The unmanned semi-submersible vehicle (USSV) developed by the unmanned surface vehicle team of the Institute of Atmospheric Physics is an unmanned, rugged, and high-endurance autonomous navigation vessel designed for the collection of long-term, continuous and real-time marine meteorological measurements, including atmospheric sounding in the lower troposphere. A series of river and sea trials were conducted from May 2016 to November 2017, and the first rocketsonde was launched from the USSV. Real-time meteorological parameters in the marine atmospheric boundary layer (MABL) were obtained, including sea surface temperature, and vertical profiles of the pressure, temperature, relative humidity, wind speed, and wind direction. These data are extremely useful and important for research on air-sea interactions, sea surface heat and latent heat flux estimations, MABL modeling, and marine satellite product validation.
Seasonal variations of rainfall microphysics in East China are investigated using data from the observations of a two-dimensional video disdrometer and a vertically pointing micro rain radar. The precipitation and rain drop size distribution (DSD) characteristics are revealed for different rain types and seasons. Summer rainfall is dominated by convective rain, while during the other seasons the contribution of stratiform rain to rainfall amount is equal to or even larger than that of convective rain. The mean mass-weighted diameter versus the generalized intercept parameter pairs of convective rain are plotted roughly around the "maritime" cluster, indicating a maritime nature of convective precipitation throughout the year in East China. The localized rainfall estimators, i.e., rainfall kinetic energy-rain rate, shape-slope, and radar reflectivity-rain rate relations are further derived. DSD variability is believed to be a major source of diversity of the aforementioned derived estimators. These newly derived relations would certainly improve the accuracy of rainfall kinetic energy estimation, DSD retrieval, and quantitative precipitation estimation in this specific region.
This study assesses the performance of temperature extremes over China in two regional climate models (RCMs), RegCM4 and WRF, driven by the ECMWF's 20th century reanalysis. Based on the advice of the Expert Team on Climate Change Detection and Indices (ETCCDI), 12 extreme temperature indices (i.e., TXx, TXn, TNx, TNn, TX90p, TN90p, TX10p, TN10p WSDI, ID, FD, and CSDI) are derived from the simulations of two RCMs and compared with those from the daily station-based observational data for the period 1981-2010. Overall, the two RCMs demonstrate satisfactory capability in representing the spatiotemporal distribution of the extreme indices over most regions. RegCM performs better than WRF in reproducing the mean temperature extremes, especially over the Tibetan Plateau (TP). Moreover, both models capture well the decreasing trends in ID, FD, CSDI, TX10p, and TN10p, and the increasing trends in TXx, TXn, TNx, TNn, WSDI, TX90p, and TN90p, over China. Compared with observation, RegCM tends to underestimate the trends of temperature extremes, while WRF tends to overestimate them over the TP. For instance, the linear trends of TXx over the TP from observation, RegCM, and WRF are 0.53°C (10 yr)-1, 0.44°C (10 yr)-1, and 0.75°C (10 yr)-1, respectively. However, WRF performs better than RegCM in reproducing the interannual variability of the extreme-temperature indices. Our findings are helpful towards improving our understanding of the physical realism of RCMs in terms of different time scales, thus enabling us in future work to address the sources of model biases.
Salinity variability and its causes in the tropical Pacific are analyzed using observations, reanalysis products and model simulations. The mixed-layer salinity (MLS) budget analyses from observations and reanalysis products indicate that its interannual evolution is closely related to ENSO and is predominantly governed by surface forcing and surface advection in the western-central equatorial Pacific. It is found that the observed MLS tendency leads Ni?o3.4 by about 12 months due to the effect of negative freshwater flux (evaporation minus precipitation). These observation-based analyses are used to evaluate the corresponding simulation using GFDL-ESM2M. It is evident that the model can simulate the spatiotemporal variations of MLS with some discrepancies compared to observations. In the warm pool of the equatorial Pacific the MLS tendency in the model is sensitive to ocean dynamics, however model biases cause the tendency to be underestimated. In particular, the freshwater flux is overestimated while the ocean surface zonal current and vertical velocity at the base of the mixed layer are underestimated. Due to model biases in representing the related physics, the effects of surface forcing on the simulated MLS budget are overestimated and those of subsurface and surface advection are relatively weak. Due to weaker surface advection and subsurface forcing than observed, the simulated compensations for surface forcing are suppressed, and the simulated MLS tendency that leads Ni?o3.4 by 8-10 months, which is shorter than the observed lead time. These results are useful for the interpretation of observational analyses and other model simulations in the tropical Pacific.
Using the Regional Ocean Modeling System, this study investigates the simulation uncertainties in the current velocity in the low-latitude North Pacific where the Kuroshio originates [i.e., the beginning of the Kuroshio (BK)]. The results show that the simulation uncertainties largely reflect the contributions of wind stress forcing errors, especially zonal wind stress errors, rather than initial or boundary errors. Using the idea of a nonlinear forcing singular vector, two types of zonal wind stress errors (but sharing one EOF mode) are identified from error samples derived from reanalysis data as having the potential to yield large simulation uncertainties. The type-1 error possesses a pattern with positive anomalies covering the two zonal bands of 0°-15°N and 25°-40°N in the Pacific Ocean, with negative anomalies appearing between these two bands; while the type-2 error is almost opposite to the type-1 error. The simulation uncertainties induced by the type-1 and -2 errors consist of both large-scale circulation errors controlled by a mechanism similar to the Sverdrup relation and mesoscale eddy-like errors generated by baroclinic instability. The type-1 and -2 errors suggest two areas: one is located between the western boundary and the meridional 130°E along 15°-20°N, and the other is located between 140°-150°E and along 15°-20°N. The reduction of errors over these two areas can greatly improve the simulation accuracy of the current velocity at BK. These two areas represent sensitive areas for targeted observations associated with the simulation of the current velocity at BK.