MODIS ATMOSPHERIC PROFILES PRODUCT IN FINLAND A FEASIBILITY STUDY

Transcription

1 MODIS ATMOSPHERIC PROFILES PRODUCT IN FINLAND A FEASIBILITY STUDY Sauli Joro Finnish Meteorological Institute P.O.Box 503, FIN Helsinki, FINLAND sauli.joro@fmi.fi ABSTRACT The prediction of convection is traditionally based on the interpretation of atmospheric stability from radio soundings. To help this task, a vast amount of different instability indices has been developed in the past. The usability of these indices varies especially with the geographical location, but also with the season. The space-based observations of the atmospheric profiles have become more and more common during the last years. One of the instruments suitable for this task is the Moderate Resolution Imaging Spectroradiometer (MODIS) with 36 spectral bands. Operational retrieval of temperature and moisture profiles, derived from cloud free MODIS radiance measurements, is administered by NASA. The algorithm averages the measured radiances from an area of 5 by 5 field-of-views (approx. 5 by 5 kilometres). As a result, estimates of temperature and dew point temperature for 20 pressure levels ranging from 1000 hpa to 5 hpa are provided. The objective of this study is to find out if the MODIS profiles could be utilized in the operational prediction of thunderstorms in Finland at summertime. The selected months are May September from 2003 and Finnish Meteorological Institute has extensive weather radar and lightning sensor networks, which offer excellent reference data sets to the study. 1. INTRODUCTION Information on atmospheric stability is a very important factor in Nowcasting and Short Term Forecasting. The prediction of thunderstorms is traditionally based on the interpretation of atmospheric temperature and moisture profiles, which are typically observed with radio soundings. The problem is that the radio sounding network is rather sparse, and soundings are made only every six hours. The interpretation of the observed profiles is a task of a duty forecaster. To help this task, a vast amount of different instability indices have been developed in the past. Quite many of the indices have been developed for areas in the United States where severe convection occurs. The climate in these areas differ a great deal from the climate in Finland. Thus, the usability of these indices vary especially with the geographical location, but also with the season.

2 Space-based observations of the atmospheric profiles have become more and more common during the last years. While the best instruments for this would be different kind of sounders, one possibility is to use 36-channel Moderate Resolution Imaging Spectroradiometer (MODIS). MODIS is on-board NASA s Earth Observation System satellites Terra and Aqua. The objective of this study is to find out if the MODIS profiles could be utilized in the operational prediction of thunderstorms in Finland at summertime. 2. MODIS ATMOSPHERIC PROFILES PRODUCT Operational retrieval of temperature and moisture profiles, derived from cloud free MODIS radiance measurements, is administered by NASA. The method is based on a synthetic statistical regression that uses a database of 12,000 global radio soundings and computes the simulated MODIS radiances using a fast forward radiative transfer model. More detailed descriptions of the MODIS product can be found in Seeman et al. (2003) and Menzel et al. (2002). The product includes temperature and dew point temperature from 20 pressure levels ranging from 1000 hpa to 5 hpa. The horizontal resolution of the product is 5 by 5 kilometres. 3. AN EARLIER INSTABILITY STUDY MADE IN FINLAND An earlier study concerning the atmospheric instability and related weather was made at the University of Helsinki by Roine (2001). The objective of this study was to find out the best stability indices capable of forecasting thunderstorms in Finland at summertime. 25 different indices were selected, and optimal threshold values for the indices were defined based on radio soundings. The method used here, presented earlier by Huntrieser et al. (1997), is based on finding the maximum value of True Skill Statistic (TSS) in the case of a simple yes or no forecast. The reference data for the study was mainly lightning data. Stability indices were calculated from 00, 06, and 12 UTC soundings for two reference areas, Luonetjärvi (06 UTC) and Jokioinen (00 and 12 UTC), shown in Figure 1. The radii for the reference areas are 170 km and 130 km, respectively. Lightnings were monitored between 09 UTC and 18 UTC. Even one lightning within the reference areas was qualified as a thunderstorm leading to a hit for indices predicting instability. Based on the results of Roine (2001), four different stability indices were selected for this study Jefferson Index (JI ), K-Index (KI ), Showalter Index calculated from the lifted condensation level (SI LCL ), and Surface-based Lifted Index (SLI ). The equations for these indices are presented in Section 4. The results of Roine (2001) for the selected indices are shown in Table 1. Notice that the threshold values used in this study are emphasized.

3 Figure 1. Two reference areas, Luonetjärvi and Jokioinen, defined for the study of Roine (2001). The radii of the areas are 170 km and 130 km, respectively. Table 1. The results of Roine (2001) calculated from 06 UTC and 12 UTC soundings. Threshold values, statistical parameters, and the rank of an index is shown. Threshold values used in this study are emphasized. Index Threshold POD FAR TSS Rank 06 UTC JI > SI LCL < SLI < KI > UTC SLI < SI LCL < JI > KI > EQUATIONS FOR THE JI, KI, SI LCL, AND SLI The numerical subscripts in the Equations (1) (4) always refer to a pressure level in hpa. T and T d refers to temperature and dew point temperature, respectively, while θ w is the wet bulb potential temperature. The unit for the temperatures is Celsius. In Equations (3) and (4), air parcel lifting is done. For example, the denotion T LCL 500 in Equation (3) stands for the temperature, which is achieved by lifting the air parcel adiabatically from the LCL to 500 hpa. As the level here is the LCL, the lift would be completely a moist adiabatic process. In Equation (4) the lift begins from the surface, thus, both dry and moist adiabatic processes occur during the lift. JI = 1.6 θ w850 T , (1)

4 KI = (T 850 T 500 ) + T d850 (T 700 T d700 ) (2) SI LCL = T 500 T LCL 500 (3) SLI = T 500 T sfc 500 (4) 5. A CASE STUDY FROM 30 JULY 2003 In Finland the morning MODIS overpass occurs around 9 10 UTC, which corresponds to local time. Typically at this time convection, occasionally even severe convection, has already started producing quite a lot of cumuliform clouds, which prevent the MODIS atmospheric profile retrieval. For this reason only a hand-full of acceptable cases were found from the summers of 2003 and The best from these, where large areas over Finland were cloud free, occurred 30 July 2003 at 09:25 UTC, and is presented next. Figure 2 presents two Advanced Very High Resolution Radiometer (AVHRR) images from NOAA 12 and NOAA 15 satellites received on 30 July 2003 at 04:36 UTC and 16:10 UTC. In the morning large areas over Finland are cloud free, while in the afternoon some thunderstorms develop especially to the western part of Finland. The southern most part of Finland remained cloud free. Figure 3 shows lightnings occurring between UTC and accumulated rain between UTC on 30 July The lightnings (very dark grey) are mapped on top of the 09:25 UTC MODIS overpass. Light gray here represents clear sky, while the darker grey Figure 2. NOAA 12 and NOAA 15 images from 30 July The overpasses were received at 04:36 UTC and 16:10 UTC, respectively.

5 Figure 3. Lightnings (very dark grey) between UTC and accumulated rain (dark grey shades) between UTC on 30 July Light grey in the lightning map represents clear sky in the MODIS overpass at 09:25 UTC, while the darker grey marks the cloudy areas. Encircled are the areas around Luonetjärvi and Jokioinen where the radio soundings are made. marks the cloudy areas where the profile retrieval cannot be done. The white spots represent no data -areas. The 12 hour accumulated rain is taken from the Finnish Meteorological Institute weather radar network. The two black rings in the radar image encircle areas around Luonetjärvi and Jokioinen. Both the lightning and the accumulated rain images in Figure 3 suggest that there was not much convection occuring in these areas during the day. Figure 4 shows the stability indices calculated from the 09:25 UTC MODIS overpass on 30 July Very dark grey in the figures marks the areas where the index is indicating instability, while the light grey areas are expected to remain stable. As in the lightning image in Figure 3, dark grey represents the cloudy areas. In Figure 4 both KI and SI LCL are indicating instability to almost all clear areas. Surface-based Lifted Index does a slightly better job having some stable areas in the central part of Finland. Nevertheless, it is indicating instability over the southern most part of Finland. As can be seen from Figure 3, this instability never occurred. Jefferson index differs remarkably from the other indices. It predicts the air mass to stay very stable. Figure 5a shows the 06 UTC sounding from Luonetjärvi (light grey) together with a MODIS profile (black) collected from the same point at 09:25 UTC. It can be seen that the MODIS temperature profile is rather good, but the retrieval fails to detect the dry layer between

6 Figure 4. Stability indices calculated from the MODIS overpass on 30 July 2003 at 09:25 UTC. Very dark grey depicts the areas where the index is indicating instability, while the light gray areas are expected to remain stable during the day. As in Figure 3, dark gray marks the cloudy areas in the MODIS overpass. White represents no data -areas. 700 and 500 hpa. This result is somewhat expected when keeping in mind that MODIS is not a sounder. Even though the small details in the moisture profile are missing, the MODIS retrieval manages to characterize the amount of moisture acceptably. Figure 5b shows MODIS profiles collected 50 and 100 kilometers to north, east, south, and west from Luonetjärvi. While the temperature profiles in Figure 5b are very much alike, none of the moisture profiles really catches the dry layer between 700 and 500 hpa. Figure 5c, showing the 06, 12, and 18 UTC soundings from Luonetjärvi and Jokioinen, suggests Figure 5. a) 06 UTC sounding from Luonetjärvi on 30 July 2003 together with a MODIS profile collected from the same point at 09:25 UTC. b) A group of MODIS profiles collected 50 km and 100 kilometers to north, east, south, and west from the point Luonetjärvi at 09:25 UTC. c) 06, 12, and 18 UTC soundings from Luonetjärvi and Jokioinen on 30 July 2003.

7 Table 2. JI, KI, SI LCL, and SLI calculated from the 06, 12, and 18 UTC soundings, and from the 09:25 UTC MODIS profiles. Capital letters refer to Luonetjärvi (L) and Jokioinen (J) areas. Index values exceeding the predefined threshold are emphasized. Sounding UTC 06:00L 09:25L 09:25J 12:00J 18:00L JI KI SI LCL SLI that the air mass remained unchanged during the day, and the dry layer between 700 and 500 hpa prevented the convection both in Luonetjärvi and Jokioinen area (see Figure 3). All of the stability indices selected for this study use temperatures and dew point temperatures from certain fixed pressure levels. This is true for quite a number of other indices as well. Table 2 presents the JI, KI, SI LCL, and SLI indices calculated from the 06, 12, and 18 UTC soundings, but also from the MODIS profiles at 09:25 UTC. The capital letters in the time stamps refer to Luonetjärvi (L) and Jokioinen (J). The values exceeding the threshold are emphasized. It can be seen that even if the indices are calculated from radio soundings they are predicting instability to both Luonetjärvi and Jokioinen area. Again, when taking a look to Figure 3, both of the areas remained quite stable during the day. This interpretation, of course, depends on one s opinion on the representativeness of a sounding if increasing the radius of the circles in Figure 3, one starts to have both lightnings and rain within the circle. 6. CONCLUSIONS The objective of this study was to find out if the MODIS atmospheric profiles product could be operationally utilized in the prediction of thunderstorms in Finland at summertime. The results of Roine (2001), an earlier instability study made in Finland, were used to select stability indices and their optimal threshold values. From the operational point of view MODIS overpasses in Finland occur too late. Quite often convection has already started producing cumuliform clouds, which in turn prevent the atmospheric profile retrieval. A case study from 30 July 2003 was presented. The MODIS overpass occurred at 09:25 UTC. None of the stability indices, when compared to the lightnings and the 12 hour accumulated rain, managed to to give good general description of the instability on that day. Both KI and SI LCL predicted instability to almost all clear areas, while the result of JI was just the opposite. Even though SLI was the best index here, it also over-forecasted instability. On the other hand, it must be noted that development of thunderstorms and/or showers is a complicated process, which is affected by many factors; an unstable air mass alone is not automatically enough. When having a large amount of

8 atmospheric profiles, like from the MODIS retrieval, it could be more useful to examine the distribution of the index values instead of having just one serving as a threshold. This kind of inspection for this case would be very interesting and should be done in the future. The MODIS profiles collected around Luonetjärvi area proved that the retrieval failed to detect a dry layer between 700 and 500 hpa, but generally speaking the amount moisture was characterized acceptably. The temperature profiles, instead, were good. The failure of detecting the dry layer, however, was not the reason for the indices over-forecasting the instability. The result was the same when calculating the indices from the actual radio soundings. This is because the selected indices use information only from a few pressure levels, which is not always enough to analyze the stability of an air mass. Furthermore, they are developed experimentally quite a few decades ago. It might be more beneficial to utilize modern radiance measurements in a more sophisticated way, e.g., by integrating them to LAPS (Albers et al., 1996). REFERENCES ALBERS, S. C., J. A. MCGINLEY, D. L. BIRKENHEUER and J. R. SMART, (1996) The Local Analysis and Prediction System (LAPS): Analysis of clouds, precipitation, and temperature. Weather and Forecasting, 11, 3, HUNTRIESER, H., H. H. SCHIESSER, W. SCHMID and A. WALDVOGEL, (1997) Comparison of traditional and newly developed thunderstorm indices for Switzerland. Weather and Forecasting, 12, 1, MENZEL, W. P., S. W. SEEMAN, J. LI and L. E. GUMLEY, (2002) MODIS Atmospheric Profile Retrieval Algorithm Theoretical Basis Document. National Aeronautics and Space Administration. < ROINE, K., (2001) Stabiilisuusindeksit ja sää Suomessa. Master s thesis, Department of Meteorology, University of Helsinki. Written in Finnish. Title translation: "Stability indices and weather in Finland". 72 pp. SEEMAN, S., J. LI, W. P. MENZEL and L. E. GUMLEY, (2003) Operational retrieval of atmospheric temperature, moisture, and ozone from modis infrared radiances. Journal of Applied Meteorology, 42, 8, ACKNOWLEDGEMENTS In order to get more familiar with the MODIS Atmospheric Profiles product, a visit to Cooperative Institute of Meteorological Satellite Studies (CIMSS) at the University of Wisconsin-Madison, was made in the beginning of May The funding for the visit was provided by The Finnish Academy of Science and Letters, Vilho Yrjö and Kalle Väisälä Foundation. I would like to thank the personnel of CIMSS, especially Dr. W. Paul Menzel and Liam E. Gumley, and the Finnish Academy of Science and Letters for making this visit possible.