Assimilation of spaceborne Doppler wind lidar observations in a mesoscale model

Šavli, Matic (author), Žagar, Nedjeljka (mentor)

A continuous improvement of weather prediction is the most important activity of the most of meteorological research. Numerical Weather Prediction (NWP) is the initial value problem, in addition dependent on the quality of numerical model. The NWP improvements rely substantially on the quality atmospheric observations. They are needed in the process of data assimilation that prepares initial conditions for the model forecast. The lack of observations of wind profiles is currently the main shortcoming of the Global Observing System (GOS). The wind information is crucial in the tropics and for small-scale processes in the extra-tropics. On 22 August 2018, a long awaited ESA’s mission, the Aeolus satellite has been launched, which marks the beginning of the new era of measuring winds using lidars from space. Aeolus will measure the so-called horizontal line-of-sight (HLOS) winds below about 30 km. This is the wind component measured in the direction of the pointing lidar and projected horizontally. The line of sight is defined by the azimuth angle from the north which is in the midlatitudes around 60o. Winds are retrieved from the light scattered on the air molecules (Rayleigh winds) and on the air particles such as aerosols and cloud particulates (Mie winds). The HLOS Aeolus winds are expected to improve the forecast skill in global models. The potential of HLOS winds in limited area models (LAMs), the main objective of this thesis, has not been yet addressed. As LAMs simulate small-scale processes, their initialization requires higher resolution observations compared to global models. Even though the Aeolus data with its default horizontal resolution of 90 km can not provide many profiles for the use in a LAM domain, they may be valuable due to the lack of wind profiles. In addition, it is possible to increase the HLOS horizontal resolution at the expense of the data accuracy. The main goal of the thesis is to assess the potential of the HLOS winds in comparison to the zonal and meridional wind components and the full wind information in a LAM domain over Europe and northern Atlantic. As a single HLOS observation contains some information on both the zonal and meridional wind components, its impact in the assimilation will project on both components depending on the azimuth and data assimilation modelling, especially the covariances of the background errors which define the spreading of observed information in the model space. The impact of HLOS profiles in a LAM was addressed using the ensemble data assimilation that provides flow-dependent background error covariance. A novel system built for the thesis is based on the 50-member ensemble using the Weather Research and Forecasting (WRF) model and the Ensemble Adjustment Kalman Filter (EAKF), nested in the state-of-the-art operational ensemble prediction system of the European Centre for medium-Range Weather Forecasts (ECMWF). The flow-dependent representation of the background-error covariances has been shown crucial for the assimilation of HLOS. This was demonstrated on the case of a cold front in the North Atlantic. It was also shown that the assimilation of HLOS winds in special cases with the EAKF may be more useful than the assimilation of full wind vector. An average potential of HLOS winds was investigated using a series of Observing System Simulation Experiments (OSSEs) that compared the impact of simulated HLOS data with the impact of full wind and its two wind components as well as temperature observations. Results show that the impact of HLOS winds is linearly distributed between the zonal and meridional wind components as defined by the applied azimuth of 30◦ from the zonal direction. The multivariate coupling has been found on average weak. Despite a weak multivariate impact, the HLOS winds have been shown promising as they provide better analysis in the zonal wind component compared to the case when only meridional winds are assimilated, and a better impact on the meridional wind compared to the assimilation of the zonal wind component only. The impact of increased resolution of Aeolus observations was addressed using sensitivity experiments with the Aeolus simulator and a global high resolution (T3999) 10-day forecast of ECMWF coupled with the CALIPSO satellite observations of optical properties of the atmosphere. It is found that the Mie winds are less sensitive on the changes in the accumulation length used to prepare a single HLOS profile then the Rayleigh winds. In particular, the Mie wind observation error is found rather constant with amplitude 1-1.2 ms−1 for the range of the accumulation lengths between 30 km and 90 km. These results suggest a significant tuning potential of the Aeolus retrieval for the need of weather prediction with high-resolution LAMs.

• Language: English
• Keywords: Aeolus satellite, horizontal line-of-sight winds, limited-area modelling, ensemble Kalman filter data assimilation
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2018
• PID: 20.500.12556/RUL-104358
• COBISS.SI-ID: 372393
• Publication date in RUL: 05.10.2018

Secondary language

• Language: Slovenian
• Title: Asimilacija satelitskih opazovanj vetra z Dopplerjevim lidarjem v mezoskalni model

Abstract

Eden glavnih ciljev raziskav v meteorologiji je izboljšanje kvalitete napovedi vremena. Numerično napovedovanje vremena (ang. Numerical Weather Prediction - NWP) predstavlja problem začetnih pogojev. Za izboljšanje NWP so predvsem potrebna kvalitetna opazovanja atmosfere, ki so osnova za pripravo začetnih pogojev za napoved. Trenutno glavna slabost globalnega opazovalnega sistema je pomankanje direktnih opazovanj profilov vetra. Informacija o vetru je posebej pomembna v tropskih predelih in tudi v zmernih zemljepisnih širinah za predstavitev procesov na majhnih skalah. To luknjo v opazovanjih, bo delno zapolnil Aeolus, satelit Evropske vesoljske agencije, ki predstavlja prvi sistem za merjenje vetra iz vesolja z Dopplerjevim lidarjem. Aeolus je bil uspešno izstreljen 21.8.2018. Lidar na Aeolus meri radialno komponento hitrosti gibanja delcev in molekul zraka, ki jo določa usmerjenost žarka lidarja, med tlemi in višino približno 30 km. Glavni produkt sistema je meritev hitrosti HLOS (ang. Horizontal Line-of-sight), ki predstavlja projekcijo radialne komponente hitrosti na horizontalno ravnino, pri predpostavki, da je vertikalna hitrost zanemarljiva. Dosedanje študije so pokazale, da bo uporaba opazovanj Aeolus zanesljivo prispevala h izboljšanju napovedi vremena z globalnimi modeli. Disertacija se ukvarja z vplivom opazovanj sistema Aeolus v modelih na omejenem območju (ang. limited area models - LAM), ki do sedaj ni raziskan. Glede na velikost območja LAM, dolžino uporabne napovedi in število opazovanj HLOS, ni jasno kakšen je pričakovan vpliv opazovanj Aeolus. Tipična horizontalna ločljivost opazovanj Aeolus je 90 km, kar je značilno večja razdalja kot je tipična ločljivost LAM, ki je nekaj km. Opazovanja Aeolus so kljub temu lahko zelo pomembna, saj v splošnem opazovanj vetra ni veliko (npr. nad oceani) in poleg tega, sistem Aeolus omogoča procesiranje opazovanj na višji ločljivosti. V okviru disertacije je razvit nov sistem za napovedovanje vremena, ki je osnovan na metodi ansambel Kalman filtra in je sklopljen z modelom ECMWF. Sistem se je uporabil pri analizi vpliva opazovanj vetra tipa HLOS v primerjavi z opazovanji celotnega vektorja horizontalnega vetra, oziroma njegovih dveh komponent. Povezanost kovariance napak prvega približka z lastnostmi toka v atmosferi, predstavlja pomemben faktor pri asimilaciji opazovanj HLOS. To je bilo prikazano na primeru hladne fronte v Severnem Atlantiku, kjer se je izkazalo, da je lahko asimilacija opazovanja HLOS, v nekaterih primerih bolj učinkovita kot asimilacija opazovanja celotnega vektorja vetra. Povprečen vpliv opazovanj HLOS na pripravo začetnih pogojev za LAM, je bil ovrednoten z vrsto eksperimentov OSSE (ang. Observing System Simulation Experiments). Pri tem se je povprečni vpliv opazovanj HLOS primerjal s povprečnim vplivom vektorja vetra, njegovih komponent in tudi temperature. Rezultati so pokazali, da je vpliv HLOS, ki je nagnjen za 30◦severno iz zonalne smeri, linearno porazdeljen med zonalno in meridionalno komponento vetra. Vpliv multivariatnih lastnosti sistema pri asimilaciji HLOS je v povprečju majhen. Kljub temu se je izkazalo, da opazovanja HLOS omogočajo pripravo bolj kvalitetnih analiz v zonalnem vetru, kot v primeru kjer se asimilira le meridionalni veter. Podobno, opazovanja HLOS omogočajo pripravo bolj kvalitetnih analiz v meridionalnem vetru, kot v primeru kjer se asimilira le zonalni veter. V namen preučevanja vpliva povečane ločljivosti opazovanj Aeolus na njihove lastnosti, se je pripravila vrsta eksperimentov občutljivosti. Eksperimenti so bili narejeni z simulatorjem Aeolus, kjer so bila vhodna polja pripravljena s pomočjo kompozicije globalne napovedi na visoki ločljivosti Evropskega centra za srednjeročno napoved (ECMWF) in satelitskimi meritvami optičnih lasnosti atmosfere sistema CALIPSO. Napaka opazovanj vetra, ocenjenega iz gibanja aerosolov in hidrometeorjev (veter Mie), je relativno neodvisna od njihove horizontalne ločljivosti. Za horizontalno ločljivost v intervalu 30-90 km je napaka opazovanj med 1 in 1.2 ms−1. Ti rezulati napovedujejo, da je za pripravo začetnih polj za LAM največji vpliv pričakovati iz opazovanj vetra Mie.

• Keywords: misija Aeolus, horizontalna komponenta radialnega vetra, mezoskalni sistem za asimilacijo, metoda ansabel Kalman filtra