Yunfei Fu

The Variability of Tropical Precipitation Profiles and Its Impact on Microwave Brightness Temperatures as Inferred from TRMM Data

Precipitation radar and microwave radiometer data collected by the Tropical Rainfall Measuring Mission (TRMM) satellite are used to study the variability of precipitation profiles and the relationship between precipitation profile and microwave brightness temperature. The variability has been examined using empirical orthogonal function (EOF) analysis and microwave emission and scattering signatures. Precipitation profiles are divided into three groups according to emission signatures at 19.4 GHz and three groups according to scattering signatures at 85.5 GHz. For stratiform rain, the differences of vertical precipitation profiles among these groups are small and are mainly seen in the slope of profiles below the freezing level. However, clear differences in vertical precipitation profiles can be found among the deep-convective rain groups. The maximum rainfall rate occurs at a considerably lower altitude when low liquid-emission or low ice-scattering signatures are observed. When emission or scattering signatures are high, precipitation profiles peak near the freezing level, a feature that is similar to the one in stratiform precipitation profiles. The three patterns of the vertical profiles derived from microwave signatures are very similar to the three patterns derived by EOF analysis. This similarity suggests that the three patterns derived by microwave signatures represent the most significant variability in vertical precipitation profiles. Results also show that, for the same near-surface rainfall rate, the pixel group with anomalously high microwave emission also shows anomalously high microwave scattering, and vice versa, suggesting that the liquid and ice water amounts in tropical rains are correlated over scales the size of a satellite pixel. It is also found that, for a given surface rainfall rate, the brightness-temperature differences among the pixel groups are large, highlighting the importance of vertical precipitation profile in determining upwelling microwave radiation and, therefore, the need to incorporate realistic precipitation profile information in rain retrieval algorithms.

Possible Misidentification of Rain Type by TRMM PR over Tibetan Plateau

Abstract Rain-type statistics derived from Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) standard product show that some 70% of raining pixels in the central Tibetan Plateau summer are stratiform—a clear contradiction to the common knowledge that rain events during summer in this region are mostly convective, as a result of the strong atmospheric convective instability resulting from surface heating. In examining the vertical distribution of the stratiform rain-rate profiles, it is suspected that the TRMM PR algorithm misidentifies weak convective rain events as stratiform rain events. The possible cause for this misidentification is believed to be that the freezing level is close to the surface over the plateau, so that the ground echo may be mistakenly identified as the melting level in the PR rain classification algorithm.

Diurnal Cycle of Summer Precipitation over Subtropical East Asia in CAM5

The simulations of summertime diurnal cycle of precipitation and low-level winds by the Community Atmosphere Model, version 5, are evaluated over subtropical East Asia. The evaluation reveals the physical cause of the observed diurnal rainfall variation in East Asia and points to the source of model strengths and weaknesses. Two model versions with horizontal resolutions of 2.88 and 0.58 are used. The models can reproduce the diurnal phase of large-scale winds over East Asia, with an enhanced lowlevel southwesterly in early morning. Correspondingly, models successfully simulated the diurnal variation of stratiform rainfall with a maximum in early morning. However, the simulated convective rainfall occurs at local noontime, earlier than observations and with larger amplitude (normalized by the daily mean). As a result, models simulated a weaker diurnal cycle in total rainfall over the western plain of China due to an out-of-phase cancellation between convective and stratiform rainfalls and a noontime maximum of total rainfall over the eastern plain of China. Over the East China Sea, models simulated the early-morning maximum of convective precipitation and, together with the correct phase of the stratiform rainfall, they captured the diurnal cycle of total precipitation. The superposition of the stratiform and convective rainfalls also explains the observed diurnal cycle in total rainfall in East Asia. Relative to the coarse-resolution model, the high-resolution model simulated slight improvement in diurnal rainfall amplitudes, due to the larger amplitude of stratiform rainfall. The two models, however, suffer from the same major biases in rainfall diurnal cycles due to the convection parameterization.

Climatic characteristics of convective and stratiform precipitation over the Tropical and Subtropical areas as derived from TRMM PR

Climatic characteristics of convective and stratiform precipitation over the Tropical and Subtropical areas are investigated based on the measurements of Tropical Rainfall Measuring Mission’s (TRMM) Precipitation Radar (PR) from 1998 to 2007. Results indicate that convective precipitation are distributed mainly over the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), the Asian Monsoon Region, regions between the South America and the Mid-America, and the Tropical Africa where the frequencies lie between 1% and 2%. But in four seasons, total area fractions of convective precipitation frequencies less than 1% all exceed 85%. The frequencies of stratiform precipitation are much higher than those of convective precipitation, and total area fractions of stratiform precipitation frequencies >1% are over 55% during four seasons. However, frequencies of the two rain types show not only remarkable regionality, but also distinct seasonal variations. Conditional rain rates of convective precipitation range from 6 to 14 mm/h whereas those of stratiform precipitation are smaller than 4 mm/h. Meanwhile, rain tops of convective precipitation are higher than those of stratiform precipitation. The mean profiles of the two rain types show significant latitudinal dependency. And the seasonal variations of precipitation profiles are displayed mainly in the variations of rain tops. The frequencies and conditional rain rates of both rain types over ocean are higher than those over land, but rain tops are just the opposite. Moreover, the seasonal variations of both rain types over ocean are weaker than those over land because of the different stable states of underlying surfaces.

A new water vapor algorithm for TRMM Microwave Imager (TMI) measurements based on a log linear relationship

[1] A new algorithm is proposed for retrieving atmospheric columnar water vapor (CWV) over ocean in the absence of rain by using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) measurements. Applying a log linear relationship between the brightness temperatures and main environmental variables including CWV, sea surface temperature, wind speed, cloud liquid water path, and cloud temperature, this five-channel-based algorithm was developed through radiative transfer model simulations. The unique advantage of this algorithm is that the retrieved CWV, derived simply from only the five channel brightness temperatures without other ancillary data, is hardly influenced by other geophysical variables. Retrievals are compared against radiosonde observations, which showed a good agreement with a bias less than 0.7 kg m ―2 and a root mean square error about 2.5 kg m ―2 , regardless of the presence of clouds. Additionally, comparison is made with CWV retrievals from the Remote Sensing Systems, which showed a high consistency between the two algorithms with a mean difference of 0.021 kg m ―2 and a root mean square difference of 2.076 kg m ―2 . Finally, the similarity of CWV global distributions based on this algorithm to those from other independent data sets suggests that the new algorithm can be applied for climatic applications.

Retrieval and characterization of cloud liquid water path using airborne passive microwave data during INDOEX

During the 1999 intensive observation period of the Indian Ocean Experiment (INDOEX), the Airborne Imaging Microwave Radiometer (AIMR) was deployed on the National Center for Atmospheric Research (NCAR) C-130 aircraft to measure up welling microwave radiation that can be used to retrieve cloud liquid water path (LWP). In this study, we present a LWP retrieval algorithm that is optimized for tropical atmospheric conditions, typical of conditions observed in INDOEX. Radiative transfer modeling and error analysis are conducted for the four AIMR channels, to guide selection of AIMR channels used for the LWP retrievals. Results show that the horizontal polarization channels outperform vertical polarization channels at both 37 and 90 GHz. Additionally, for LWP less than ∼300 g m−2, the best results are expected from the 90 GHz horizontal polarization channel, while the 37 GHz horizontal polarization channel performs better for higher LWPs. On the basis of these findings we formulated the LWP retrieval algorithm from the combination of the retrievals of 37 and 90 GHz horizontal polarization channels. Results of several indirect validations show that in nearly clear condition the LWP retrievals have essentially no bias and a random error of about 28 g m−2. The image of the retrieved LWP compares well with observations by a 0.64 μm visible channel, and the magnitude of the retrieved LWP for large convective cells is comparable to the estimation based on in situ measurements. It is also shown that the retrieved LWPs for convective cells are smaller than those estimated by assuming adiabatic process while the two have a similar trend in the LWP versus cloud top temperature diagram. By analyzing all available AIMR observations, it is found that the mean LWP for cloudy pixels measured during the INDOEX experiment is about 50 g m−2. A significant north-south gradient of the mean LWP is found in INDOEX domain during this period, with the mean LWP in the region south of 5°S being twice as high as that in the region north of 50N. The LWP frequency distribution shows that clouds with larger LWPs occur more often in the southern region than in the northern region.

The biophysical responses of the upper ocean to the typhoons Namtheun and Malou in 2004

The responses of the upper ocean to typhoons were investigated by the observations of sea surface wind (SSW), sea surface temperature (SST), sea surface height anomaly (SSHA), chlorophyll-a (Chl-a) and Argo floats. Typhoon Namtheun had notable impacts on the upper ocean along its track from July to August 2004. The local processes (entrainment and upwelling) dominated the upper ocean responses in the regions of the pre-existing cold eddy and beneath the typhoon track, where the observed locations of upwelling, SSHA changes, SST cooling, and Chl-a enhancement were consistent with each other. Besides, there were cold tongues extending from the cold centres. The trajectories of Argo floats, along with the cold tongues, indicated that the surface advections induced such non-local responses. On the other hand, the following weak typhoon Malou had few impacts on the upper ocean. Finally, the mechanisms of the Chl-a concentration enhancement were sketched as the effects of both the local upwelling and the non-local advection. This study implies that some non-local processes, e.g. horizontal advections, may play a notable role in the upper ocean responses to the typhoons.The responses of the upper ocean to typhoon Namtheun in July 2004 are investigated by sea surface measurements and vertical profiles. Pre-typhoon ocean environment played an important role in this case. There were two extreme cooling regions located at cyclonic eddy A and typhoon wake B. Although the magnitudes of SST cooling at A and B were similar, other physical and biophysical responses were quite different. Combining multi-satellite data with vertical profile data, it is found that the upwelling dominated the responses at A and the vertical mixing dominated the responses at B. This study implies that to insight into the ocean surface responses to typhoon, the subsurface dynamics need to be analyzed via both the in situ and satellite-based observations, and the physical and biological models.

Characteristics of tropopause‐penetrating convection determined by TRMM and COSMIC GPS radio occultation measurements

Distribution and influence of convection in the upper troposphere and lower stratosphere have been investigated case by case or on regional to global scale. However, previous studies were limited by using proxies for convection or the bias of the tropopause data. Here the tropopause-penetrating convection is investigated based on the sole use of observational products from Tropical Rainfall Measuring Mission (TRMM) precipitation radar data and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). The result shows that the frequency of precipitation-top heights above the monthly mean tropopause in the tropics is reduced logarithmically if the cold-point tropopause is adopted instead of the lapse-rate tropopause. Using the collocated COSMIC and precipitation radar observations, the tropopause-penetrating convection, i.e., the convection with the precipitation-top height above the lapse-rate tropopause, can be found over the summer monsoon regions and some continental regions. The averaged relative precipitation-top heights of tropopause-penetrating convective clusters are about 0.2–0.5 km without significant land-ocean difference, while equivalent radii of clusters are 2.7–3.5 km over land and 0.2–0.5 km larger than those over ocean. These areal and vertical extents are smaller than those reported by previous studies. Furthermore, the collocated temperature profiles show that the tropopause-penetrating convection generates warming in the upper troposphere and cooling near the lapse-rate tropopause and in the lower stratosphere. Moreover, the tropopause-penetrating convection leads to a rapid (within 20 min) lift of the lapse-rate tropopause by the adiabatic lofting within the convection (within a 10 km radius).

1997/98 El Niño–Induced Changes in Rainfall Vertical Structure in the East Pacific

AbstractThe 1997/98 El Nino–induced changes in rainfall vertical structure in the east Pacific (EP) are investigated by using collocated Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and associated daily SST and 6-hourly reanalysis data during January, February, March, and April of 1998, 1999, and 2000. This study shows that there are five key parameters, that is, surface rain rate, precipitation-top height (or temperature), and precipitation growth rates at upper, middle, and low layers to define a rainfall profile, and those five key parameters are strongly influenced by both SST and large-scale dynamics. Under the influence of 1997/98 El Nino, the precipitation-top heights in the EP were systematically higher by about 1 km than those under non–El Nino conditions, while the freezing level was about 0.5 km higher. Under the constraints of rain type, surface rain rate, and the precipitation top, the shape of rainfall profile still showed significant differences: the rain growth was r...

The optical properties and longwave radiative forcing in the lateral boundary of cirrus cloud

Through observations from the Cloud-Aerosol Lidar with Orthogonal Polarization onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, we detected a common feature of narrow and subvisible lateral boundary layer in cirrus cloud. In this layer the lidar backscatter, the depolarization ratio, the ice water content, the effective radius of ice particles, and the cloud optical depth all decrease sharply toward the cloud edge. In general, the width of this layer (6.4 ± 3.1 km over land) decreases with increasing ambient temperature. The estimated longwave radiative forcing associated with the layer is about 10 W/m2. Due to its extremely small optical depth (less than 0.3), such lateral boundary layer may be missed by conventional satellite passive optical sensors. As a consequence, the mentioned radiative forcing has not been credited with its deserved share in the Earths radiative energy budget.