View source for Earth:Tropical cyclone rainfall forecasting

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[[Image:Rita5dayqpf.png|thumb|right|250 px|Hurricane QPF]]
{{Tropicalcyclone}}
'''Tropical cyclone rainfall forecasting''' involves using scientific models and other tools to predict the precipitation expected in [[Physics:Tropical cyclone|tropical cyclone]]s such as hurricanes and typhoons.  Knowledge of [[Earth:Tropical cyclone rainfall climatology|tropical cyclone rainfall climatology]] is helpful in the determination of a tropical cyclone rainfall forecast.  More rainfall falls in advance of the center of the cyclone than in its wake.  The heaviest rainfall falls within its [[Earth:Central dense overcast|central dense overcast]] and eyewall.  Slow moving tropical cyclones, like Hurricane Danny and Hurricane Wilma, can lead to the highest rainfall amounts due to prolonged heavy rains over a specific location.  However, vertical [[Physics:Wind shear|wind shear]] leads to decreased rainfall amounts, as rainfall is favored downshear and slightly left of the center and the upshear side is left devoid of rainfall.  The presence of hills or mountains near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, {{wipe|Madagascar}}, Réunion, {{wipe|China}}, and {{wipe|Japan}} act to magnify amounts on their windward side due to forced ascent causing heavy rainfall in the mountains.  A strong system moving through the mid latitudes, such as a [[Physics:Cold front|cold front]], can lead to high amounts from tropical systems, occurring well in advance of its center.  Movement of a tropical cyclone over cool water will also limit its rainfall potential.  A combination of factors can lead to exceptionally high rainfall amounts, as was seen during Hurricane Mitch in Central America.<ref>Federal Emergency Management Agency. [http://www.fema.gov/areyouready/hurricanes.shtm Are You Ready?] {{Webarchive|url=https://web.archive.org/web/20060629081031/http://www.fema.gov/areyouready/hurricanes.shtm |date=2006-06-29 }} Retrieved on 2006-04-05.</ref>

Use of forecast models can help determine the magnitude and pattern of the rainfall expected.  [[Earth:Climatology|Climatology]] and persistence models, such as r-CLIPER, can create a baseline for tropical cyclone rainfall [[Forecast skill|forecast skill]].  Simplified forecast models, such as the Kraft technique and the eight and sixteen-inch rules, can create quick and simple rainfall forecasts, but come with a variety of assumptions which may not be true, such as assuming average forward motion, average storm size, and a knowledge of the rainfall observing network the tropical cyclone is moving towards.  The forecast method of TRaP assumes that the rainfall structure the tropical cyclone currently has changes little over the next 24&nbsp;hours.  The global forecast model which shows the most skill in forecasting tropical cyclone-related rainfall in the {{wipe|United States}} is the ECMWF IFS (Integrated Forecasting System).<ref>{{Cite web|url=http://www.atmos.albany.edu/facstaff/tang/tcguidance/|title=Tropical Cyclone Guidance|access-date=2017-09-10|archive-date=2017-09-10|archive-url=https://web.archive.org/web/20170910173511/http://www.atmos.albany.edu/facstaff/tang/tcguidance/|url-status=live}}</ref><ref>{{Cite web|url = https://arstechnica.com/science/2017/09/us-forecast-models-have-been-pretty-terrible-during-hurricane-irma/|title = US forecast models have been pretty terrible during Hurricane Irma|date = 8 September 2017|access-date = 10 September 2017|archive-date = 10 September 2017|archive-url = https://web.archive.org/web/20170910121200/https://arstechnica.com/science/2017/09/us-forecast-models-have-been-pretty-terrible-during-hurricane-irma/|url-status = live}}</ref>

==Rainfall distribution around a tropical cyclone==
[[Image:Typhoonsizes.jpg|right|frame|The relative sizes of Typhoon Tip, Cyclone Tracy, and the United States.]]
A larger proportion of rainfall falls in advance of the center (or [[Physics:Eye (cyclone)|eye]]) than after the center's passage, with the highest percentage falling in the right-front [[Cartesian coordinate system#Cartesian coordinates in two dimensions|quadrant]].  A tropical cyclone's highest rainfall rates can lie in the right rear quadrant within a training (non-moving) inflow band.<ref>Ivan Ray Tannehill. '''Hurricanes.''' Princeton University Press: Princeton, 1942. Pages 70-76.</ref>  Rainfall is found to be strongest in their inner core, within a degree of [[Earth:Latitude|latitude]] of the center, with lesser amounts farther away from the center.  Most of the rainfall in hurricanes is concentrated within its radius of gale-force winds.<ref name="CORENE">Corene J. Matyas. [http://ams.confex.com/ams/pdfpapers/108831.pdf Relating Tropical Cyclone Rainfall Patterns to Storm Size.] {{Webarchive|url=https://web.archive.org/web/20061025121109/http://ams.confex.com/ams/pdfpapers/108831.pdf |date=2006-10-25 }} Retrieved on 2007-02-14.</ref>  Larger tropical cyclones have larger rain shields, which can lead to higher rainfall amounts farther from the cyclone's center.<ref name="CORENE"/>  Storms which have moved slowly, or loop, lead to the highest rainfall amounts.  Riehl calculated that {{convert|33.97|in|mm}} of rainfall per day can be expected within one-half degree, or {{convert|35|mi|km}}, of the center of a mature tropical cyclone.<ref>Herbert Riehl.  '''Tropical Meteorology.''' McGraw-Hill Book Company, Inc.:  New York, 1954.  Pages 293-297.</ref>  Many tropical cyclones progress at a forward motion of 10&nbsp;knots, which would limit the duration of this excessive rainfall to around one-quarter of a day, which would yield about {{convert|8.50|in|mm}} of rainfall.  This would be true over water, within {{convert|100|mi|km}} of the coastline,<ref>Russell Pfost. [http://www.srh.noaa.gov/mfl/newpage/tampa/index.htm Tropical Cyclone Quantitative Precipitation Forecasting.] {{Webarchive|url=https://web.archive.org/web/20061231124107/http://www.srh.noaa.gov/mfl/newpage/tampa/index.htm |date=2006-12-31 }} Retrieved on 2007-02-25.</ref> and outside topographic features.  As a cyclone moves farther inland and is cut off from its supply of warmth and moisture (the ocean), rainfall amounts from tropical cyclones and their remains decrease quickly.{{US States Tropical Cyclone Point Maxima}}

===Vertical wind shear===
[[Image:Floyd1999RadarPANYNJDMP.gif|thumb|right|250 px|Circulation around the east side of Floyd forcing rainfall near and behind a front to its northeast]]
Vertical [[Physics:Wind shear|wind shear]] forces the rainfall pattern around a tropical cyclone to become highly asymmetric, with most of the precipitation falling to the left and downwind of the shear vector, or downshear left.  In other words, southwesterly shear forces the bulk of the rainfall north-northeast of the center.<ref>Shuyi S. Chen, John A. Knaff, and Frank D. Marks, Jr. [http://rammb.cira.colostate.edu/resources/docs/chen_etal_2006.pdf Effects of Vertical Wind Shear and Storm Motion on Tropical Cyclone Rainfall Asymmetries Deduced from TRMM.] {{Webarchive|url=https://web.archive.org/web/20071129132638/http://rammb.cira.colostate.edu/resources/docs/chen_etal_2006.pdf |date=2007-11-29 }} Retrieved on 2007-03-28.</ref>  If the wind shear is strong enough, the bulk of the rainfall will move away from the center leading to what is known as an exposed circulation center.  When this occurs, the potential magnitude of rainfall with the tropical cyclone will be significantly reduced.

===Interaction with frontal boundaries and upper level troughs===
As a [[Physics:Tropical cyclone|tropical cyclone]] interacts with an upper-level [[Astronomy:Trough (meteorology)|trough]] and the related [[Earth:Surface weather analysis|surface front]], a distinct northern area of precipitation is seen along the front ahead of the axis of the upper level trough.  Surface fronts with precipitable water amounts of {{convert|1.46|in|mm}} or more and upper level divergence overhead east of an upper level trough can lead to significant rainfall.<ref name="WESWET">Norman W. Junker. [http://www.wpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page907.htm Original Maddox et al. MCS archetypes associated with flash flooding.] {{Webarchive|url=https://web.archive.org/web/20151029220711/http://www.wpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page907.htm |date=2015-10-29 }} Retrieved on 2007-06-24.</ref>  This type of interaction can lead to the appearance of the heaviest rainfall falling along and to the left of the tropical cyclone track, with the precipitation streaking hundreds of miles or kilometers downwind from the tropical cyclone.<ref name="WES">Norman W. Junker. [http://www.wpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page1637.htm Hurricanes and extreme rainfall.] {{Webarchive|url=https://web.archive.org/web/20130530195500/http://www.wpc.ncep.noaa.gov/research/mcs_web_test_test_files/Page1637.htm |date=2013-05-30 }} Retrieved on 2006-02-13.</ref>

===Mountains===
Moist air forced up the slopes of coastal hills and mountain chains can lead to much heavier rainfall than in the coastal plain.<ref>Yuh-Lang Lin, S. Chiao, J. A. Thurman, D. B. Ensley, and J. J. Charney. [http://ams.confex.com/ams/10Mountain/techprogram/paper_40695.htm Some Common Ingredients for heavy Orographic Rainfall and their Potential Application for Prediction.] {{Webarchive|url=https://web.archive.org/web/20071007055225/http://ams.confex.com/ams/10Mountain/techprogram/paper_40695.htm |date=2007-10-07 }} Retrieved on 2007-04-26.</ref>  This heavy rainfall can lead to landslides, which still cause significant loss of life such as seen during Hurricane Mitch in Central America, where several thousand perished.<ref>John L. Guiney and Miles B. Lawrence. [http://www.nhc.noaa.gov/1998mitch.html  Hurricane Mitch.] {{Webarchive|url=https://web.archive.org/web/20140216194107/http://www.nhc.noaa.gov/1998mitch.html |date=2014-02-16 }} Retrieved on 2007-04-26.</ref>

==Tools used in preparation of forecast==
[[Image:Isabel2003rcliper.jpg|thumb|right|150px|r-CLIPER for Isabel (2003)]]

===Climatology and persistence===
The Hurricane Research Division of the [[Organization:Atlantic Oceanographic and Meteorological Laboratory|Atlantic Oceanographic and Meteorological Laboratory]] created the r-CLIPER (rainfall [[Earth:Climatology|climatology]] and persistence) model to act as a baseline for all verification regarding [[Physics:Tropical cyclone|tropical cyclone]] rainfall.  The theory is, if the global forecast models cannot beat predictions based on climatology, then there is no [[Forecast skill|skill]] in their use.  There is a definite advantage to using the forecast track with r-CLIPER because it could be run out 120&nbsp;hours/5&nbsp;days with the forecast track of any tropical cyclone globally within a short amount of time.<ref>Frank Marks. [http://gpm.gsfc.nasa.gov/4thworkshop/files/48-Marks.ppt GPM and Tropical Cyclones.] {{webarchive|url=https://web.archive.org/web/20061006230302/http://gpm.gsfc.nasa.gov/4thworkshop/files/48-Marks.ppt |date=2006-10-06 }} Retrieved on 2007-03-15.</ref>  The short range variation which uses persistence is the '''T'''ropical '''Ra'''infall '''P'''otential technique ('''TRaP''') technique, which uses satellite-derived rainfall amounts from [[Physics:Microwave|microwave]] [[Astronomy:Satellite imagery|imaging satellites]] and extrapolates the current rainfall configuration forward for 24&nbsp;hours along the current forecast track.<ref>Elizabeth Ebert, Sheldon Kusselson, and Michael Turk. [https://wayback.archive-it.org/all/20071129132639/http://www.bom.gov.au/bmrc/wefor/staff/eee/TRaP_Australia.pdf Validation of Tropical Rainfall Potential (TRaP) Forecasts for Australian Tropical Cyclones.] Retrieved on 2007-03-28.</ref>  This technique's main flaw is that it assumes a [[Steady state|steady state]] tropical cyclone which undergoes little structural change with time, which is why it is only run forward for 24&nbsp;hours into the future.<ref>Stanley Q. Kidder, Sheldon J. Kusselson, John A. Knaff, and Robert J. Kuligowski. [http://amsu.cira.colostate.edu/kidder/11thSatmetPaper.pdf Improvements to the Experimental Tropical Rainfall Potential (TRaP) Technique.] {{webarchive|url=https://web.archive.org/web/20070817025800/http://amsu.cira.colostate.edu/kidder/11thSatmetPaper.pdf |date=2007-08-17 }} Retrieved on 2007-03-15.</ref>
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[[Image:GFSisabel2003.jpg|thumb|right|150px|GFS for Isabel (2003)]]

===Numerical weather prediction===
Computer models can be used to diagnose the magnitude of tropical cyclone rainfall.  Since forecast models output their information on a grid, they only give a general idea as to the areal coverage of moderate to heavy rainfall.  No current forecast models run at a small enough grid scale (1&nbsp;km or smaller) to be able to detect the absolute maxima measured within tropical cyclones.  Of the {{wipe|United States}} forecasting models, the best performing model for tropical cyclone rainfall forecasting is known as the '''GFS''', or '''G'''lobal '''F'''orecasting '''S'''ystem.<ref>Timothy P. Marchok, Robert F. Rogers, and Robert E. Tuleya. [http://www.ofcm.gov/ihc04/presentations/e_session3/06_marchok_ihc58_final.ppt Improving the Validation and Prediction of Tropical Cyclone Rainfall.] {{webarchive|url=https://web.archive.org/web/20061010130335/http://www.ofcm.gov//ihc04/presentations/e_session3/06_marchok_ihc58_final.ppt |date=2006-10-10 }} Retrieved on 2007-03-15.</ref>  The [[Organization:Geophysical Fluid Dynamics Laboratory|GFDL]] model has been shown to have a high bias concerning the magnitude of heavier core rains within tropical cyclones.<ref>Robert E. Tuleya, Mark DeMaria, and Robert J. Kuligowski.  [http://www.gfdl.noaa.gov/reference/bibliography/2007/rt0701.pdf Evaluation of GFDL and Simple Statistical Model Rainfall Forecasts for U. S. Landfalling Tropical Storms.] {{Webarchive|url=https://web.archive.org/web/20070824134341/http://www.gfdl.noaa.gov/reference/bibliography/2007/rt0701.pdf |date=2007-08-24 }}</ref>  Beginning in 2007, the NCEP Hurricane-WRF became available to help predict rainfall from tropical cyclones.<ref>WRF Program Coordinator. [http://wrf-model.org/development/wexob/meetings/Monthly-PC-Report-31Oct04.pdf Monthly Report of the WRF Program Coordinator.] {{Webarchive|url=https://web.archive.org/web/20071011063630/http://wrf-model.org/development/wexob/meetings/Monthly-PC-Report-31Oct04.pdf |date=2007-10-11 }} Retrieved on 2007-04-10.</ref>  Recent verification shows that both the European ECMWF forecast model and [[Earth:North American Mesoscale Model|North American Mesoscale Model]] (NAM) show a low bias with heavier rainfall amounts within tropical cyclones.<ref name="DR032009">David M. Roth [http://www.wpc.ncep.noaa.gov/research/roth/TC_QPF_talk032009.ppt Tropical Cyclone Rainfall (July 2007 presentation).] Retrieved on 2009-05-07.</ref>
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===Kraft rule===
During the late 1950s, this [[Rule of thumb|rule of thumb]] came into being, developed by R. H. Kraft.<ref>Frank Marks. [http://ams.confex.com/ams/last2000/techprogram/paper_12681.htm WSR-88D Derived Rainfall Distributions in Hurricane Danny (1997).] {{Webarchive|url=https://web.archive.org/web/20110609165241/http://ams.confex.com/ams/last2000/techprogram/paper_12681.htm |date=2011-06-09 }} Retrieved on 2007-04-13.</ref>  It was noted from rainfall amounts (in imperial units) reported by the first order rainfall network in the {{wipe|United States}} that the storm total rainfall fit a simple equation:  100 divided by the speed of motion in knots.<ref name="WES 2">Norman W. Junker. [http://www.wpc.ncep.noaa.gov/research/mcs_web_test_test_files/page0024.htm Hurricanes and Extreme Rainfall.] Retrieved on 2007-03-15.</ref>  This rule works, even in other countries, as long as a tropical cyclone is moving and only the first order or synoptic station network (with observations spaced about {{convert|60|mi|km}} apart) are used to derive storm totals.  Canada uses a modified version of the Kraft rule which divides the results by a factor of two, which takes into account the lower [[Physics:Sea surface temperature|sea surface temperature]]s seen around Atlantic Canada and the prevalence of systems undergoing vertical wind shear at their northerly latitudes.<ref name="DR032009"/>  The main problem with this rule is that the rainfall observing network is denser than either the synoptic reporting network or the first order station networks, which means the absolute maximum is likely to be underestimated.  Another problem is that it does not take the size of the tropical cyclone or topography into account.

==See also==
*[[Earth:Tropical cyclone forecasting|Tropical cyclone forecasting]]
*[[Earth:Tropical cyclone observation|Tropical cyclone observation]]

==References==
{{reflist|2}}

==External links==
*[http://www.wpc.ncep.noaa.gov/tropical/rain/web/cliqr.html CLIQR tool - finds historical matches to ongoing tropical cyclones near North America and Puerto Rico]

{{DEFAULTSORT:Tropical Cyclone Rainfall Forecasting}}
[[Category:Tropical cyclone meteorology]]

{{Sourceattribution|Tropical cyclone rainfall forecasting}}