Propagation and influence on tropical precipitation of intraseasonal variation over mid-latitude East Asia in boreal winter

The authors investigate the characteristics of propagation and the influence on tropical precipitation of 9–29-day intraseasonal variation over midlatitude East Asia during boreal winter, and find that the intraseasonal wind signal can propagate both eastward and southward. In the case of eastward propagation, the intraseasonal wind signal is mainly confined to the midlatitudes, featuring eastward migration of anomalous cyclones and anticyclones. In the case of southward propagation, intraseasonal meridional wind perturbations may extend from the mid to the low latitudes, and even the equatorial region. The accompanying wind convergence/divergence induces anomalous precipitation in the near-equatorial regions, forming a north–south dipole precipitation anomaly pattern between the southern South China Sea and the eastern China– Japan region. An anomalous meridional overturning circulation plays an important role in linking tropical and midlatitude intraseasonal wind and precipitation variations.

Estimating Tropical Cyclone Precipitation from Station Observations

In this paper, an objective technique for estimating the tropical cyclone （TC） precipitation from station observations is proposed. Based on a comparison between the Original Objective Method （OOM） and the Expert Subjective Method （ESM）, the Objective Synoptic Analysis Technique （OSAT） for partitioning TC precipitation was developed by analyzing the western North Pacific （WNP） TC historical track and the daily precipitation datasets. Being an objective way of the ESM, OSAT overcomes the main problems in OOM, by changing two fixed parameters in OOM, the thresholds for the distance of the absolute TC precipitation （D0） and the TC size （D1）, into variable parameters.Case verification for OSAT was also carried out by applying CMORPH （Climate Prediction Center MORPHing technique） daily precipitation measurements, which is NOAA＇s combined satellite precipitation measurement system. This indicates that OSAT is capable of distinguishing simultaneous TC precipitation rain-belts from those associated with different TCs or with middle-latitude weather systems.

Contribution of El Nino amplitude change to tropical Pacific precipitation decline in the late 1990s

Equatorial central Pacific precipitation experienced a prominent decline in the late 1990 s.This change was previously attributed to a La Nina-like mean sea surface temperature(SST)change in the Pacific Ocean associated with a phase switch of the Interdecadal Pacific Oscillation.Here,using a series of model experiments,the authors reveal that the El Nino-related interannual SST anomalies contributed largely to the precipitation decrease over the equatorial central Pacific.This El Nino SST effect was due to the change in the amplitude of El Nino events in the late 1990 s.The 1980-98 decade had more large-amplitude El Nino events than the 1999-2014 decade.The nonlinear precipitation response to SST anomalies resulted in a larger decadal mean precipitation in the 1980-98 decade than in the 1999-2014 decade.The results highlight the importance of El Nino amplitude change in future climate change related to global warming.

ANALYSIS OF TROPICAL CYCLONE PRECIPITATION FOR DIFFERENT INTENSITY CLASS IN NORTHWEST PACIFIC WITH TRMM DATA

Combined with TRMM products and Tropical Cyclone(TC) best track data in Northwest Pacific from 1 January 2003 to 31 December 2009,a total of 118 TCs,including 336 instantaneous TC precipitation observations are established as the TRMM TC database,and the database is stratified into four intensity classes according to the standard of TC intensity adopted by China Meteorological Administration(CMA):Severe Tropical Storm(STS),Typhoon(TY),Severe Typhoon(STY) and Super Typhoon(SuperT Y).For each TC snapshot,the mean rainfall distribution is computed using 10-km annuli from the TC center to a 300-km radius,then the axisymmetric component of TC rainfall is represented by the radial distribution of the azimuthal mean rain rate;the mean rain rates,rain types occurrence and contribution proportion are computed for each TC intensity class;and the mean quadrantal distribution of rain rates along TCs motion is analyzed.The result shows that:(1) TCs mean rain rates increase with their intensity classes,and their radial distributions show single-peak characteristic gradually,and furthermore,the characteristics of rain rates occurrence and contribution proportion change from dual-peak to single-peak distribution,with the peak rain rate at about 5.0mm/h;(2) Stratiform rain dominate the rain type in the analysis zone,while convective rain mainly occurred in the eye-wall region;(3) The values of mean rain rate in each quadrant along TCs motion are close to each other,relatively,the value in the right-rear quadrant is the smallest one.

Relationship over southern China between the summer rainfall induced by tropical cyclones and that by monsoon

This study investigates the relationship between tropical cyclone-induced precipitation（P_（TC）） and summer monsoon-induced precipitation（P_（SM）） in southern China（SC） during June-August.The spatial patterns of the first interannual mode are uniform in sign over SC,with positive anomalies for P_（TC） and negative anomalies for P_（SM）.The background of an increase in cyclonic vorticity,an increase in RH,and a decrease in vertical wind shear over the South China Sea（SCS）-western north Pacific（WNP） provides favorable conditions for more TC genesis.The positive equatorial central Pacific SST anomaly and negative North Indian Ocean SST anomaly contribute to the anomalous cyclone over the SCS-WNP,which causes decreasing P_（SM） in SC together with an anomalous anticyclone over eastern China-Japan.By contrast,whilst the spatial patterns of the first interdecadal mode are also uniform in sign over SC,there are positive anomalies for both P_（TC） and P_（SM）.The first interdecadal principal component features significant positive correlation with the number of TCs forming in the SCS.There is a significant increase in P_（TC） and P_（SM） after early 1990 s.A positive tropical Indian Ocean（TIO） SST anomaly persists from the preceding winter to summer.During the preceding winter and spring after the early 1990 s,a positive western Pacific SST anomaly can result in TIO SST warming through vertical circulation.Then,the positive TIO SST anomaly triggers an anomalous WNP anticyclone and contributes to the interdecadal increase in SC P_（SM） in the succeeding summer.The persistent heating source over SC from May to summer related to an earlier onset of the SCS monsoon may strengthen the East Asian subtropical summer monsoon.

Evaluation of Latest TMPA and CMORPH Precipitation Products with Independent Rain Gauge Observation Networks over High-latitude and Low-latitude Basins in China

The Tropical Rainfall Measuring Mission （TRMM） Multi-satellite Precipitation Analysis （TMPA） and National Oceanic and Atmospheric Administration （NOAA） Climate Prediction Center （CPC） morphing technique （CMORPH） are two important multi-satellite precipitation products in TRMM-era and perform important functions in GPM-era. Both TMPA and CMORPH systems simultaneously upgraded their retrieval algorithms and released their latest version of precipitation data in 2013. In this study, the latest TMPA and CMORPH products （i.e., Version-7 real-time TMPA （T-rt） and gauge-adjusted TMPA （T-adj）, and Version- 1.0 real-time CMORPH （C-rt） and Version-l.0 gauge-adjusted CMORPH （C-adj）） are evaluated and intercompared by using independent rain gauge observations for a 12-year （2000--2011） period over two typical basins in China with different geographical and climate conditions. Results indicate that all TMPA and CMORPH products tend to overestimate precipitation for the high-latitude semiarid Laoha River Basin and underestimate it for the low-latitude humid Mishui Basin. Overall, the satellite precipitation products exhibit superior performance over Mishui Basin than that over Laoha River Basin. The C-adj presents the best performance over the high-latitude Laoha River Basin, whereas T-adj showed the best performance over the low-latitude Mishui Basin. The two gauge-adjusted products demonstrate potential in water resource management. However, the accuracy of two real-time satellite precipitation products demonstrates large variability in the two validation basins. The C-rt reaches a similar accuracy level with the gauge-adjusted satellite precipitation products in the high-latitude Laoha River Basin, and T-rt performs well in the low-latitude Mishui Basin. The study also reveals that all satellite precipitation products obviously overestimate light rain amounts and events over Laoha River Basin, whereas they underestimate the amount and events over Mishui Basin. The findings of the precision characteristics associated with the latest TMPA and CMORPH precipitation products at different basins will offer satellite pre- cipitation users an enhanced understanding of the applicability of the latest TMPA and CMORPH for water resource management, hydrologic process simulation, and hydrometeorological disaster prediction in other similar regions in China. The findings will also be useful for IMERG algorithm development and update in GPM-era.

Contribution of tropical cyclone rainfall at categories to total precipitation over the Western North Pacific from 1998 to 2007

Based on 10 years precipitation data from Tropical Rainfall Measurement Mission （TRMM） Multi-satellite Precipitation Analysis （TMPA） 3B42 and the best track data from China Meteorological Administration （CMA）, the seasonal, monthly and annual contribution of tropical cyclone （TC） precipitation to the total rainfall are analyzed over the Western North Pacific （WNP） during 1998 to 2007 from May to December. The results show that： （1） TC seasonal rainfall contribution ranges from 4% in inland regions to above 40% in ocean-regions of 15°N-25°N. TCs at higher categories contribute much more to the total precipitation. （2） On monthly scale, TCs contribute 60% to the total rainfall regionally during whole TC season, which is the maximum contribution. The peak contribution of TC rainfall averaged in multi-months of the ten years occurs in August （28%） over the whole ocean impacted by TC and in December （23%） over the whole land impacted by TC, respectively. （3） On annual scale, the maximum contribution of TC precipitation to the total rainfall are in 2004 （-30%） over ocean and in 1998 （-20%） over land, respectively. （4） The contribution of TC precipitation to the total rainfall increases 6% （decreases 6%） in El Nino （La Nifia） years compared with neutral years.

Relationship of tropical-cyclone-induced remote precipitation with tropical cyclones and the subtropical high

This study concerns the precipitation induced by a tropical cyclone （TC） before the TC arrives, which will be referred to as TC remote precipitation （TRP）. Based on the distribution characteristics of the non-rotational wind and the divergent-wind vertical circulation related to TC, the subtropical high, and TRP of 45 TRP events during June, July, and August of 2000-2009, the relationships among these three entities （TC, subtropical high, and TRP） can be categorized into four patterns. The first pattern accounts for the highest proportion of the TRP events （59%）, and a conceptual model is then provided for this pattern. The primary characteristics of this model are as follows： TC, the subtropical high, and TRP can interact with each other through the divergent-wind secondary circulation at both sides of the ridge line of the subtropical high （between the subtropical high and TC, and between the subtropical high and TRP）. At the upper level （150 or 200 hPa）, the northward non-rotational wind from the TC converged toward the subtropical high ridge line and subsided, and at 950 hPa the divergent wind from the ridge line of the subtropical high converged toward TC; these constructed the secondary circulation between TC and the subtropical high. In the meantime, the southward non- rotational wind at the upper level （150 or 200hPa） from TRP and the divergent wind at 950hPa from the subtropical high ridge line toward TRP constructed the secondary circulation between TRP and the subtropical high. As TC and TRP interacted with each other, the subtropical high ridge line was usually under the down- draft area of the whole atmosphere. The other three patterns are different from the first pattern mainly in terms of the intensity and position of the non-rotational-wind secondary circulation. The numerical simulation of the Beijing 7.21 rainstorm confirmed the relationship amongTC, the subtropical high, and TRP, indicating that when the interaction weakened, the TRP also weakened.

Contribution of Winter SSTA in the Tropical Eastern Pacific to Changes of Tropical Cyclone Precipitation over Southeast China

Tropical cyclone precipitation(TCP)accounts for 10%-40%of the boreal summer precipitation that occurs over Southeast China(SEC),causing flood disasters and serious damage.On the decadal scale,TCP increases significantly in SEC while TC frequency decreases in the western North Pacific(WNP)during 1980-2019.Therefore,variations in TCP and the corresponding physical mechanism are investigated in this study.First,an empirical statistical method is introduced to quantify the TCP amount based on accumulated cyclone energy(ACE)and TC frequency with the TCP anomaly decomposed into three items(rainfall frequency,rainfall intensity,and nonlinear item).ACE,as the integration of TC intensity and frequency,is a more effective index than TC frequency for depicting the characteristics of TCP because the contribution of rainfall frequency represented by ACE is higher than that of TC frequency.Then,the physical mechanism affecting the WNP TC activities and TCP in SEC are inspected.Positive sea surface temperature anomaly(SSTA)over the tropical eastern Pacific(TEP)in winter can trigger variations of air-sea interaction over the tropical Pacific,including low-level divergent winds,mid-tropospheric descent flows,high-level convergent winds coupled with negative anomalies of vorticity and humidity over the tropical western Pacific(TWP)in the next summer.These dynamic conditions provide unfavorable environments for TC activities in the WNP and constrain TCP in SEC.Furthermore,more significantly negative SSTA events in the TEP facilitate enhanced ACE along with positive relative vorticity,relative humidity,and upwelling vertical winds anomalies over the coast of SEC after 1998,which is a reasonable explanation for the increasing TCP in SEC.

Estimation of mean water vapour residence time during tropical cyclones using a Lagrangian approach

Tropical cyclone(TC)-related rainfall mostly depends on the atmospheric moisture uptake from local and remote sources.In this study,the mean water vapour residence time(MWVRT)was computed for precipitation related to TCs in each basin and on a global scale by applying a Lagrangian moisture source diagnostic method.According to our results,the highest MWVRT was found for the TCs over the South Indian Ocean and South Pacific Ocean basins(~3.08 days),followed by the Western North Pacific Ocean,Central and East North Pacific Ocean,North Indian Ocean,and North Atlantic Ocean basins(which exhibited values of 2.98,2.94,2.85,and 2.72 days,respectively).We also found a statistically significant(p<0.05)decrease in MWVRT,at a rate of~2.4 h/decade in the North Indian Ocean and~1.0 h/decade in the remaining basins.On average,the MWVRT decreased during the 24 h before TCs made landfall,and the atmospheric parcels precipitated faster after evaporation when TCs moved over land than over the ocean.Further research should focus on the relationship between global warming and MWVRT of atmospheric parcels that precipitate over TC positions.

Fundamental Characteristics of Tropical Rain Cell Structures as Measured by TRMM PR

Rain cells are the most elementary unit of precipitation system in nature.In this study,fundamental geometric and physical characteristics of rain cells over tropical land and ocean areas are investigated by using 15-yr measurements of the Tropical Rainfall Measuring Mission(TRMM)Precipitation Radar(PR).The rain cells are identified with a minimum bounding rectangle(MBR)method.The results indicate that about 50%of rain cells occur at length of about 20 km and width of 15 km.The proportion of rain cells with length>200 km and width>100 km is less than1%.There is a a log-linear relationship between the mean length and width of rain cells.Usually,for the same horizontal geometric parameters,rain cells tend to be square horizontally and lanky vertically over land,while vertically squatty over ocean.The rainfall intensity of rain cells varies from 0.4 to 10 mm h-1 over land to 0.4–8 mm h-1 over ocean.Statistical results indicate that the occurrence frequency of rain cells decreases as the areal fraction of convective precipitation in rain cells increases,while such frequency remains almost invariant when the areal fraction of stratiform precipitation varies from 10%to 80%.The relationship between physical and geometric parameters of rain cells shows that the mean rain rate of rain cells is more frequently associated with the increase of their area,with the increasing rate over land greater than that over ocean.The results also illustrate that heavy convective rain rate prefers to occur in larger rain cells over land while heavy stratiform rain rate tends to appear in larger rain cells over ocean.For the same size of rain cells,the areal fraction and the contribution of convective precipitation are about10%–15%higher over land than over ocean.