basic tools in meteorology and climatology rené garreaud

Basic Tools inMeteorology and Climatology

René Garreaudwww.dgf.uchile.cl/rene

Since pressure always decrease with height, sometimes we use pressure as the vertical coordinate (instead of m ASL). In this case, geopotential height (Z) is a variable. For instance, at the 500 hPa level, Z usually varies between 5300-5600 m.

Atmospheric pressure:Weight of the air column above you

Units: hPa = mb = 100 Pa = 100 * N/m2

p(z) p0exp(-z/H*) ~ 1013*exp(z/8000) [hPa if z in meters], H* = R[T]/g height scale

Air temperature ~ kinetic energy of air moleculesChanges because heat flux divergence

Radiosonde (p,T,RH,…)with height

Average (time, space) temperature profile of the earth atmosphere. Note and explain the three maxima.

T usually decrease with height (unstable condition)Inversion layers (stable air) where T increase with height

ff [m/s]dd [°]

streamline

streamline

Local wind

u > 0v > 0

u < 0v < 0u < 0

v < 0

Blowing from….SW

SE

NE

Wind is a vector: magnitude (speed) and blowing direction (° wrt north)Alternatively, zonal (u, west-east) and meridional (v, south-north) components

Mapa de Observaciones Carta del tiempo

Every day meteorologist have quite a lot of data that need to be synthesized to detect weather patterns. Analysis: Hand-made (old) or objective (computer)

Isobars: lines of constant pressure at a given level(e.g. sea level pressure chart) and time (snapshot).

High (anticyclones) and Low (cyclones) pressure centers.

HH

L

L

SLP map, October 17 2010

Isotherms: lines of constant air temperature at a given level(z or p) and time (snapshot).

The temperature field usually decreases poleward and often exhibits elongated bands of strong thermal gradient (baroclinicity) called fronts

T @ 850 hPa (1500 m ASL)October 17, 2010

Wind @ 300 hPa (9000 m ASL)

Map showing wind vectors (arrows) and wind speed (colors)Note the presence of wind maxima (jet streams)

Jet stream

Mostly zonal flow Mostly meridional flow

XX: Lines of constant geopotential height at a given pressure levelTrough: relative minimum pressure. Ridge: relative maximum pressure

In both cases “relative” looking at a same latitude

Z @ 300 hPa (9000 m ASL)

Trough

Ridge

pi1 = pi2 = pi3 pi1 = pi2 = pi3 pi1 > pi2 < pi3

ps1 = ps2 = ps3 ps1 < ps2 > ps3 ps1 < ps2 > ps3

psup

pinf

C

What causes the pressure gradients (or geopotential gradients) in the atmosphere? Case 1: Thermal contrasts.

Combining hydrostatic and ideal gas equations one can show:

Z = RTg-1*log10(pinf/psup)

1. Inicialmente, las tres columnas son identicas2. La columna central se calienta diferencialmente3. Aire en col. más cálida se expande... Aparece gradiente de presión en altura4. Viento diverge, sacando masa de la columna5. Presión cae en superficie...parece gradiente cerca de la superficie6. Viento converge, agregando masa de la columna

0 km

12 km

L

HL

H

Warmer Cooler

The mass field (pressure) adjust rapidly to the thermal forcingNote that pressure remains unchanged at the level of warming

Eventually, atmospheric circulation will flatten pressure field

Z = RTg-1*log10(pinf/psup)

We now can reinterpret troughs and ridges seen in the geopotential height in the upper troposphere (e.g., 300 hPa) as tongues of cold air abnormally at low latitudes and warm air abnormally at high latitudes

Geopotential @ 300 hPa (contours)Air temperature averaged between 700 and 400 hPa (colors)

Trayectoria desdesistema terrestre

t = 0 t = dT

Trayectoria desdesistema inercial

Fh=0Observador

en Tierra

Air will flow from high pressure areas toward low pressure areasEarth rotation, however, complicate things…apparently

Earth rotation introduce an apparent force that deviate air parcels from its expected trajectory. This force is called the Coriolis force and obeys the following rules:

• Acts over moving air and water parcels• Deflects air parcels to the left in the SH (right in the NH)• Its magnitude is zero at the equator and increase to a maximum at the poles.

Partículas flotando sobre superficieparten impulsivamente al ecuador….

A AA

En el ecuador o enplaneta sin rotación

Muy cerca del ecuador orotación planetaria muy lenta

Lejos del ecuador (>20) para movimientos lentos

¿Como circula el aire en torno a los centros de alta y baja presión (HS)?

B BB

Air will flow from high pressure areas toward low pressure areasEarth rotation, however, complicate things…apparently

zf

gkp

fkV

y

pfu

x

pfv

Fgpvdt

vd

Fam

g

R

ˆ1ˆ

11

12

Atmospheric dynamics (same for ocean)

Second Newton’s law:

Which in the atmospherebecomes...

Scale analysis for large-scaleflow leads to…

So we can define the geostrophicwind, very close to actual wind in large scale systems

CoriolisPressuregradient Friction

HL H

Geostrophic wind (close to actual wind) rules of operation (SH):

• Clock-wise around a low, anti-clockwise around a high• Stronger in areas of tight isobars (or geopotential lines)• Not very usefull at low latitudes

B

A

B

B

B

A

A

Isobaras

¿Porque nos gustan tanto los mapas del tiempo?

Climate data issues

• Temporal resolution (daily, monthly, yearly)• Temporal coverage and continuity• Quality and QControl• Station data versus gridded products• Spatial context• Multi variable records (T, P, ….)• Data sources• Data formats (ASCII, NC, never Excel!)• Data sharing etiquette

Surface (land/ocean) Synoptic Stations Met. Observations (T,Td,P,V,…) @ 0, 6, 12, 18 UTC are transmitted in real-time to

WMO and Analysis Centers

From where do we get climate data? Almost all climate data is initially meteorological data, acquired to assist weather nowcast and forecast (especially for aviation)

Red de Radiosondas (OMM, GTS)

Perfiles verticales (20 km) de T, HR, viento, presión, cada 12 / 24 hr

All stations (anytime, any length)

Century-long stations (Ti<1905, Tf>1995, missdata<20%)

Precipitation Mean Temperature

Global Historical Climate Network (GHCN)

Surface and UpperAir Observations

Satellite Products

Assimilationsystem

Gridded Analysis

Griddingmethod

DATA SOURCES AND PRODUCTS

Table 1. Main features of datasets commonly used in climate studies

Dataset Key referencesInput data -Variables

Spatial resolution -Coverage

Time span -Time resolution

Station GHCN

Peterson and Vose (1997)

Sfc. Obs Precip and SAT

N/A Land only

1850(*) – present Daily and Monthly

GriddedGHCN

Peterson and Vose (1997)

Sfc. Obs Precip and SAT

5° 5° lat-lon Land only

1900 – present Monthly

GriddedUEA-CRU

New et al. 2000Sfc. Obs Precip and SAT

3.75° 2.5° lat-lon Land only

1900 – present Monthly

GriddedUEA-CRU05

Mitchell and Jones (2005)

Sfc. Obs Precip and SAT

0.5° 0.5° lat-lon Land only

1901 – present Monthly

GridddedU. Delware

Legates and Willmott (1999a,b)

Sfc. Obs Precip and SAT

0.5° 0.5° lat-lon Land only

1950 – 1999 Monthly

GriddedSAM-CDC data

Liebmann and Allured (2005)

Sfc. Obs Precip

1° 1° lat-lon South America

1940 – 2006 Daily and Monthly

GriddedCMAP

Xie and Arkin (1987)Sfc. Obs.; Sat. data Precip

2.5° 2.5° lat-lon Global

1979 – present Pentad and Monthly

GriddedGCPC

Adler et al. (2003)Sfc. Obs.; Sat. data Precip

2.5° 2.5° lat-lon Global

1979 – present Monthly

NCEP-NCAR Reanalysis (NNR)

Kalnay et al. 1996Kistler et al. 2001

Sfc. Obs.; UA Obs; Sat. data Pressure, temp., winds, etc.

2.5° 2.5° lat-lon,17 vertical levels Global

1948 – present 6 hr, daily, monthly

ECMWF Reanalysis(ERA-40)

Uppala et al. (2005)Sfc. Obs., UA Obs, Sat. data Pressure, temp., winds, etc.

2.5° 2.5° lat-lon,17 vertical levels Global

1948 – present 6 hr, daily, monthly

http://www.cdc.noaa.gov

http://www.ncdc.noaa.gov

http://iridl.ldeo.columbia.edu/

Because analysis are produced in real-time, some data is not assimilated, but it was archived. In the 90’s the NCEP-NCAR (USA) began a major project in which they re-run their assimilation system with all the available data.

The result is the widely used “Reanalysis” data, including many fields (air temperature, wind, pressure) on a regular 2.5°x2.5° lat-lon grid, from 1948 to present every 6 hours (also available daily, monthly and long-term-mean means). Fields are 2- or 3-Dimensional. Preferred data format: NetCDF. Freely available.

Reanalysis?!

Reanalysis system also includes a meteorological model from which precipitation and other not-observed variables (e.g., vertical motion) are derived.

Reanalysis data is great for studying interannual and higher frequency variability. Interdecadal variability and trends are not so well depicted (we don’t trust much before the 70’s, particularly in the SH).

European Center (ECMWF) did a similar effort (ERA-15 and ERA-40). Higher horizontal resolution (1.25°x1.25°), but harder to get.

Reanalysis