Model Simulations of Mesoscale Eddies and Deep Convection in the Labrador Sea

Deep convection in the Labrador Sea is confined within a small region in the southwest part of the basin.The strength of deep convection in this region is related to the local atmospheric and ocean characteristics,which favor processes of deep convection preconditioning and intense air-sea exchange during the winter season.In this study,we explored the effect of eddy-induced flux transport on the stratification of the Labrador Sea and the properties of deep convection.Simulations from an eddy-resolving ocean model are presented for the Labrador Sea.The general circulation was well simulated by the model,including the seasonal cycle of the deep Labrador Current.The simulated distribution of the surface eddy kinetic energy was also close to that derived from Topex-Poseidon satellite altimeter data,but with smaller magnitude.The energy transfer diagnostics indicated that Irminger rings are generated by both baroclinic and barotropic processes; however,when they propagate into the interior basin,the barotropic process also disperses them by converting the eddy energy to the mean flow.In contrast to eddy-permitting simulations,deep convection in the Labrador Sea was better represented in the eddyresolving model regarding their lateral position.Further analysis indicated that the improvement might be due to the lateral eddy flux associated with the resolved Irminger rings in the eddy-resolving model,which contributes to a realistic position of the isopycnal dome in the Labrador Sea and correspondingly a realistic site of deep convection.

Seasonal to sub-seasonal variations of the Asian Tropopause Aerosols Layer affected by the deep convection,surface pollutants and precipitation

The Asian Tropopause Aerosols Layer(ATAL)refers to an accumulation of aerosols in the upper troposphere and lower stratosphere during boreal summer over Asia,which has a fundamental impact on the monsoon system and climate change.In this study,we primarily analyze the seasonal to sub-seasonal variations of the ATAL and the factors potentially influencing those variations based on MERRA2 reanalysis.The ability of the reanalysis to reproduce the ATAL is well validated by CALIPSO observations from May to October 2016.The results reveal that the ATAL has a synchronous spatiotemporal pattern with the development and movement of the Asian Summer Monsoon.Significant enhancement of ATAL intensity is found during the prevailing monsoon period of July-August,with two maxima centered over South Asia and the Arabian Peninsula.Owing to the fluctuations of deep convection,the ATAL shows an episodic variation on a timescale of 7-12 days.Attribution analysis indicates that deep convection dominates the variability of the ATAL with a contribution of 62.7%,followed by a contribution of 36.6%from surface pollutants.The impact of precipitation is limited.The ATAL further shows a clear diurnal variation:the peak of ATAL intensity occurs from 17:30 to 23:30 local time(LT),when the deep convection becomes strongest;the minimum ATAL intensity occurs around 8:30 LT owing to the weakened deep convection and photochemical reactions in clouds.The aerosol components of the ATAL show different spatiotemporal patterns and imply that black carbon and organic carbon come mainly from India,whereas sulfate comes mainly from China during the prevailing monsoon period.

Numerical simulations of Atlantic meridional overturning circulation(AMOC)from OMIP experiments and its sensitivity to surface forcing

Atlantic meridional overturning circulation(AMOC)plays an important role in transporting heat meridionally in the Earth’s climate system and is also a key metrical tool to verify oceanic general circulation models.Two OMIP(Ocean Model Intercomparison Project phase 1 and 2)simulations with LICOM3(version 3 of the LASG/IAP Climate System Ocean Model)developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics(LASG),Institute of Atmospheric Physics(IAP),are compared in this study.Both simulations well reproduce the fundamental characteristics of the AMOC,but the OMIP1 simulation shows a significantly stronger AMOC than the OMIP2 simulation.Because the LICOM3 configurations are identical between these two experiments,any differences between them must be attributed to the surface forcing data.Further analysis suggests that sea surface salinity(SSS)differences should be mainly responsible for the enhanced AMOC in the OMIP1 simulation,but sea surface temperature(SST)also play an unignorable role in modulating AMOC.In the North Atlantic,where deep convection occurs,the SSS in OMIP1 is more saline than that in OMIP1.We find that in the major region of deep convection,the change of SSS has more significant effect on density than the change of SST.As a result,the SSS was more saline than that in OMIP2,leading to stronger deep convection and subsequently intensify the AMOC.We conduct a series of numerical experiments with LICOM3,and the results confirmed that the changes in SSS have more significant effect on the strength of AMOC than the changes in SST.

Analysis of Deep Convective Towers in a Southwest-Vortex Rainstorm Event

The structure and organization of the extreme-rain-producing deep convection towers and their roles in the formation of a southwest vortex(SWV)event are studied using the intensified surface rainfall observations,weather radar data and numerical simulations from a high-resolution convection-allowing model.The deep convection towers occurred prior to the emergence of SWV and throughout its onset and development stages.They largely resemble the vortical hot tower(VHT)commonly seen in typhoons or hurricanes and are thus considered as a special type of VHT(sVHT).Each sVHT presented a vorticity dipole structure,with the upward motion not superpose the positive vorticity.A positive feedback process in the SWV helped the organization of sVHTs,which in turn strengthened the initial disturbance and development of SWV.The meso-γ-scale large-value areas of positive relative vorticity in the mid-toupper troposphere were largely induced by the diabatic heating and tilting.The strong mid-level convergence was attributed to the mid-level vortex enhancement.The low-level vortex intensification was mainly due to low-level convergence and the stretching of upward flow.The meso-α-scale large-value areas of positive relative vorticity in the low-level could expand up to about 400 hPa,and gradually weakened with time and height due to the decaying low-level convergence and vertical stretching in the matured SWV.As the SWV matured,two secondary circulations were formed,with a weaker mean radial inflow than the outflow and elevated to 300-400 hPa.

Deep Convective Clouds over the Northern Pacific and Their Relationship with Oceanic Cyclones

Based on combined CloudSat/CALIPSO detections, the seasonal occurrence of deep convective clouds （DCCs） over the midlatitude North Pacific （NP） and cyclonic activity in winter were compared. In winter, DCCs are more frequent over the central NP, from approximately 30~N to 45~N, than over other regions. The high frequencies are roughly equal to those occurring in this region in summer. Most of these DCCs have cloud tops above a 12 km altitude, and the highest top is approximately 15 km. These wintertime marine DCCs commonly occur during surface circulation conditions of low pressure, high temperature, strong meridional wind, and high relative humidity. Further, the maximum probability of DCCs, according to the high correlation coefficient, was found in the region 10^-20~ east and 5^-10~ south of the center of the cyclones. The potential relationship between DCCs and cyclones regarding their relative locations and circulation conditions was also identified by a case study. Deep clouds were generated in the warm conveyor belt by strong updrafts from baroclinic flows. The updrafts intensified when latent heat was released during the adjustment of the cyclone circulation current. This indicates that the dynamics of cyclones are the primary energy source for DCCs over the NP in winter.

Weak ENSO Asymmetry Due to Weak Nonlinear Air–Sea Interaction in CMIP5 Climate Models

State-of-the-art climate models have long-standing intrinsic biases that limit their simulation and projection capabilities. Significantly weak ENSO asymmetry and weakly nonlinear air-sea interaction over the tropical Pacific was found in CMIP5 （Coupled Model Intercomparison Project, Phase 5） climate models compared with observation. The results suggest that a weak nonlinear air-sea interaction may play a role in the weak ENSO asymmetry. Moreover, a weak nonlinearity in air-sea interaction in the models may be associated with the biases in the mean climate--the cold biases in the equatorial central Pacific. The excessive cold tongue bias pushes the deep convection far west to the western Pacific warm pool region and suppresses its development in the central equatorial Pacific. The deep convection has difficulties in further moving to the eastern equatorial Pacific, especially during extreme El Nifio events, which confines the westerly wind anomaly to the western Pacific. This weakens the eastern Pacific El Nifio events, especially the extreme E1 Nifio events, and thus leads to the weakened ENSO asymmetry in climate models. An accurate mean state structure （especially a realistic cold tongue and deep convection） is critical to reproducing ENSO events in climate models. Our evaluation also revealed that ENSO statistics in CMIP5 climate models are slightly improved compared with those of CMIP3. The weak ENSO asymmetry in CMIP5 is closer to the observation. It is more evident in CMIP5 that strong ENSO activities are usually accompanied by strong ENSO asymmetry, and the diversity of ENSO amplitude is reduced.

Establishment of the South Asian High over the Indo-China Peninsula During Late Spring to Summer

The establishment of the upper-level South Asian high （SAH） over the Indo-China Peninsula （ICP） during late boreal spring and its possible causes are investigated using long-term NCEP-NCAR and ERA-40 reanalysis and satellite-observed OLR data. Results show that, from early March to mid-April, deep convection stays south of -6°N over the northern Sumatran islands. As the maximum solar radiation moves over the latitudes of the ICP （10-20°N） in late April, the air over the ICP becomes unstable. It ascends over the ICP and descends over the adjacent waters to the east and west. This triggers deep convection over the ICP that induces large latent heating and strong updrafts and upper-level divergence, leading to the formation of an upper-level anticyclonic circulation and the SAH over the ICE During early to mid-May, deep convection over the ICP intensifies and extends northwards to the adjacent waters. Strong latent heating from deep convection enhances and maintains the strong updrafts and upper-level divergence, and the SAH is fully established by mid-May. Thus, the seasonal maximum solar heating and the land-sea contrast around the ICP provide the basic conditions for deep convection to occur preferentially over the ICP, which leads to the formation of the SAH over the ICP from late April to mid-May. Simulations using RegCM4 also indicate that the diabatic heating over the ICP is conducive to the generation and development of upper-level anticyclonic circulation, which leads to an earlier establishment of the SAH.

Variability of Atlantic Meridional Overturning Circulation in FGOALS-g2

The variability of Atlantic Meridional Overturning Circulation （AMOC） in the pre-industrial control experiment of the Flexible Global Ocean-Atmosphere-Land System model, Grid-point Version 2 （FGOALS-g2） was investigated using the model outputs with the most stable state in a 512-yr time window from the total 1500-yr period of the experiment. The period of AMOC in FGOALS-g2 is double peaked at 20 and 32 years according to the power spectrum, and 22 years according to an auto-correlation analysis, which shows very obvious decadal variability. Like many other coupled climate models, the decadal variability of AMOC in FGOALS-g2 is closely related to the convection that occurs in the Labrador Sea region. Deep convection in the Labrador Sea in FGOALS-g2 leads the AMOC maximum by 3-4 years. The contributions of thermal and haline effects to the variability of the convection in three different regions [the Labrador, Irminger and Greenland-Iceland- Norwegian （GIN） Seas] were analyzed for FGOALS-g2. The variability of convection in the Labrador and Irminger Seas is thermally dominant, while that in the colder GIN Seas can be mainly attributed to salinity changes due to the lower thermal expansion. By comparing the simulation results from FGOALS-g2 and 11 other models, it was found that AMOC variability can be attributed to salinity changes for longer periods （longer than 35 years） and to temperature changes for shorter periods.

Convective Rainfall in Lake Victoria Watershed and Adjacent Equatorial Africa

An integrated satellite precipitation estimation dataset, namely, the Climate Prediction Center morphing method (CMORPH), was used to analyze precipitation regimes across Equatorial Africa between 3°S - 1°N and 24°E - 42°E from 2000 to 2014. This region includes the Rift Valley, part of the Congo Forest, and the Lake Victoria (LV) basin, the second largest lake in the area of the world. Hovmöller diagrams were obtained for all organized convective systems to estimate their spans, duration, and phase speeds. The analysis included 33,189 episodes of westward propagating convective systems. Within the study area, lake and land breezes tend to trigger convection and precipitation over LV as well as mountain-valley circulation trigger thunderstorms over the mountains east of LV and western Rift Valley. The statistics of convective systems streaks on longitude-time diagrams were obtained for yearly frequencies of starting and ending longitudes and times among other morphologic variables. Results indicate organized precipitation episodes tend to move westward across Rift valley and Congo forest with an average phase speed of 10.3 m·s-1. More than 50% of them are triggered over LV and propagate more than 600 km at an average phase speed of 12.1 m·s-1. These convective systems tend to produce high rainfall rates hundreds of kilometers away into the Congo Forest. Half of all episodes of organized convection analyzed have phase speeds between 8 m·s-1 and 16 m·s-1, lasting 8 hr to 16 hr. Most precipitating systems start east of LV and west of Rift Valley in the afternoon to early morning and propagates less than 400 km. Finally, hourly precipitation accumulation and lightning density analysis indicate three preferable regions for convective initiation: 1) The mountain range east of LV;2) Midwest of LV, and;3) The Congo Forest mountain range.

A CASE STUDY OF THE UNCORRELATED RELATIONSHIP BETWEEN TROPICAL TROPOPAUSE TEMPERATURE ANOMALIES AND STRATOSPHERIC WATER VAPOR ANOMALIES

Using the measurements from the Halogen Occultation Experiment(HALOE) and the European Centre for Medium-Range Weather Forecasts(ECMWF) Interim reanalysis data for the period 1994-2005, we analyzed the relationship between tropical tropopause temperature anomalies and stratospheric water vapor anomalies. It is found that tropical tropopause temperature is correlated with stratospheric water vapor, i.e., an anomalously high(low) tropical tropopause temperature corresponds to anomalously high(low) stratospheric water vapor during the period 1994-2005,except for 1996. The occurrence frequency and strength of deep convective activity during the‘mismatched'months is less and weaker than that during the‘matched'months in 1996. However, the instantaneous intensity of four short periods of deep convective activity, caused by strong surface cyclones and high sea surface temperatures, are greater during the ‘mismatched'months than during the ‘matched'months. Water vapor is transported from the lower troposphere to the lower stratosphere through a strong tropical upwelling, leading to an increase in stratospheric water vapor. On the other hand, deep convective activity can lift the tropopause and cool its temperature. In short, the key factor responsible for the poor correlation between tropical tropopause temperature and stratospheric water vapor in1996 is the instantaneous strong deep convective activity. In addition, an anomalously strong Brewer-Dobson circulation brings more water vapor into the stratosphere during the‘mismatched'months in 1996, and this exacerbates the poor correlation between tropical tropopause temperature and stratospheric water vapor.

Application of the rotating-convection paradigm for tropical cyclones to interpreting medicanes:An example

The rotating-convection paradigm for tropical cyclone behaviour is shown to provide an attractive and consistent framework for interpreting the dynamics of formation and intensification of at least some medicanes.The ideas are illustrated by a case study of the medicane that formed over the eastern Mediterranean in mid-December 2020.This case study is based on analyses of data from the European Centre for Medium Range Weather Forecasts(ECMWF),imagery from the European geostationary meteorological satellite,Meteosat Second Generation,and output from a convection permitting numerical simulation of the event using the United Kingdom(UK)Met Office regional model with the RAL2 physics configuration.Limitations of the currently widely accepted interpretation of medicanes in terms of the so-called Wind-Induced Surface Heat Exchange(WISHE)intensification mechanism are discussed.

Intensified Impact of the Equatorial QBO in August–September on the Northern Stratospheric Polar Vortex in December–January since the Late 1990s

This study reveals an intensified impact of the equatorial quasi-biennial oscillation(QBO)in August–September(QBO_AS)on the northern stratospheric polar vortex(SPV)in December–January(SPV_DJ)since the late 1990s.The unstable relationship may be related to the differences in the deep convection anomaly over the tropical western Pacific and Indian Oceans in October–November(ON)related to the QBO_AS prior to and after the late 1990s.During 1998–2017,the easterly phase of the QBO_AS is accompanied by a colder tropical tropopause in ON,which enhances the deep convective activity over the tropical western Pacific and suppresses it over the Indian Ocean.The deep convection anomaly generates anomalous Rossby waves that propagate into the northern mid-to-high latitudes to constructively interfere with the climatological wavenumber-1 and wavenumber-2 components,thereby resulting in enhanced upward-propagating tropospheric planetary-scale waves and a weakened SPV_DJ anomaly.During1979–1997,however,the deep convection anomaly over the tropical western Pacific and Indian Oceans in ON related to the easterly phase of the QBO_AS is weaker and shifts eastward,which excites the anomalous Rossby waves to constructively/destructively interfere with the climatological wavenumber-1 component in the midlatitudes/high latitudes,thereby weakening the upward-propagating planetary-scale waves and leading to a weaker linkage with the SPV_DJ.Further analyses reveal that the unstable relationship may be associated with the interdecadal differences in deep convection over the tropical western Pacific and Indian Oceans and the upward-propagating tropospheric planetary-scale waves in ON.

Diagnostic Analysis of the Quasi-Balanced Flow of a Mesoscale Vortex During the 12 June 2008 Guangxi Rainstorm

By using the high-resolution observation data and MM5 model simulation data,the analysis on the 12 June 2008 Guangxi flash-flood rainstorm shows that the associated major mesoscale weather system of this event is a quasi-stationary mesoscale vortex,which resulted from the interaction between the midlatitude synoptic-scale waves in the westerly belt and the low-latitude warm-moist flow under the terrain effect.The genesis,development,and movement of the mesoscale vortex have significant impacts on the intensity and persistence of the severe precipitation from the Guangxi flash-flood rainstorm.This vortex is characterized by the coexistence of strong vorticity and divergence with the same order of magnitude.Well organized,deep,and moist convection was observed for a long period of time,and was produced by the interaction between the mesoscale vortex and the gravity waves.The latter was generated by the terrain effect and the ageostrophic effect of high winds in the low-level jet.According to the quasi-balanced dynamical theory,quasi-balanced flow must have existed in the mesoscale motions with both divergent and rotational winds.Thus,based on the diagnosis of the quasi-balanced flow,the PV-ω inversion method is employed to analyze the organized moist convection.The results show that 50%-70% of the vertical circulation in the rainstorm areas was quasi-balanced,so the quasi-balanced flow could well reflect features of the strong vertical motions associated with the coexistence of vorticity and divergence during this event.