DYCOMS-II Satellite: DMSP SSM/I Multisensor Composite Imagery (JPG) 1.0 General Information The DMSP SSM/I Multisensor Composite Imagery (JPG) is one of several satellite products collected as part of the Dynamics and Chemistry of Marine Stratocumulus Phase II: Entrainment Studies (DYCOMS-II) project field catalog operated by the University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS; http://www.joss.ucar.edu/dycoms/catalog/). The images were produced by the Naval Research Laboratory (NRL) from the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/I). Images are available at DMSP satellite overpass times which during DYCOMS-II were typically around 01-06 UTC and 14-19 UTC. The products included in each file include GOES-10 infrared and water vapor products and DMSP wind and vapor products. The products cover the period from 1 - 30 July 2001 and cover the region from about 25 - 50 N and 110 - 140 W. The products were acquired from NRL (http://kauai.nrlmry.navy.mil/sat-bin/display10?PHOT=yes&AREA= pacific/eastern/west_coast&PROD=composite2&NAV=composite2&CGI= composite2.cgi&ARCHIVE=Latest&MOSAIC_SCALE=15%&CURRENT=LATEST.jpg) All images are in JPG format. 2.0 Data Contact Jeff Hawkins (hawkins@nrlmry.navy.mil) Tom Lee (lee@nrlmry.navy.mil) 3.0 Product Information These composites show surface wind speed and integrated water vapor from the Special Sensor Microwave Imager (SSM/I) aboard the DMSP satellites. Also shown are images from the geostationary satellites that cover the same area. The surface wind speed (no direction) is derived from the ocean surface roughness that is sensed by the SSM/I. It is not valid for land, coastlines, or areas of precipitation (blacked-out on images). The integrated water vapor shows the vapor content from the surface of the ocean to the top of the atmosphere. It does not give moisture at a specific level. 3.1 Introduction This composite image of SSM/I wind speed, SSM/I integrated water vapor, and GOES infrared and water vapor gives valuable information about the marine operating environment. The information coming from two different satellite sensors gives users an enhanced perspective that is impossible to get from a single satellite alone. The SSM/I products give information about the near-surface wind field and moisture. The GOES products give a broader view. The Special Sensor Microwave Imager (SSM/I) is a passive microwave imager aboard the DMSP satellite series. It has seven different channels measuring the earth's radiation field at different frequencies. Scientists have combined the brightness temperatures, which represent radiation measured at these frequencies, in algorithms to produce geophysical parameters, such as surface wind speed and integrated water vapor. The SSM/I wind speed parameter estimates the ocean wind speed by sensing the roughness of the ocean's surface caused by the surface wind. It does not give wind direction. It is valid for ocean areas 50 km (25 nm) or a greater distance away from the nearest coastline. Unless there is precipitation, the accuracy of the wind speed parameter is 2 m/s (4 kt) or better. The presence of rain between the satellite and the ocean surface obscures the satellite's view of the ocean and can degrade the accuracy of wind speed estimates by large amounts. In this document rain-flagged areas are "blacked-out" to avoid misinterpretation by the user. Wind speeds are not reliable for speeds greater than about 20 m/s (40 kt). In the examples here high wind speeds, like precipitation contamination and coastal contamination, are blacked out. Thus, the SSM/I wind speed parameter can not be used to estimate wind speeds in especially intense storm systems or tropical cyclones. In these storms, rain contamination and high winds often combine to render the SSM/I wind parameter nearly useless. The surface wind speed represents the wind speed 19.5 meters above the sea surface. The surface wind speed is based on the roughening of the sea surface caused by the wind. The higher the wind, the more the sea surface roughening, and the higher the brightness temperature sensed by the SSM/I channels. This translates into a higher computed wind. The SSM/I integrated water vapor parameter (IWV) shows the water vapor contained in a vertically-integrated column above the surface of the ocean. It is measured in kilograms per meter squared. It is valid for ocean areas only. It strongly represents conditions near the surface of the ocean. In other words, high values of IWV mean that the boundary layer is moist, as in the tropics. Low values indicate that the boundary layer is dry, as near the poles. It is a very different from the geostationary water vapor image, shown in the same composite. The geostationary image represents conditions in the mid troposphere: it doesn't "see" down to surface moisture conditions as does the SSM/I. This difference explains the different appearances of the two vapor products. The IWV product does not give the height of the different layers of atmospheric moisture. Thus, it is not a sounder. It can only estimate a top-to-botton value. 3.2 Advantages The SSM/I wind speed parameters allows observation of fine-scale detail in the surface wind structure. It is particularly useful in regions where ship reports are lacking. In particular, it can show detail that global models (like NOGAPS) can not resolve. It can be used to check the accuracy of mesoscale models (like COAMPS). It is especially useful in the observation of topographically-forced winds, for example gap winds, in coastal regions. When multiple SSM/I's are operating on multiple DMSP satellites, it is possible to observe changes in the wind field from one product to the next. The Integrated Water Vapor (IWV) product is useful to assess how moist the lowest layers of the atmosphere are. It often correlates well with the Sea Surface Temperature (SST). Where the SST is high the IWV is often high, suggesting a warm humid environment. Where the SST is low, the IWV is often low, suggesting cool, often dry conditions. The IWV is often a useful indicator of atmospheric fronts. A strong gradient often indicates a frontal boundary. High values equatorward of the front indicate warm, moist air. Low values poleward of the front indicate cooler, dryer, postfrontal air. 3.3 Limits The SSM/I wind speed has several disadvantages. It is only good for speeds up to about 40 knots. It is not useful within about 50 km (25 nm) of coastlines. It will not work where precipitation is falling. In the displays shown here all all these problem areas are blacked out, so that only valid values are shown. However, with the current algorithm, there is an additional problem: in areas of heavy cloudiness or drizzle, the wind speeds may be overestimates. Unfortunately, these areas are sometimes not blacked out. Future algorithms promise to correct this problem. Perhaps the biggest problem is that the SSM/I can only provide wind speed, not wind direction. This can make the viewing of SSM/I winds confusing for some forecasters. Another spacecraft sensor, called a "scatterometer", is truly an active microwave (radar) system. It can show both wind speed and direction. The disadvantages of the Integrated Water Vapor (IPW) display are several: 1. it only gives one value representing moisture conditions in the entire troposphere; it does not give mositure by level; 2. it is not available over land or ice; 3. it does not give relative humidity; in other words, it can not assess how close a given air mass is to saturation, i.e., clouds. 4.0 Quality Control Procedures UCAR/JOSS conducted no quality checks on these data. 5.0 File Naming Convention The file names are structured as follows: dmsp.yyyymmddhhmm.multisensor2.f-13.jpg Where dmsp is the type of satellite used yyyy is the four digit year mm is the two digit month dd is the two digit day hh is the hour (UTC) mm is the minute multisensor2.f-13 is the name of the product and the satellite (f-13, f-14, and f-15). 6.0 References None.