporal association with time lags, which has not been done before in the literatures. It reveals that heatwave mostly occurs after or on the onset day of blocking and ends after or at the end of blocking. It indicates that blocking is more of a favorable environmental condition to trigger heatwave than maintain it. Lastly, the impact of blocking on the characteristics of heatwave events is explored on an event basis, using the 3D object model newly proposed by this study. Blocking related heatwave events are more likely to be more persistent, extensive and intense than unrelated events.</p>