Editing NEXRAD (section)

==Scan strategies==
The NEXRAD radar system continually refreshes its three-dimensional database via one of several predetermined scan patterns. These patterns have differing PRFs to fit the respective use, but all have a constant resolution. Since the system samples the atmosphere in three dimensions, there are many variables that can be changed, depending on the desired output. With all traditional VCPs, the antenna scans at a maximum of 19.5 degrees in elevation, and a minimum of .5, with some coastal sites scanning as low as .2 or lower. Due to the incomplete elevation coverage, a phenomenon known as "The Cone of Silence" is present with all NEXRAD radars.<ref>{{cite web|url=https://www.roc.noaa.gov/wsr88d/Engineering/NEXRADTechInfo.aspx|title=NEXRAD Technical Information|website=www.roc.noaa.gov|access-date=13 April 2018}}</ref> The term describes the lack of coverage directly above the radar sites.

There are currently seven Volume Coverage Patterns (VCP) available to NWS meteorologists, with an eighth in the process of replacing one of the existing seven. Each VCP is a predefined set of instructions that control antenna rotation speed, elevation angle, transmitter pulse repetition frequency and pulse width. The radar operator chooses from the VCPs based on the type of weather occurring:

* Clear Air or Light Precipitation: VCP 31, 32, and 35
* Shallow Precipitation: VCP 35, 112, and 215
* Non-Tropical Convection: VCP 12, 212, and 215
* Tropical System Convection: VCP 212, 215, 112, and 121<ref>{{Cite web|url=http://www.nws.noaa.gov/os/notification/tin15-49wsr-88dbuild18.htm|title=Technical Implementation Notice 15–49 National Weather Service Headquarters Washington DC|date=Oct 22, 2015|access-date=May 23, 2016}}</ref><ref name=":2">{{Cite web|url=http://www.roc.noaa.gov/WSR88D/PublicDocs/NewTechnology/New_VCP_Paradigm_Public_Oct_2015.pdf|title=WSR-88D Volume Coverage Pattern (VCP) Improvement Initiatives|date=Oct 22, 2015|publisher=National Weather Service|access-date=May 23, 2016}}</ref>

{| class="wikitable" style="text-align:center; width: 100%; font-size: normal;"
!VCP
!Scan time (min)
!Elevation scans
!Elevation angles (°)
!Usage
!SAILS available?
|-
|12
|4.2<ref name=":0" />
| rowspan="3" |14
| rowspan="3" |0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4, 5.1, 6.4, 8, 10, 12.5, 15.6, 19.5
|Severe weather, including tornadoes, located closer to the radar (within 85 miles for storms traveling up to 55 MPH, but shorter distances for faster-moving precipitation)
| rowspan="2" |Yes (up to three per volume scan)<ref name=":0">{{Cite web|url=http://www.roc.noaa.gov/wsr88d/PublicDocs/NewTechnology/MESO-SAILS_Intial_Description_v02_Feb_2014.pdf|title=MESO-SAILS (Multiple Elevation Scan Option for SAILS) Initial Description Document|publisher=National Weather Service|access-date=May 23, 2016}}</ref>
|-
|212
|4.5<ref>{{Cite web|url=https://www.weather.gov/jetstream/vcp_max|title=NWS JetStream MAX - Doppler Radar Volume Coverage Patterns (VCPs)|last=US Department of Commerce|first=NOAA|website=www.weather.gov|language=EN-US|access-date=2019-10-16}}</ref>
|Severe weather, including tornadoes, over 70 miles away from the radar, or widespread severe convection. Best VCP for MRLE use. Completion time for VCP 212 + 1 SAILS scan is similar to VCP 12 + 2 SAILS scans
|-
|112
|5.5<ref name=":3">{{Cite web|url=https://www.roc.noaa.gov/WSR88D/PublicDocs/NewTechnology/Theory_ConOps_VCP112.pdf|title=Theory and Concept of Operations for Multi-PRF Dealiasing Algorithm's VCP 112|date=March 19, 2019|website=National Weather Service|access-date=October 16, 2019}}</ref>
|Variant of VCP 212 designed for tropical systems and strong, non-severe wind shear events. Uses a combination of MPDA and SZ-2 to form a contiguous velocity display.<ref name=":3" /> MRLE use is not possible with this VCP
|Yes (up to one per volume scan)
|-
|215
|6<ref name=":2" />
|15
|0.5, 0.9, 1.3, 1,8, 2.4, 3.1, 4, 5.1, 6.4, 8, 10, 12, 14, 16.7, 19.5
|General-purpose precipitation, including tropical systems capable of producing tornadoes. Most vertical resolution of any VCP
|Yes (up to one per volume scan)
|-
|121
|6
|9
|0.5, 1.5, 2.4, 3.4, 4.3, 6, 9.9, 14.6, 19.5
|Legacy VCP, originally designed for tropical systems. Has significant gaps in vertical resolution above 6°. Scan strategy ensures 20 rotations in six minutes, heavily wearing antenna mechanical components. Similar completion time to VCP 215. To be replaced by VCP 112
|No
|-
|31
| rowspan="2" |10
| rowspan="2" |5
| rowspan="2" |0.5, 1.5, 2.4, 3.4, 4.3
|Long-pulse clear air mode designed for maximum sensitivity. Excellent for detecting light snow or subtle boundaries. Prone to detecting ground clutter. May be prone to detecting [[virga]]
|No
|-
|32
|Short-pulse clear air mode designed for clear air or isolated light rain and/or wintry precipitation. Ideal to use when no precipitation is in the radar range, to reduce wear on antenna mechanical components
|No
|-
|35
|7<ref name=":2" />
|7
|0.5, 0.9, 1.3, 1,8, 2.4, 3.1, 4, 5.1, 6.4
|Short-pulse clear air VCP designed for scattered to widespread light to moderate precipitation from non-convective cloudforms, especially [[Nimbostratus cloud|nimbostratus]]. Not recommended for convection, except for pop-up thundershowers produced by [[Cumulus congestus cloud]]s located 30 miles or more away from the radar
|Yes (up to one per volume scan)
|}

The specific VCP currently in use at each NEXRAD site is available.<ref>{{cite web |title=Current VCP in use for each Site |url=https://www.roc.noaa.gov/branches/operations-branch/current-vcps.php |access-date=25 August 2024 |website=www.roc.noaa.gov}}</ref>