Tower-based observation of air-sea momentum flux:comparisons between onshore and offshore winds

We investigate the air-sea momentum flux in the marine atmospheric boundary layer using a tower-based direct measurement method.First,we compare the collected data with previous observations,and the results are roughly consistent.Next,in the low-to-moderate winds,the exchange coefficients(or drag coefficients)deviate between onshore and offshore winds,which exhibits the influence of surface wave on the momentum flux.Furthermore,we use a surface-wave-involved parameterization scheme to explain the dependence of momentum flux on surface wave.The results consolidate the influence of surface wave on momentum flux on the one hand,and validate the surface-wave-involved parameterization scheme on the other hand.

Maintenance and Sudden Change of a Strong Elevated Ducting Event Associated with High Pressure and Marine Low-Level Jet

Capture of a strong elevated ducting event,especially its maintenance and sudden change,is of great value to airborne radar to achieve its beyond-the-line-of-sight detection.However,the knowledge is not easily accessible over the open ocean and hence very rare.During the Air–Sea Interaction Survey(ASIS)over the western North Pacific(WNP)in May 2016,a strong elevated ducting event with a long-life period and sudden change in its evolution was observed.Measurements from the ASIS,images from the Himawari-8 satellite,reanalysis data from the ECMWF,and Weather Research and Forecasting(WRF)model,were used to analyze the maintenance and sudden change of this strong ducting event,together with the model performance on simulating it.The results showed that the maintenance of strong elevated ducts,with their tops ranging from 750 to 1050 m and average strength of approximately 38 M units,was caused by a strong dry air mass capping over the wet marine atmospheric boundary layer(MABL),together with the subsidence inversion associated with high pressure.The WRF model performs well in simulating them.However,a sudden increase in duct height with a slight decrease of strength was recorded by the subsequent GPS radiosonde,which was finally contributed to the mechanical turbulent inversion and hydrolapse associated with the marine low-level jet(MLLJ).The height of the maximum horizontal wind speed(Umh)of the MLLJ corresponds well with the bottom of the trapping layer.However,these jet-relevant ducts are generally weak and it is difficult to accurately simulate them by using the mesoscale numerical model,since the wind-shear produced eddies are too small to be properly parameterized.

A New Observation Operator for the Assimilation of Satellite-Derived Relative Humidity:Methodology and Experiments with Three Sea Fog Cases over the Yellow Sea

Assimilation of satellite-derived relative humidity(Satellite-RH)is capable of improving sea fog forecasts by saturating the background in the observed foggy areas.Previous studies have achieved saturation by increasing the moisture only(Method-q).However,this method can lead to large wetting and warming biases within the marine atmospheric boundary layer(MABL).A new method using an RH observation operator(Method-RH)is designed to alleviate these biases by simultaneously adjusting the moisture and the temperature.For comparison,saturation is also achieved by decreasing the temperature only(Method-t).The three Satellite-RH assimilation methods are implemented within the Gridpoint Statistical Interpolation-based three-dimensional variational system and examined for three sea fog cases over the Yellow Sea.The three cases on 28 April 2007,9 April 2009,and 29 March 2015 fail to be predicted without the Satellite-RH assimilation as their MABLs have both warming and drying,drying,and warming biases,respectively.Intercomparisons and evaluations show that Method-RH has the best overall performance of the three methods in terms of the forecast of sea fog and MABL structures as only Method-RH can fully or partially address all the bias scenarios in forecasting sea fog.Compared with Method-q,Method-RH produces more well-defined sea fog areas by adding a smaller amount of moisture as well as decreasing the temperature.Compared with Methodt,Method-RH enlarges the sea fog areas by increasing the amount of moisture in addition to the cooling.