SSM/I Windspeed Composite - Alaska Tutorial

	NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil

SSM/I Windspeed Composite - Alaska Tutorial

Click thumbnails to view original full-sized images.

Introduction

Example of SSM/I 4-Panel

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. See Background, Advantages, and Limits below for more information. 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 cannot 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 IVW 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-bottom value. For more information on the GOES satellite, see: Menzel and Purdom BAMS Article NOAA NESDIS University of Wisconsin CIMMS University of Wisconsin SSEC

Example of SSM/I 4-Panel

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. See Background, Advantages, and Limits below for more information. 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 cannot 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 IVW 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-bottom value.

For more information on the GOES satellite, see:

Menzel and Purdom BAMS Article

NOAA NESDIS

University of Wisconsin CIMMS

University of Wisconsin SSEC

Background

Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat Applications: Tutorial

Advantages

The SSM/I wind speed parameter 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) cannot 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, such as 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 equator-ward of the front indicate warm, moist air. Low values pole-ward of the front indicate cooler, dryer, postfrontal air.

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 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 moisture by level.

2. It is not available over land or ice.

3. It does not give relative humidity. In other words, it cannot assess how close a given air mass is to saturation, i.e. clouds.

Examples

SSM/I Winds show tight gradient between fast and slow winds	NOGAPS Surface Winds cannot capture mesoscale detail

The SSM/I Wind speed panel (left, lower left panel) shows a large region of fast winds (oranges) between 40 and 50 N. On its northeast edge, notice the tight gradient with much slower winds just south of Alaska. This illustrates the ability of the SSM/I wind speed parameter to locate narrow gradients impossible to resolve by the NOGAPS winds (right). The SSM/I IWV parameter (left, lower right panel) shows a relatively dry surface layer (blues) north of the polar front.

SSM/I Winds show tight gradient
between fast and slow winds

NOGAPS Surface Winds cannot capture mesoscale detail

The SSM/I Wind speed panel (left, lower left panel) shows a large region of fast winds (oranges) between 40 and 50 N. On its northeast edge, notice the tight gradient with much slower winds just south of Alaska. This illustrates the ability of the SSM/I wind speed parameter to locate narrow gradients impossible to resolve by the NOGAPS winds (right). The SSM/I IWV parameter (left, lower right panel) shows a relatively dry surface layer (blues) north of the polar front.

SSM/I winds show low-winds at center of cyclone south of Alaska	Corresponding NOGAPS Surface Winds

The SSM/I composite (left, lower left panel) shows fast winds (oranges) virtually throughout. But notice the small blue area just south of Alaska surrounded by a black ring. The black ring shows rain-flagged regions where it is impossible to retrieve wind speeds. The blue area shows low speeds in the center of a cyclonic cloud swirl shown in the infrared image (left, upper left panel). NOGAPS wind (right) can resolve such small- scale details.

SSM/I winds show low-winds at center
of cyclone south of Alaska

Corresponding NOGAPS Surface Winds

The SSM/I composite (left, lower left panel) shows fast winds (oranges) virtually throughout. But notice the small blue area just south of Alaska surrounded by a black ring. The black ring shows rain-flagged regions where it is impossible to retrieve wind speeds. The blue area shows low speeds in the center of a cyclonic cloud swirl shown in the infrared image (left, upper left panel). NOGAPS wind (right) can resolve such small- scale details.

SSM/I Winds show "Safe Haven" in the lee of Kodiak Island	NOGAPS Winds show little detail south of Alaska

The SSM/I wind product (left, lower left panel) shows a small region of sheltered seas (blue and yellow) just to the lee (southeast) of Kodiak Island off the Alaska coast. This small region is potentially a safe haven for ships and boats in the area because the north winds off the Alaskan mainland can cause high seas and ship superstructure icing. On either side of this sheltered region, reds indicate high winds of 30-40 knots. These winds appear on the backside (west) of a low in the vicinity- -the cloud spiral can be seen in the infrared image (left, upper left panel). The SSM/I water vapor image (left, upper right panel) shows the dry air associated with the offshore flow--blue values are about 10 kg per square meter. The NOGAPS surface winds (right) barely capture the small disturbance south of Alaska. Thus, the SSM/I winds can be a great advantage in assessing winds near topography or in small-scale disturbances.

SSM/I Winds show "Safe Haven"
in the lee of Kodiak Island

NOGAPS Winds show little detail south of Alaska

The SSM/I wind product (left, lower left panel) shows a small region of sheltered seas (blue and yellow) just to the lee (southeast) of Kodiak Island off the Alaska coast. This small region is potentially a safe haven for ships and boats in the area because the north winds off the Alaskan mainland can cause high seas and ship superstructure icing. On either side of this sheltered region, reds indicate high winds of 30-40 knots. These winds appear on the backside (west) of a low in the vicinity- -the cloud spiral can be seen in the infrared image (left, upper left panel). The SSM/I water vapor image (left, upper right panel) shows the dry air associated with the offshore flow--blue values are about 10 kg per square meter.

The NOGAPS surface winds (right) barely capture the small disturbance south of Alaska. Thus, the SSM/I winds can be a great advantage in assessing winds near topography or in small-scale disturbances.

Author: Tom Lee
Last Updated: Tue Dec 10 16:45:13 2002
Produced by: The Composer (Ver: 1.1.2 )

NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil