Three-dimensional variational data assimilation of WindSat ocean surface winds for the genesis and forecasting of tropical storm Henri

With available high-resolution ocean surface wind vectors retrieved from the U.S.Naval Research Laboratory's WindSat on Coriolis,the impact of these data on genesis and forecasting of tropical storm Henri is examined using the non-hydrostatic,fifth-generation mesoscale model(MM5) of Pennsylvania State University-National Center for Atmospheric Research plus its newly released three-dimensional variational data assimilation(3DVAR) system.It is shown that the assimilation of the WindSat-retrieved ocean surface wind vectors in the 3DVAR system improves the model initialization fields by introducing a stronger vortex in the lower troposphere.As a result,the model reproduces the storm formation and track reasonably close to the observations.Compared to the experiment without the WindSat surface winds,the WindSat assimilation reduced an error between the model simulated track and observations of more than 80 km and also improved the storm intensity by nearly 2 hPa.It suggests that these data could provide early detection and prediction of tropical storms or hurricanes.

Analysis of diurnal boundary layer development in boreal forests: measurements and simulations

Aims Combining field data analysis and modeling,this study investigates factors influencing the diurnal boundary layer(BL)development in boreal forest.Methods Field data analysis used both air sounding and surface flux measurements collected during the Boreal Ecosystem–Atmosphere Study field campaigns in central Canada.Model study applied a non-local transilient turbulence theory(TTT)to simulate the impact of the heterogeneous boundary conditions together with initial conditions on the BL development at the Candle Lake and Thompson release sites over boreal forests.Boundary conditions were characterized by the integrated surface flux measurements from different forest stands.The lake effect was included in constructing the surface fluxes at Candle Lake release site.Important Findings Analyses of serial upper air sounding data and tower flux data indicate strong linear impacts of surface sensible heat forcing on the diurnal BL development above boreal forests.The regression slopes on the relationship between the BL development and the surface fluxes reflect the influences of initial boundary conditions to the BL developments.Both the modeled and the measured diurnal BLs show that lakes reduce sensible heat flux,leading to a shallower boundary in Candle Lake than in Thompson.Comparison of the model results and field measurements on the BL profiles indicates that the TTT model has the capability to simulate the BL development above boreal forests for sunny,rainfall or cloudy days.This study demonstrates the importance of lake on surface fluxes and BL development.The modeling effort shows the potential to couple the transilient theory with a land surface process model to study land surface and atmosphere interaction in boreal forest.

TROPICAL CYCLONE AMOS (2016) FORECASTING CHALLENGES: A MODEL'S PERSPECTIVE

The tropical cyclone(TC)named Amos(2016)that impacted the Samoan Islands on 23 April 2016 was a particularly dif f icult storm to forecast.Both the intensity changes and the track of Amos represent a signif icant challenge for forecasters and this is briefl y summarized in this report.Model forecasts initially indicated that the cyclone would track south of the Samoan Islands.However,the forecasts generally changed to a direct hit over Samoa as a Category 4 storm at approximately 0000 U TC 24April based on model cycles initialized at 0000 UTC 23 April.TC Amos’central pressure dropped from 983 hPa to 957 hPa between 0000 UTC 21 April and 0000 UTC 23April.The models did not pick up on this rapid intensif ication until the intensif ication had already begun around0000 UTC 21 April.The models also struggled to capture the rapid weakening of TC Amos due to vertical wind shear that began 0000 UTC 24 April as the cyclone continued to move north of the islands.Because of the initially ominous track forecasts for TC Amos to hit land,preparations for a Category 3 or Category 4 cyclone were underway in the Samoan islands and the population prepared for the worst.After the center of the storm moved north of the islands as a weaker storm than anticipated,the residents of the Samoan Islands were both surprised and relieved that the cyclone only gave a"glancing blow"to the islands and that the impacts were not as bad as originally feared.An in-depth evaluation of this particular tropical cyclone helps to shed some light on model def iciencies and can be used to help determine future model changes.