| Warm rain refers to the rain produced within clouds with cloud top temperature above 0?.It is one of the primary precipitation types which widely spread and contributes appreciable amounts to total precipitation throughout the tropical and subtropical Pacific Ocean.Warm rain also has a significant influence on regulating the radiative balance,which is a strong forcing of the climate,especially in the troposphere.This prevalent marine cloud can increase the uncertainty in climate model simulations,because a full understanding of its cloud parameters and radiative forcing characteristics is currently lacking.Shipborne or ground-based observations provide only a limited detection capability with regard to oceanic warm rain.In our study,the work is mainly based on the merged datasets from TRMM PR and VIRS and their retrieved cloud parameters dataset.On this basis,precipitation and cloud characteristics of warm rain over the tropical and subtropical Pacific Ocean are investigated by using the merged dataset.Their radiative effects are also simulated by SBDART radiative transfer model.The differences of two extreme warm precipitations(extreme-light and extreme-heavy)are also calculated in their precipitation,cloud and radiative forcing effects.1.Precipitation and spectral characteristics of warm precipitationMost of warm rain occurs in central ITCZ and its flanks which include SPCZ and northwest Pacific.Most of precipitation in southeast Pacific and east of Hawaii are the warm rain.There are three warm rain types(phased,pure and mixed)exist over these regions.The phased warm rain,which occurs during the developing or declining stage of precipitation weather systems,is located over the central to western Intertropical Convergence Zone,South Pacific Convergence Zone,and Northwest Pacific.Its occurrence frequency peaks at midnight and minimizes during daytime with a 5.5 km maximum echo top.The frequency of this warm rain type is about 2.2%,and it contributes to 40%of the regional total rainfall.The pure warm rain is characterized by typical stable precipitation with an echo top lower than 4 km,and mostly occurs in Southeast Pacific.Although its frequency is less than 1.3%,this type of warm rain accounts for 95%of the regional total rainfall.Its occurrence peaks before dawn and it usually disappears in the afternoon.For the mixed warm rain,some may develop into deep convective precipitation,while most are similar to those of the pure type.The mixed warm rain is mainly located over the ocean east of Hawaii.Its frequency is 1.2%,but this type of warm rain could contribute to 80%of the regional total rainfall.The results also uncover that the mixed and pure types occur over the regions where SST ranges from 295 to 299 K,accompanied by relatively strong downdrafts at 500 hPa.Both the mixed and pure warm rains happen in a more unstable atmosphere,compared with the phased warm rain.2.Cloud characteristics of warm precipitating cloudMost of warm precipitating clouds have an averaged Re of 15.6 ?m,Tau of 16,and LWP of 140 g/m2 over tropical and subtropical Pacific.In the western Pacific,the cloud particles are larger(?16 ?m),and the corresponding Tau(?13)and LWP(?150 g/m2)are relatively low.During summer and autumn,the Tau(?9)in high-frequency warm pool region is significantly lower than that in winter and spring.The thin and high warm precipitating cloud has smaller Re and higher Tau with 273?283 K BT4 and 4-5 km echo top height.While for the thick and high warm precipitating cloud,the Re is?5.6 ?m and the LWP is 300 g/m2 with the echo top height lower than 4 km.The Re of thin and low type is larger with the increased echo top,especially in the height higher than 5 km.Most of the thick and low warm precipitating cloud has larger Re and higher brightness temperature with height lower than 3 km.Smaller Re of warm precipitating cloud more appears in 0900?1000 LST and 1200?1300 LST.When the accumulated sun radiative heat increases ocean atmosphere moisture,the Re of warm precipitating cloud become larger and Tau becomes higher.In addition,the upper layers of the Re are larger than the lower layers during morning and moon,the falling process of cloud particles creates much larger Re and stronger reflectivity in lower height of Tau.During 1500?1600 LST,the Tau and LWP are higher than the other period.Moreover,the cloud particles in upper layers are larger than in lower layers.The Re becomes smaller and the echo intensity is weaker during the descending process.3.Precipitation,cloud and radiative effects of extreme light and heavy warm precipitating cloudThe distributions of higher frequency for extreme-light and extreme-heavy precipitating clouds are northwest Pacific,warm pool,ITCZ and SPCZ,which is similar to the total warm precipitating cloud.In addition,most of extreme-light precipitating cloud has higher ratio to total warm precipitating cloud in Pacific warm pool,which has more instability in the region and more accompanied by stronger convective motion.While most of extreme-heavy warm precipitating clouds in total warm precipitating clouds are distributed in Indonisian of ocean,SPCZ and ocean in west of Mexico,which accounts for more than 16%of the total warm precipitating cloud.Most of warm precipitations in SPCZ are phased warm rain.Extreme-light type is a little larger averaged Re than extreme-heavy type,and its Tau and LWP are 14 and120 g/m2,while the extreme-heavy type has 15.6 ?m of Re,26 of Tau and 250 g/m2 of LWP.Echo top is generally decreasd with the increase of BT4.When echo top is lower than 3.5 km,the extreme-heavy warm precipitating cloud generates stronger precipitation through larger Re coalescence.However,this is not obvious in extreme-light type possibly due to a certain echo height to generate a light precipitation.Extreme-light type induces a cooling of approximately-0.5 W/m2 for radiative forcing,which is-3.0 W/m2 less than the extreme-heavy type.The Re in 15.8 ?m corresponds to the highest negative forcing about-0.45 W/m2.Most of the radiative effects for warm precipitationg cloud are influenced by cloud fraction and Tau. |
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