Simulation of Seawinds Measurements in the Presence of Rain using Collocated TRMM PR Data

The scatterometer Sea Winds on QuikSCAT measures ocean winds via the relationship between the wind and the normalized radar backscatter cross-section (aO) from the ocean surface. Scattering and attenuation from falling rain droplets along with ocean surface perturbations due to rain change the backscatter signature of the waves induced by near-surface winds. A simple model incorporates the effects of rain on ocean aO. Colocated data from the precipitation radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite is used to simulate the effects of rain as seen by Sea Winds. PRderived backscatter, atmospheric rain attenuation, and rain rates are averaged over the Sea Winds footprint. The enhancement in backscatter from rain striking the ocean surface is estimated as a function of rain rate using a least-squares technique. QuikSCAT aO values are simulated from the PR-derived parameters and numerical weather prediction wind data using the simple backscatter model. The simple model estimates 90% of the observed rain-contaminated QuikSCAT aO values to within 3 dB

An Improved Ocean Vector Winds Retrieval Approach Using C- And Ku-band Scatterometer And Multi-frequency Microwave Radiometer Measurements

This dissertation will specifically address the issue of improving the quality of satellite scatterometer retrieved ocean surface vector winds (OVW), especially in the presence of strong rain associated with tropical cyclones. A novel active/passive OVW retrieval algorithm is developed that corrects Ku-band scatterometer measurements for rain effects and then uses them to retrieve accurate OVW. The rain correction procedure makes use of independent information available from collocated multi-frequency passive microwave observations provided by a companion sensor and also from simultaneous C-band scatterometer measurements. The synergy of these active and passive measurements enables improved correction for rain effects, which enhances the utility of Ku-band scatterometer measurements in extreme wind events. The OVW retrieval algorithm is based on the next generation instrument conceptual design for future US scatterometers, i.e. the Dual Frequency Scatterometer (DFS) developed by NASA’s Jet Propulsion Laboratory. Under this dissertation research, an end-to-end computer simulation was developed to evaluate the performance of this active/passive technique for retrieving hurricane force winds in the presence of intense rain. High-resolution hurricane wind and precipitation fields were simulated for several scenes of Hurricane Isabel in 2003 using the Weather Research and Forecasting (WRF) Model. Using these numerical weather model environmental fields, active/passive measurements were simulated for instruments proposed for the Global Change Observation Mission- Water Cycle (GCOM-W2) satellite series planned by the Japanese Aerospace Exploration Agency. Further, the quality of the simulation was evaluated using actual hurricane measurements from the Advanced Microwave Scanning Radiometer and iv SeaWinds scatterometer onboard the Advanced Earth Observing Satellite-II (ADEOS-II). The analysis of these satellite data provided confidence in the capability of the simulation to generate realistic active/passive measurements at the top of the atmosphere. Results are very encouraging, and they show that the new algorithm can retrieve accurate ocean surface wind speeds in realistic hurricane conditions using the rain corrected Ku-band scatterometer measurements. They demonstrate the potential to improve wind measurements in extreme wind events for future wind scatterometry missions such as the proposed GCOM-W2

A new approach to estimation of global air-sea gas transfer velocity fields using dual-frequency altimeter backscatter

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C11003, doi:10.1029/2006JC003819.A new approach to estimating air-sea gas transfer velocities based on normalized backscatter from the dual-frequency TOPEX and Jason-1 altimeters is described. The differential scattering of Ku-band (13.6 GHz) and C-band (5.3 GHz) microwave pulses is used to isolate the contribution of small-scale waves to mean square slope and gas transfer. Mean square slope is derived for the nominal wave number range 40–100 rad m−1 by differencing mean square slope estimates computed from the normalized backscatter in each band, using a simple geometric optics model. Model parameters for calculating the differenced mean square slope over this wave number range are optimized using in situ optical slope measurements. An empirical relation between gas transfer velocity and mean square slope, also based on field measurements, is then used to derive gas transfer velocities. Initial results demonstrate that the calculated transfer velocities exhibit magnitudes and a dynamic range which are generally consistent with existing field measurements. The new algorithm is used to construct monthly global maps of gas transfer velocity and to illustrate seasonal transfer velocity variations over a 1-year period. The measurement precision estimated from >106 duplicate observations of the sea surface by TOPEX and Jason-1 altimeters orbiting in tandem is better than 10%. The estimated overall uncertainty of the method is ±30%. The long-term global, area-weighted, Schmidt number corrected, mean gas transfer velocity is 13.7 ± 4.1 cm h−1. The new approach, based on surface roughness, represents a potential alternative to commonly used parameterizations based on wind speed.Financial support for this research from the National Aeronautics and Space Administration through Jet Propulsion Laboratory contract 961425 and the NOAA Global Carbon Cycle Program under grant NA16GP2918, Office of Global Programs is gratefully acknowledged

Simulation and Interpretation of the Genesis of Tropical Storm Gert (2005) as Part of the NASA Tropical Cloud Systems and Processes Experiment

Several hypotheses have been put forward for the how tropical cyclones (tropical storms and hurricanes in the Atlantic) first develop circulation at the surface, a key event that needs to occur before a storm can begin to draw energy from the warm ocean. One hypothesis suggests that the surface circulation forms from a "top-down" approach in which a storm s rotating circulation begins at middle levels of the atmosphere and builds down to the surface through processes related to light "stratiform" (horizontally extensive) precipitation. Another hypothesis suggests a bottom-up approach in which deep thunderstorm towers (convection) play the major role in spinning up the flow at the surface. These "hot towers" form in the area of the mid-level circulation and strongly concentrate this rotation at low levels within their updrafts. Merger of several of these hot towers then intensifies the surface circulation to the point in which a storm forms. This paper examines computer simulations of Tropical Storm Gert (2005), which formed in the Gulf of Mexico during the National Aeronautics and Space Administration s (NASA) Tropical Cloud Systems and Processes (TCSP) Experiment, to investigate the development of low-level circulation and, in particular, whether stratiform or hot tower processes were responsible for the storm s formation. Data from NASA satellites and from aircraft were used to show that the model did a good job of reproducing the formation and evolution of Gert. The simulation shows that a mix of both stratiform and convective rainfall occurred within Gert. While the stratiform rainfall clearly acted to increase rotation at middle levels, the diverging outflow beneath the stratiform rain worked against spinning up the low-level winds. The hot towers appeared to dominate the low-level flow, producing intense rotation within their cores and often being associated with significant pressure falls at the surface. Over time, many of these hot towers merged, with each merger adding to the rotation of the storm and the pressure falls at the surface. This process continued to increase the strength of the storm until the storm made landfall on the east coast of Mexico. These results support the bottom-up hypothesis for development

Revealing the Winds under the Rain. Part I: Passive Microwave Rain Retrievals Using a New Observation-Based Parameterization of Subsatellite Rain Variability and Intensity—Algorithm Description

Scatterometer ocean surface winds have been providing very valuable information to researchers and operational weather forecasters for over 10 years. However, the scatterometer wind retrievals are compromised when rain is present. Merely flagging all rain-affected areas removes the most dynamic and interesting areas from the wind analysis. Fortunately, the Advanced Earth Observing Satellite II (ADEOS-II) mission carried a radiometer [the Advanced Microwave Scanning Radiometer (AMSR)] and a scatterometer, allowing for independent, collocated retrievals of rain. The authors developed an algorithm that uses AMSR observations to estimate the rain inside the scatterometer beam. This is the first in a series of papers that describe their approach to providing rain estimation and correction to scatterometer observations. This paper describes the retrieval algorithm and evaluates it using simulated data. Part II will present its validation when applied to AMSR observations. This passive microwave rain retrieval algorithm addresses the issues of nonuniform beam filling and hydrometeor uncertainty in a novel way by 1) using a large number of soundings to develop the retrieval database, thus accounting for the geographically varying atmospheric parameters; 2) addressing the spatial inhomogeneity of rain by developing multiple retrieval databases with different built-in inhomogeneity and rain intensity, along with a ‘‘rain indicator’’ to select the most appropriate database for each observed scene; 3) developing a new cloud-versus-rain partitioning that allows the use of a variety of drop size distribution assumptions to account for some of the natural variability diagnosed from the soundings; and 4) retrieving atmospheric and surface parameters just outside the rainy areas, thus providing information about the environment to help decrease the uncertainty of the rain estimates.Keywords: Precipitation, Satellite observations, Algorithms, Radiative transfer, Microwave observations, Remote sensin

Buoy and satellite observation of wind induced surface heat exchange in the intraseasonal oscillation over West Pacific and Indian Ocean

The importance of wind-driven latent heat fluxes for supporting tropical intraseasonal precipitation variability is analyzed. Tropical intraseasonal variability in the west pacific and Indian ocean is analyzed for northern and southern hemisphere summer during 1999-2005 using satellite and buoy observations. A composite analysis of QuikSCAT ocean vector winds and TRMM precipitation for the intraseasonal oscillation (ISO) indicate that enhanced precipitation is associated with anomalous surface westerly flow over the west Pacific and the Indian oceans. Anomalous westerly flow associated with the ISO is also accompanied by enhanced wind speed over the west Pacific and Indian ocean. Enhanced wind speed during westerly phases in the west Pacific and Indian oceans suggests an increase in the wind-driven component of latent heat flux. An analysis using TAO buoys and TRMM precipitation shows a significant correlation between intraseasonal (30-100 day) latent heat flux and TRMM precipitation in the region where enhanced precipitation occurs during the ISO westerly phase. Latent heat flux anomalies are about 20% of precipitation anomalies, a magnitude approximately sufficient as suggested by previous studies to support a flux-driven convective instability. The correlation of QuikSCAT wind speed and TRMM precipitation was analyzed over the west Pacific and it also showed a correlation of similar magnitude as the correlation of latent heat flux and precipitation. As a sensitivity test to determine how much of the latent heat flux anomalies were wind-driven, latent heat flux anomalies were recalculated at the location of strong correlation by fixing SST, boundary layer relative humidity, and boundary layer temperature to their sixty days running averages. This sensitivity analysis shows that most of the latent heat flux anomaly is wind-driven, indicating that intraseasonal precipitation may be supported by a wind-evaporation feedback mechanism. Wind speed and precipitation anomalies also show a significant correlation in the Indian ocean. This result suggests that if latent heat flux anomalies in the Indian ocean are primarily wind-driven, then a wind-evaporation feedback mechanism may also be supported there

Tropical cyclones in the South-West Indian Ocean : intensity changes, oceanic interaction and impacts

Includes abstract.Includes bibliographical references (p. 229-253).This study investigates the climatology, intensification and ocean atmosphere interaction in relation to the passage of tropical cyclones (TCs) in the South-West Indian Ocean (SWIO). A Climatology of TCs in the SWIO including landfall in the area of Mozambique and Madagascar was developed for the 1952-2007 and 1980-2007 periods

Time variability of sea surface parameters in the tropical Atlantic using satellite and in situ data

The influence of the tropical Atlantic Ocean over the climate of Europe, Africa and America is well known today. However, several questions about high-frequency processes in this region remain open. This thesis addresses the characterisation of the diurnal and other short timescale variability of the meteo-ocean variables measured in the tropical Atlantic Ocean by the PIRATA array, as well as derived air-sea heat fluxes. By combining the complementarity and mitigating the disadvantages of using the high temporal resolution of in situ data in conjunction with the excellent spatial coverage of satellite based data, this work also aims to investigate the characteristics of the Tropical Instability Waves in the tropical Atlantic. The satellite data validation process used in this study assesses each of the buoys individually, to take into account possible regional biases. A complete picture of the mean diurnal cycle and the seasonal variability of the diurnal signal is performed for the first time for the whole tropical Atlantic basin. The SST diurnal signal presents strong characteristics during the respective summer in both hemispheres. However, through the wavelet technique used in this analysis, a significant diurnal signal at the equator could be noticed during the second half of each year, indicating a possible modulation of the diurnal signal by processes with different timescales. It is suggested that Tropical Instability Waves could be one of these processes. The results presented here show that the TIW clearly vary their position and time of activity, depending on the degree of development of the equatorial cold tongue. The most active year analysed in this study was 2001, when the spectral characteristics could be observed as far north as 4oN. The imprints of the TIW are well marked in the wind fields, showing that clearly there are coupled mechanisms associated with the TIW. Moreover, this study confirms that a coupling mechanism suggested for the Pacific Ocean is also applicable to the tropical Atlantic basin. The measurements made by the TMI sensor, in conjunction with the Qscat wind data showed that the atmospheric fields are highly correlated with the SST fields at the timescale associated with the TIW. The analysis of the cross-scale relationship suggests that the passage of instability waves might affect the diurnal amplitude of SST, skin-SST and latent heat flux. The mechanisms that interact on the eastern and western side of the equatorial Atlantic tend to be distinct, especially due to the local oceanographic and meteorological conditions, and due to the different level of TIW activity.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

An evaluation of novel remotely sensed data to improve and verify ocean- atmosphere forecasting.

The aim of this study is to evaluate the use of novel remote observations and spatial data analysis to improve the skill of an ocean forecasting system for the central Mediterranean Sea. A high-resolution (0.042 by 0.042ๆ ocean forecasting system was setup consisting of an atmosphere model (NCEP Eta model) that was coupled to an ocean model (Princeton Ocean Model). This coupling consisted of the provision of surface atmospheric fluxes predicted at 3-hourly intervals to drive forward the ocean model. This research study dealt with a variety of aspects to improve this forecasting system using an inter-disciplinary approach. The main aspect of this thesis is an evaluation of novel, remotely- sensed data acquired by an orbiting passive microwave sensor as a tool to assess and improve ocean forecasting. Thus, SST derived by the Tropical Microwave Imager onboard the TRMM satellite was evaluated for its potential to define one of the lower boundary conditions of the Eta model. The impact was positive, and resulted in an average improvement of the skill of the model to predict lower surface marine winds by approximately 10%. TMI-data proved extremely useful to derive instantaneous turbulent heat fluxes and other surface geophysical fields that were needed to diagnose and fine-tune the skill of the Eta model to forecast these fields. The TMI SST product also proved to be a valuable data source for data assimilation by the ocean model. An optimised data assimilation scheme was derived resulting in a bias of just -0.05 С after a 15-day model integration run. This thesis shows how spatial data analysis can provide more detailed information about the high-resolution forecasts and their quality in addition to standard verification tools. Routines that explore the spatial data of the forecasts, observations and their relationship were developed and applied. Geostatistical analysis was used to model the spatial structure of the residual fields of the predictions and observations, and to translate the degree of spatial correlation in numerical and graphical terms