tropical forecasting final
what months are SSTS warmest
August & Sept
convective to mesoscale
0-100km minutes to hours thunderstorms, tornadoes, dust storms, squall lines
planetary scale
10,000km monsoons, el Nino, MJO weeks/months/years
sub-seasonal oscillations
2 weeks - 3 months MJO
tropical storm winds
34-63kt
threshold for high seas
8 feet
MJO phases and 850mb wind anomalies
8 phases westerly wind anomalies in eastern tropical Pacific associated with phases 8, 1, 2 easterly wind anomalies in eastern tropical pacific are associated with phases 4, 5, 6
variables of the MJO
850 hPa zonal wind 200 hPa zonal wind outgoing longwave radiation OLR (cloudiness)
Saharan Air Layer
A mass of very dry, dusty air which forms over the Sahara Desert
what is generally the busiest period for genesis of tropical cyclones from AEWs? how about in the western caribbean?
AEW: early to mid-september caribbean: october
decadal oscillations
AMO
seasonal oscillations
AO, NAO
AEWs form as instabilities on the
African Easterly Jet form ahead of troughs
annual oscillations
ENSO
ENSO
El Niño Southern Oscillation 2-7 year climate cycle characterized by SST and pressure anomalies across the Pacific 3 phases- El Nino (warm), La Nina (cold), and neutral atmospheric "see-saw" signal back and forth across Pacific
how do forecasters distinguish between a Monsoon trough and the ITCZ?
ITCZ: northern and southern hemisphere trade winds converge monsoon: trade winds and seasonal monsoonal winds converge
what is the general direction of the winds near the surface between 0-30N and what are they called
NE/E - easterly trade winds
should MEOWs and MOMs be considered forecasts?
NO they should be considered as risk maps they are a composite of storms
Probabilistic Storm Surge
P-surge storm surge probabilities based on NHC official advisories real-time forecast accounts for uncertainty in: track/tandfall location, size, forward speed, intensity available 48 hrs before arrival of TS winds
forces on a wind around a hurricane
PGF - inward centrifugal force - outward coriolis force- to the right of wind
arctic oscillation
a see-saw in pressure between the arctic and northern mid-latitudes. Similar to the North Atlantic oscillation, but confined to the middle and higher latitudes. Time scale of decades but its phases are irregular. phases correlate directly with NAO
definition of a tropical cyclone
a warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-developed center
why do tropical storms need to be triggered?
above boundary layer, RH is 60-70% downdrafts can bring drier air to the surface, impeding new clouds need to overcome "resistance" by moistening in the middle troposphere
how is the atmosphere cooled in deep convection
absorption and scattering of solar radiation cooling during downdrafts
scatterometer
active sensor that measures ocean surface winds by measuring backscatter to determine roughness of surface
cold cloud processes
aggregation- ice crystals sticking together deposition- WV changes to ice riming- ice crystals grow by colliding into supercooled water droplets, which freeze onto them
define rapid intensification
an increase in the maximum sustained winds of a tropical cyclone of at least 30 kt in a 24 hour period (NHC) hard to predict!
define wind shear
any change in wind speed and direction along a straight line
synoptic scale
around 1000km days tropical cyclones
cyclone phase space looks at what 2 parameters
asymmetry vs symmetry warm core vs cold core (thermal wind)
factors affecting storm surge
central pressure (only minimal) intensity (wind speed) forward speed size (radius of max winds) angle of approach width and slope of shelf local features- bays, rivers, islands, etc
relative radii of cloud particles
cloud condensation nucleus: 0.1 micron typical cloud droplet: 10 microns borderline cloud/rain: 100 microns typical raindrop: 1000 microns
deep convection
clouds extend well above freezing level heavy precipitation, thunder, lightning strong updrafts and downdrafts frozen recip melts at freezing level anvils of ice particles cooling and drying stabilizes air beneath clouds
Atlantic Multidecadal Oscillation AMO
coherent mode of natural variability of SSTs in Atlantic average anomaly of SSTs
how is the atmosphere warmed in deep convection
condensation = release of latent heat sinking in adjacent region (adiabatic warming)
internal dynamics that govern TC motion
convective asymmetry mesoscale systems coupling of upper/lower level circulations outflow layer instability
how does the MJO affect TC activity
convectively enhanced phase of MJO increases TC activity in Indian Ocean, Australian, NW Pacific and NE Pacific basins possible correlation btw more major hurricanes and MJO phase 1-3 (these phases relate to lower vertical shear anomalies vs high anomalies in phase 5-7)
what is data assimilation
data assimilation aims at accurate re-analysis, estimation, and prediction of an unknown, true state by merging observed information into a model -find some way to blend a model "first guess" with observations to best estimate the truth
P-3 lower fuselage radar
doppler radar below plane horizontal view
surface wind maximum is located
downshear
strongest updrafts on ________ side of eye
downshear left
expendables
dropsondes, buoy
forecast model types
dynamical models statistical models global vs regional ensemble
North American Monsoon
early june to late july provides a lot of Arizona's annual rainfall and 60% of northern Mexico's
Walker circulation
easterly trade winds on either side of ITCZ warm waters pushed to west pacific warm pool increased precipitation above WPWP depth of warm waters is greater over warm pool (deep thermocline)
Madden-Julian Oscillation
eastward-moving anomalies of precipitation and westerly wind (and clouds and pressure) traverses planet in the tropics, returns starting point approx. every 30-60 days intraseasonal (multiple events per season) modeling/forecasting is HARD
ENSO and TCs
el nino: more hurricanes in east pacific (less wind shear) and less in Atlantic (more shear) la nina: few hurricanes in east pacific (more shear, drier), more hurricanes in Atlantic (less shear)
peak hurricane szn
end of august - mid sept
2 phases of MJO
enhanced rainfall (convective) suppressed rainfall
as air parcels rise, they
entrain air from the environment this entrained air is unsaturated, cooler than air parcel
how does entrainment affect the parcel's buoyancy
entrainment reduces buoyancy in updraft
external influences that govern TC motion
environmental "steering" flow- main factor subtropical highs, approaching troughs, interactions with nearby TCs
scales of airborne sampling
environmental structure (moisture content) vortex structure (eyewall) convective structure microphysical structure
3 types of factors that influence TC motion
external influences (main) internal dynamics interactive dynamics
tropical vs extratropical cyclone
extratropical derive energy from temp gradient, tropical derive energy from latent heat and sensible heat at the air-sea interface
what are the typical spatial scales for the following types of structure in a tropical cyclone: eyewall, rainbands, cirrus outflow? what are the corresponding words for these scales?
eyewall: 1km-ish, convective scale rainbands: 10-100 kms, mesoscale cirrus outflow: 100-1000s km, synoptic scale
does a TC move faster or slower when it transitions to an extratropical cyclone?
faster- increased forward motion heavy rainfall and strong winds remain
IPCC extreme events expected
fewer cold days and nights more warm days and nights more land regions where the number of heavy precip events has increased than where it has decreased extreme sea levels have increased since 1970
how/why does landfall weaken a TC?
friction causes spin-down moisture availability is low- dry air near surface intrudes TC circulation decrease in surface evaporation rate and latent heat interaction begins when outer rainbands reach land
hurricane genesis location
from africa in aug/sept gulf/west caribbean in october
tail doppler radar
gives vertical winds
global vs regional models
global are the whole world, have lower resolution, parameterized physics such as clouds regional are just certain regions, higher resolution, explicitly resolving clouds and thunderstorms inconsitent physics with "parent" domain
what is the name of the horizontal balance in a mature hurricane?
gradient wind balance
intermediate convection
heavy showers ex. cumulus congestus does not extend far enough vertically to contain ice rain produced by collision-coalescence- warm rain
OLR anomalies
high = convectively supressed, less rainstorms low = cloudy convectively active, lots of rainstorms
under what circumstances can tropical storms form?
high RH high thermodynamic instability low wind shear enough vorticity (not on equator, not too far north) substantial potential intensity
Atlantic hurricanes vs global warming
higher rainfall rates more intense on average (medium confidence) uncertain how the annual number will change tropical cyclone surge levels should increase with sea level rise (all else equal)
confident effects of global warming
higher storm surge increased rainfall rates
primary circulation
horizontal flow (tangential) gradient and thermal wind balance symmetric for strong storms
challenges of ensemble forecasting
how to provide accurate probabilistic forecasts (will it actually hit miami 30% of the time) how to provide probabilistic info that is easy for decision-makers to interpret (when to inform the public) even when info is correct, how does one make a yes/no decision based on a probability!
cold clouds- 3/4 stages in the lifecycle of a liquid cloud droplet
ice crystals and supercooled water droplets co-exist (<0C) ice nucleation- hard, fewer nuclei than liquid nucleation condensation- WV deposition grows ice cloud droplets growth- ice crystals grow faster than water droplets
secondary circulation
in-up-out radial and vertical forced by heat and angular momentum sustains primary circulation against friction and radiative cooling also symmetric
as a tropical cyclone moves northward... it encounters
increased temperature gradients increased moisture gradients increased vertical wind shear decreased SSTs and increased SST gradients increased coriolis force
visible imagery
indicated amount of solar radiation reflected albedo, thickness
IFEX and 3 goals
intensity forecast experiment forecasts, nowcasts, research
interactive dynamics that govern TC motion
interaction between primary circulation and vorticity gradient
extratropical transition of a tropical cyclone ways
landfall moving over cooler water increased vertical wind shear (can have dry air intrusion) absorbed by extratropical low/trough
nose doppler radar
least important
datum
like a reference point for storm surge 20 feet of storm surge = 20 feet above datum mean seal level MLLW MHHW
predictions of TCs with climate change
low confidence in basin-scale projections of changes in the intensity and frequency of TCs we have no clue!! modes of climate variability that have led to variations in the intensity, frequency, and structure of TCs across the globe are very likely to continue influencing TCs
shallow TCs are steered by
lower-layer flow ex 500-850hPa mean wind
MJO and Atlantic TCs
lower-tropospheric MJO signal is lost when it hits the Atlantic, but upper level (200mb) divergence may remain strong this leads to reduced vertical wind shear and enhanced vertical motion --> more TCs!
large scale influences on TCs
madden julian oscillation ENSO north atlantic oscillation atlantic multidecadal oscillation (AMO) climate change
what serves as the trigger for TCs?
many TCs develop from African Easterly Waves first seen in April/May, continue until Oct/Nov
ensemble forecasting
many forecasts! reveals possible range of scenarios can make quantitative probabilistic estimates (30% chance it will hit Miami)
MEOW
maximum envelope of water product available on SLOSH pre-computed composite of the maximum storm surge for all surge simulations over a given set of parameters use when you can narrow down to specific scenarios (category, speed, direction, initial tide)
MOMs
maximum of the MEOWs pre-computed used to design evacuation zones and when uncertainty is high
define intensity of a TC
maximum surface wind speed at any point, sustained over a 1 minute period (from NHC)
forecast model cycle
model "first guess" --> observational preparation --> data assimilation --> initial conditions
more shear =
more sensitivity (to other factors that could destroy the storm)
how does deep convection occur
need to destabilize environmental air heating/moistening lifting from approaching a cold front, sea breeze, outflow boundary surface friction leads to convergence in boundary layer --> L, upward flow
where is there a net gain/loss of radiation
net gain in tropics, net loss near poles
are eyewalls vertical
no, stadium effect, there's a slope
is shear all bad?
no, there could be an optimal amount of shear that helps outflow
shallow convection
non-precipitating "trade wind cumuli", 1-2km high
how have monsoon criteria changed
now everyone wants their own monsoon, there's even a "north american monsoon" but there is no winter component so it's debatable
warm clouds- 4 stages in the life cycle of a liquid cloud droplet
nucleation- cloud droplet air is supersaturated and condenses) condensation- droplets grow when more WV condenses on them collision/coalescence- droplet becomes sufficiently heavy so they fall and sweep up other droplets on the way down break up- the droplets break up as they fall
air parcels must overcome ___ and be brought to ____ for deep convection to occur
overcome CIN, reach LFC
convection: dry thermals
parcels do no reach level of saturation mostly in boundary, RH<100% no clouds, invisible on satellite
passive vs active sensors
passive- receive emissions produced by Earth's atmosphere (visible, IR, microwave) active- sensor transmits a pulse and this signal is reflected/transmitted/scattered by Earth's atmosphere (precipitation radar, cloud profile radar, scatterometer)
different ways to make ensembles
perturb initial conditions to create appropriate "spread"- uses same model perturb model physics to represent uncertainty and errors in model- uses same model ensembles of different models!
AO positive phase
polar low pressure system (polar vortex) over the arctic is stronger stronger upper level westerlies cold arctic air is forced to remain farther north mid-latitude storm track is further north
microwave imagery
polar orbiting satellites measure radiances in microwave wavelength bands temperature profiles retrieved from radiances passive MIMIC (what we use to look at eyewall development)
AO negative phase
polar vortex is weaker weaker upper-level westerlies cold arctic air can push further south into USA mid-latitude storm track remains further south
AMO and TCs
positive AMO = warmer SSTs = more TCs
NAO and tropical cyclones
positive phase- more southern Azores high --> more zonal TC tracks into gulf of mexico negative phase- more northern Azores high --> more tracks recurving western North Atlantic
classical criteria for a monsoon
prevailing wind shifts 120 degrees btw Jan and July average frequency of prevailing wind > 40% speed of mean wind exceeds 3 m/s pressure patterns satisfy a steadiness criterion
potential storm surge flooding map
provides a quantitative risk assessment for decision makers shows height above ground that water could reach depicts reasonable worst-case scenario 10% chance of exceedence
rainfall in slow vs fast moving storms
rainfall is NOT correlated with intensity! slower moving storms produce more rain over a specific location
ET process
reduced sensible and latent heating, loss of warm core inner core loses symmetric appearance broad asymmetries, increase in area development of fronts faster forward speed
how does moving over cooler water affect TC
reverse of positive feedback loop already discussed; reduction of total heat flux into atmosphere --> decrease in intensity--> weaker heat and moisture fluxes
does the tropospheric air rise or sink at the equator? at 30N
rise at the equator sinks at 30N
how is sea level in the eye compared to the rest of the sea
sea level is higher in the eye bc low pressure above allows water to be up higher
North Atlantic Oscillation
sea-level pressure difference btw subtropical (Azores) high and subpolar low
how does large vertical wind shear affect storms
shear forces dry, low energy air from environment into the core at mid-levels (ventilation) convective towers are tilted which weakens convection (storm loses circular symmetry) concentration of heat becomes difficult
slower vs faster forward speed compared to storm surge
slow speed = more inland penetration fast speed = higher maximum
the atmosphere regulates itself in response to
solar radiation earth's rotation differences in ocean and land surface forcing natural and anthropogenic changes
SHIPS
statistical model, 5 day forecasts of intensity shear, SST, potential intensity, relative humidity, etc
deep layer mean
steering motion is the mean wind in the 250-850hPa (deep) layer
SFMR
stepped frequency microwave radiometer detects microwave radiation from foam at the sea surface (surface winds)
"southern oscillation"
strength of easterly trade winds modified by swings of mass and pressure across the basin higher pressure in east = stronger easterlies higher pressure in west = weak easterlies not well known if this southern oscillation is a cause or effect of el nino, but the 2 are intimately coupled
la nina phase
stronger easterlies, warm pool and convection concentrated in west cooler SSTs in east Pac thermocline slope across pacific increases - more upwelling along South American coast
positive NAO
sup-polar low and subtropical high are stronger than normal stronger pressure gradient/jet stream more northern storm track wet north america
negative NAO
sup-polar low and subtropical high are weaker than normal weaker pressure gradient dry US
IR imagery
surface and cloud top temperatures highest and coldest cloud tops appear as blue
remote sensors
tail doppler radar SFMR
el nino --> la nina transition
takes a lot longer easterly trade winds must re-establish and drive the upwelling off the South American coast
vortical hot towers
tall and skinny circulating storms morph with mesoscale convective system to help it grow form into mesoscale convective vortex
tangential vs radial wind
tangential = swirling, azimuthal radial = inward vertical = up/down
in the tropopause, what happens to pressure and temp as we increase altitude
temp and pressure decrease pressure always decreases
storm surge models are strongly dependent on
the accuracy of the meteorological input
significant wave height
the average height of the highest one-third of the waves
Mean lower low water MLLW and MHHW
the average of the peak low/high tides
what causes a monsoon?
the seasonal oscillation of solar heating with net heating in the summer hemisphere, which leads to migration of the equatorial trough and the ITCZ the differential heating between land and ocean and the resulting pressure gradient the swirl introduced to the winds by earth's rotation moisture processes and convection
how are the tropics defined
the tropics encompass the region of relatively low surface pressure located between high pressures belts in the subtropics. This definition emphasizes the dynamic nature of atmospheric circulations as a response, primarily, to solar heating of the earth, and, secondarily, to other factors such as surface properties.
storm surge warning
there is a danger of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area generally within 36 hours life-threatening situation
storm surge watch
there is the possibility of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area generally within 48 hours
how do eyewall replacement cycles affect rapid intensification
they bring intensification to a halt almost half of tropical cyclones are undergoing rapid intensification as they begin an ERC
why is it hard to predict hurricane changes
they're rare events climate models are limited attribution is difficult because of all the other large-scale oscillations and factors we don't know the accurate count of storms from earlier years, especially shorter ones or ones in the middle of the Atlantic
limitation of scatterometer
thick gaps in coverage contamination by rain
forecast improvements over time
track forecasts have constantly improved, less error intensity forecasts are barely better than they used to be
how have TC tracks already changed
translational speed is down by 10% storms are slower from 1949
TAFB
tropical analysis and forecast branch
multi-cell thunderstorms
updrafts and downdrafts support each other new cells form from lifting air from gust front of adjacent cell longer lasting, bring heavier precipitation
dynamical models
use "primitive equations" of fluid dynamics and physics of the atmosphere, coupled to ocean ex. GFS, ECMWF, HWRF give u a picture
data assimilation 101 with T
use least squares theory to produce minimum error
statistical models
use weighted combinations of parameters relevant to intensity change ex. ships just numbers
vertical wind shear
vector difference between 200hPa and 850hPa wind vectors often expressed as a magnitude
environmental influences on TCs
vertical wind shear ingestion of dry air outflow interactions air-sea interactions
el nino phase effects across globe
warm and dry west pacific & Australia wet and warm south america warm north america wet over california
hurricane intensification process
warm core builds as water condenses this release of heat induces stronger azimuthal (swirling) winds central pressure drops- increasing radial pressure gradient stronger inflow and updrafts stronger winds = more water vapor exchange from ocean --> building warm core
favorable conditions for hurricane intensification
warm ocean deep convection low wind shear moist middle troposphere enhanced upper-tropospheric outlfow
single cell thunderstorms
warm, buoyant plume of rising air creates clouds single updraft and downdraft mature stage had heavy downdraft with rain and hail downdraft cuts off updraft and storm extinguishes itself, leaving only anvil
el nino phase
weaker easterlies --> warm pool spreads out warmer SSTs in east pacific increased surface convergence over central/eastern pacific increases precipitation thermocline gets deeper in east, shallower in west pac
what is the general direction of the winds in the upper troposphere at around 30N
westerly (from west to east)
la nina --> el nino transition
westerly wind burst in the western pacific leads to quick transition wind burst can be initiated by any large-scale phenomenon (MJO) transitions usually occur in spring
width and slope of shelf compared to storm surge
wide & gentle slope = the worst - water just rolls up and huge waves hit homes narrow and sharp slope = barrier waves must overcome to reach land, much less storm surge
radius of max winds compared to storm surge
wider = worse acts like a big wall and just pushes it all onto land
in-situ data
wind, pressure, temp in flight recorded
when is the enso signal usually the strongest?
winter! december, jan, feb
what is the typical height (in km) of the tropopause in the tropics vs that in the mid-latitudes?
~20km in the tropics, <16km difference is usually greater than 3-4km
