Weizhong Zheng

A SAR Observation and Numerical Study on Ocean Surface Imprints of Atmospheric Vortex Streets

The sea surface imprints of Atmospheric Vortex Street (AVS) off Aleutian Volcanic Islands, Alaska were observed in two RADARSAT-1 Synthetic Aperture Radar (SAR) images separated by about 11 hours. In both images, three pairs of distinctive vortices shedding in the lee side of two volcanic mountains can be clearly seen. The length and width of the vortex street are about 60-70 km and 20 km, respectively. Although the AVSs in the two SAR images have similar shapes, the structure of vortices within the AVS is highly asymmetrical. The sea surface wind speed is estimated from the SAR images with wind direction input from Navy NOGAPS model. In this paper we present a complete MM5 model simulation of the observed AVS. The surface wind simulated from the MM5 model is in good agreement with SAR-derived wind. The vortex shedding rate calculated from the model run is about 1 hour and 50 minutes. Other basic characteristics of the AVS including propagation speed of the vortex, Strouhal and Reynolds numbers favorable for AVS generation are also derived. The wind associated with AVS modifies the cloud structure in the marine atmospheric boundary layer. The AVS cloud pattern is also observed on a MODIS visible band image taken between the two RADARSAT SAR images. An ENVISAT advance SAR image taken 4 hours after the second RADARSAT SAR image shows that the AVS has almost vanished.

Coexistence of Atmospheric Gravity Waves and Boundary Layer Rolls Observed by SAR

AbstractBoth atmospheric gravity waves (AGW) and marine atmospheric boundary layer (MABL) rolls are simultaneously observed on an Environmental Satellite (Envisat) advanced synthetic aperture radar (ASAR) image acquired along the China coast on 22 May 2005. The synthetic aperture radar (SAR) image covers about 400 km × 400 km of a coastal area of the Yellow Sea. The sea surface imprints of AGW show the patterns of both a transverse wave along the coastal plain and a diverging wave in the lee of Mount Laoshan (1133-m peak), which indicate that terrain forcing affects the formation of AGW. The AGW have a wavelength of 8–10 km and extend about 100 km offshore. Model simulation shows that these waves have an amplitude over 3 km. Finer-scale (~2 km) brushlike roughness features perpendicular to the coast are also observed, and they are interpreted as MABL rolls. The FFT analysis shows that the roll wavelengths vary spatially. The two-way interactive, triply nested grid (9–3–1 km) Weather Research and Forecasti...

Ocean Upwelling Along the Yellow Sea Coast of China Revealed by Satellite Observations and Numerical Simulation

Satellite observations reveal that an ocean cooling event happened along the Yellow Sea coast of China intermittently in spring 2008, which lasted for days. During this period, the sea surface temperature (SST) dropped 3 °C-4 °C and the chlorophyll A (Chl-a) content increased by 0.5-1 mg/m3, as determined from satellite-derived products. The cold water also suppressed the sea surface capillary waves and made the ocean surface smooth, a distinct feature shown as dark patches observed in the synthetic aperture radar image acquired during this period of time. The surface wind direction varied between alongshore and offshore. We implemented an interactively coupled ocean (regional ocean modeling system) and atmosphere (Weather Research and Forecasting model) model to capture the dynamical processes of this seemingly wind-driven cooling event. When the wind changed direction such that the alongshore component blew with the land on its left side, stronger upwelling occurred; and when the wind blew offshore with no alongshore component, the upwelling still occurred in this area, but with less strength. Two simulations with idealized alongshore and offshore winds show that the upwelling can be set up within several hours. The alongshore wind is more effective than the offshore wind in transporting upper level water offshore and triggering upwelling and causing SST cooling areas that are relatively large in size, although the maximum SST cooling they cause is on the same order of magnitude.

Improving the Stable Surface Layer in the NCEP Global Forecast System

AbstractThis study examines the performance of the NCEP Global Forecast System (GFS) surface layer parameterization scheme for strongly stable conditions over land in which turbulence is weak or even disappears because of high near-surface atmospheric stability. Cases of both deep snowpack and snow-free conditions are investigated. The results show that decoupling and excessive near-surface cooling may appear in the late afternoon and nighttime, manifesting as a severe cold bias of the 2-m surface air temperature that persists for several hours or more. Concurrently, because of negligible downward heat transport from the atmosphere to the land, a warm temperature bias develops at the first model level. The authors test changes to the stable surface layer scheme that include introduction of a stability parameter constraint that prevents the land–atmosphere system from fully decoupling and modification to the roughness-length formulation. GFS sensitivity runs with these two changes demonstrate the ability o...

Synergistic Use of Satellite Observations and Numerical Weather Model to Study Atmospheric Occluded Fronts

Synthetic aperture radar (SAR) images reveal the surface imprints of atmospheric occluded fronts. An occluded front is characterized as a low-wind zone located between and within two zones of higher winds blowing in the opposite directions on the left and right sides of the occluded front. A group of four SAR images reveal that the width of an individual occluded frontal zone and the wind magnitudes outside fronts vary greatly from case to case. In this paper, we performed a case study to analyze an occluded front observed by an Environmental Satellite (Envisat) Advanced SAR and ASCAT scatterometer along the west coast of Canada on November 24, 2011. The two-way interactive, triply nested grid (9-3-1 km) Weather Research and Forecasting (WRF) model was utilized to simulate the evolution of the occluded front. The occluded front moved toward the east during a 24-h model simulation, and the movement between 18:00 and 21:00 UTC matched the occluded front positions derived from the concurrently collected surface weather maps; from the National Oceanic and Atmospheric National Weather Service archives. The WRF-simulated low-wind zone associated with the occluded front and ocean surface wind speed match well with the SAR and scatterometer wind retrievals. High wind outside the front zone became weaker during the front evolution, whereas the width of the occluded frontal zone was contracted laterally. Analysis of the WRF model derived potential temperature field suggests that the occlusion process occurred below the 800-mb level. The structure of the occluded front studied here not only follows the conventional conceptual model and also supports the findings of a novel wrap-up conceptual model for an atmospheric frontal occlusion process.

Multisatellite observations and numerical simulation of an along‐coast cumulus cloud line induced by sea‐breeze circulation

A coastal cumulus cloud‐line formation along the east coast of the USA was observed on a National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellite (POES) Advanced Very High Resolution Radiometer (AVHRR) satellite image from 17 August 2001. The cloud line starts to form at about 16:00 UTC (local 12:00 noon) and follows the coastline from Florida to North Carolina. The length and width of the cloud line are about 850 km and 8.5 km, respectively. A 15‐min interval sequence of NOAA Geostationary Operational Environmental Satellite (GOES) images shows that the cloud line maintains the shape of the coastline and penetrates inland for more than 20 km over the next 6‐h timespan. Model simulation with actual atmospheric conditions as inputs shows that the cloud line is formed near the land–sea surface temperature (SST) gradient. The synoptic flow at all model levels is in the offshore direction prior to 16:00 UTC whereas low‐level winds (below 980 hPa) reverse direction to blow inland after 16:00 UTC. This reversal is due to the fact that local diurnal heating over the land takes place on shorter time‐scales than over the ocean. The vertical wind at these levels becomes stronger as the land–SST increases during the summer afternoon, and the leading edge of the head of the inland wind ascends from 920 hPa to about 850 hPa in the 3 h after 16:00 UTC. Model simulation and satellite observations show that the cloud line becomes very weak after 21:00 UTC when the diurnal heating decreases.

SAR imaging and numerical simulation of upwelling processes near the coastal area of Qingdao in China

SST cooling episodes in the early Spring are often observed by various remote sensing measurement systems near the coast of Qingdao, a city in Chinas Shandong province (Figure 1). Several wind-SST interaction mechanisms that can cause colder deep water to force its way upward and drive away and subsequently replace the warmer surface water, also known as upwelling, have been previously documented. Most of the previous coastal upwelling work has been focused on the mechanism of upwelling processes on a large scale and caused by winds blowing parallel to the coast and Ekman transport. Upwelling on small local scales such as the one shown in Figure 1 has not been extensively studied. However, on a more local and smaller scale, not associated with Coriolis force or Ekman transport, winds blowing offshore can also push water mechanically away from land to produce upper level divergence and upwelling. But it is not clear if the recurring upwelling in Figure 1 can be attributed to the along-coast winds or the offshore winds or the local bathymetry, or a combination of several factors. We will look into the possible mechanisms.

A preliminary assessment of the impact of SMAP Soil Moisture on numerical weather Forecasts from GFS and NUWRF models

NASA Soil Moisture Active/Passive (SMAP) satellite was launched on January 31st, 2015 and has been providing global soil moisture (SM) data products since April 2015. One of the primary justifications of the mission was to improve numerical weather predictions. With the SMAP SM data becoming available, it is anxiously expected that SMAP SM data could be demonstrated to significantly improve weather forecasts from numerical weather prediction (NWP) models. In this study, the NOAA Global Forecast System (GFS) and NASA Unified Weather Research and Forecast (NUWRF) model coupled with NASA Land Information System are used to carry out the demonstration. A hardwired Ensemble Kalman filter is implemented within GFS to assimilate surface SM observations. For assimilating SM data into NUWRF model, NASA Land Information System (LIS) is coupled with the NUWRF model. In this paper preliminary results of SMAP soil moisture impact on GFS and NUWRF forecasts are presented after the assimilation algorithms and system designs are introduced. Plans for more comprehensive assessment of the satellite soil moisture data impact on NWP models will be discussed.

Fetch imaged by SAR and simulated by WRF model

In this paper, we present the synthetic aperture radar (SAR) observation of the detailed sea surface wind patterns associated with fetch in the Bohai Sea, China. We then implemented the WRF model to simulate the entire processes of this weather event. WRF model results show the dynamics and evolution of this event.

Sea Fetch Observed by Synthetic Aperture Radar

Two satellite synthetic aperture radar (SAR) observations of the fetch in the Bohai Sea of China are presented. The sea surface winds derived from SAR data indicated a high wind of 15-16 m/s that occurred in the fetch zone. The winds are shown to have immediate direct mechanical forcing impacts on the significant wave heights (Hs) of ocean surface gravity waves. Buoy measurements and numerical wave modeling results show that the Hs increased to a maximum of 3.5 m in the semienclosed sea, 3 h after the passage of the fetch winds, and the high Hs in the sea was sustained for a total of 6 h. The Weather Research and Forecasting (WRF) model implemented in our modeling simulation captured the wind field responsible for the evolution of the fetch event. The model-simulated surface horizontal winds agree with the SAR-derived winds. In addition, the vertical wind distribution reveals that the fetch wind field reached the 800 hPa level, and the event lasted less than one day. This study demonstrates the synergy of using SAR imagery and the WRF model as effective tools to investigate the lateral and vertical structures of coastal wind.