Satellite-Based Detection of Fog and Very Low Stratus

Transcription

1 Satellite-Based Detection of Fog and Very Low Stratus A High-Latitude Case Study Centred on the Helsinki Testbed Experiment J. Cermak 1, J. Kotro 2, O. Hyvärinen 2, V. Nietosvaara 2, J. Bendix 1 1: Laboratory for Climatology and Remote Sensing (LCRS), Marburg/Germany 2: Finnish Meteorological Institute (FMI) 7 September 2006 Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

2 Outline 1 Introduction Fundamentals The Problem 2 Approach and Data Helsinki Testbed Satellite Algorithm 3 Tests and Analysis Data Comparison Conclusions Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

3 Motivation Introduction Fundamentals Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

4 Aims Introduction Fundamentals Fog detection system should offer High spatial resolution (fog margins) High temporal resolution (nowcasting) Long time series (climatology) Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

5 Aims Introduction Fundamentals Fog detection system should offer High spatial resolution (fog margins) High temporal resolution (nowcasting) Long time series (climatology) Only geostationary satellite data suited! Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

6 Introduction Fundamentals Satellite system: Meteosat 8 SEVIRI 3 km spatial resolution 15 min repeat rate 11 spectral channels Operational since 2004 Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

7 High Latitudes: Finland Introduction The Problem Latitude southern Finland: 61 Satellite zenith angle: 70 Reduced spatial resolution Low solar elevation Enhanced atmospheric effect Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

8 Introduction High Latitudes: Difficulties The Problem Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

9 Introduction The Problem High Latitudes: Difficulties Sensor sensitivity CO 2 absorbance Wavelength (µm) Terra MODIS ch. 20 NOAA 16 AVHRR ch. 3 Meteosat SEVIRI ch. 4 CO 2 absorbance Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

10 Introduction High Latitudes: Difficulties The Problem Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

11 Research Question Introduction The Problem Problem Is the detection of fog and very low stratus possible from a geostationary platform at high latitudes? To be investigated with detailed measurements. Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

12 Helsinki Testbed Approach and Data Helsinki Testbed Figure: J. Poutiainen, 2006 Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

13 Helsinki Testbed Approach and Data Helsinki Testbed Mesoscale weather research System integration Model development Data and info on Figure: J. Poutiainen, 2006 Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

14 Measurements Approach and Data Helsinki Testbed Targeted measurements for Nowcasting applications Precipitation type Sea breeze Convection Stable boundary layer Figure: J. Poutiainen, 2006 Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

15 SOFOS Approach and Data Satellite Algorithm SOFOS Satellite-Based Operational Fog Observation Scheme Fog as a cloud in the water phase low above ground with a stratiform surface impairing visibility at the ground. Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

16 SOFOS Approach and Data Satellite Algorithm SOFOS Satellite-Based Operational Fog Observation Scheme Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

17 SOFOS Approach and Data Satellite Algorithm SOFOS Satellite-Based Operational Fog Observation Scheme Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

18 Tests and Analysis Test Procedure: Data Selection Data Comparison 12 days, November 2005 February 2006 Data availability Data quality Includes fog / low stratus situations Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

19 Tests and Analysis Data Comparison Test Procedure: Point/Pixel Comparisons Ground station observations Low clouds High clouds Clear Satellite classification Low water clouds Other clouds Clear Visibility and ceilometer data for low cloud presence Compared with satellite classification (no cloud / very low stratus cloud / high cloud) Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

20 Tests and Analysis Data Comparison Test Procedure: Contingency Tests Observation Yes Observation No Classification Yes A B Classification No C D Hit Rate: HR = A A+C [0 1] False Alarm Rate: FAR = B A+B [0 1] Critical Success Index: CSI = A A+B+C [0 1] Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

21 Test Results: By Day Tests and Analysis Data Comparison Indicator Value Best Worst HR FAR CSI n 9374 Hit rate False alarm rate Figure: Results by day Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

22 Tests and Analysis Test Results: By Station Data Comparison Indicator Value Best Worst HR FAR CSI n 9374 Hit rate False alarm rate Figure: Results by station Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

23 Analysis Tests and Analysis Data Comparison Possible reasons for misclassifications: Missed situations: Cloud overlap Low cloud cover False alarms: Station data not representative of pixel Station data quality (Visibility and ceiling don t always agree) Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

24 Tests and Analysis Test Results: CSI Distribution Data Comparison Lowest category not present No obvious spatial pattern Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

25 Tests and Analysis Data Comparison European Context: CSI Distribution METAR stations 24 days 2 nd half satellite scenes No latitudinal effect! Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

26 Conclusions Tests and Analysis Conclusions High latitude detection of very low stratus works well Small-scale situations may be missed Coordinated measurement efforts useful Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

27 Outlook Tests and Analysis Conclusions Ground fog will be tested for Helsinki testbed Qualitative analysis of cases to follow Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25

28 Contact Tests and Analysis Conclusions Helsinki Testbed: Laboratory for Climatology and Remote Sensing: cermak [at] lcrs.de Cermak et al. (LCRS, FMI) High Latitude Fog Detection 7 September / 25