24
How low can you go? Retrieval of light precipitation in mid-latitudes Chris Kidd School of Geography, Earth and Environmental Science The University of Birmingham

How low can you go? Retrieval of light precipitation in mid-latitudes

  • Upload
    wilbur

  • View
    20

  • Download
    0

Embed Size (px)

DESCRIPTION

How low can you go? Retrieval of light precipitation in mid-latitudes. Chris Kidd School of Geography, Earth and Environmental Science The University of Birmingham. 3 rd IPWG workshop, Melbourne, Australia. 23-28 October 2006. Truth? 8 Gauge / 4 MRR comparison. - PowerPoint PPT Presentation

Citation preview

Page 1: How low can you go? Retrieval of light precipitation in mid-latitudes

How low can you go?Retrieval of light precipitation in mid-latitudes

Chris Kidd

School of Geography, Earth and Environmental Science

The University of Birmingham

Page 2: How low can you go? Retrieval of light precipitation in mid-latitudes

Truth? 8 Gauge / 4 MRR comparison

0.0

2.5

5.0

7.5

10.0

12.5

15.0

1 6 11 16 21 26 31 36 41 46 51 56

Time (Minutes after 16:00, 06/07/06)

Ac

cu

mu

lati

on

(m

m)

12345678MRR1MRR2MRR3MRR4

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 3: How low can you go? Retrieval of light precipitation in mid-latitudes

0.0

2.5

5.0

7.5

10.0

12.5

15.0

1 6 11 16 21 26 31 36 41 46 51 56

Time (Minutes after 16:00, 06/07/06)

Ac

cu

mu

lati

on

(m

m)

12345678MRR1MRR2MRR3MRR4

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 4: How low can you go? Retrieval of light precipitation in mid-latitudes

0.0

2.5

5.0

7.5

10.0

12.5

15.0

1 6 11 16 21 26 31 36 41 46 51 56

Time (Minutes after 16:00, 06/07/06)

Ac

cu

mu

lati

on

(m

m)

12345678MRR1MRR2MRR3MRR4

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 5: How low can you go? Retrieval of light precipitation in mid-latitudes

0.0

2.5

5.0

7.5

10.0

12.5

15.0

1 6 11 16 21 26 31 36 41 46 51 56

Time (Minutes after 16:00, 06/07/06)

Ac

cu

mu

lati

on

(m

m)

12345678MRR1MRR2MRR3MRR4

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 6: How low can you go? Retrieval of light precipitation in mid-latitudes

0.0

2.5

5.0

7.5

10.0

12.5

15.0

1 6 11 16 21 26 31 36 41 46 51 56

Time (Minutes after 16:00, 06/07/06)

Ac

cu

mu

lati

on

(m

m)

12345678MRR1MRR2MRR3MRR4

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 7: How low can you go? Retrieval of light precipitation in mid-latitudes
Page 8: How low can you go? Retrieval of light precipitation in mid-latitudes

Zonal distribution of light precipitation

• Light rainfall becomes increasingly important towards the polar regions

• COADS data shows light precipitation occurrence >80%; ~50% in mid-latitudes

• European radar suggests ~85% of precipitation <1 mmh-1 (35%<0.1 mmh-1)

• Accumulation of light precipitation is small however… particularly in the Tropics

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 9: How low can you go? Retrieval of light precipitation in mid-latitudes

-0.5

Rain/no-rain induced biases

-1.0

• Differences in rain/no-rain boundaries reveal regional variations that do not exist in reality

• Further complicated since rain/no-rain boundaries tend to differ over land/sea areas

Problem:

• if a satellite estimate cannot retrieve light rain, it should underestimate rain totals

• corrections are usually made to correct this bias, so that it matches the 'real' rainfall

• this can lead to unintended regional biases

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 10: How low can you go? Retrieval of light precipitation in mid-latitudes

Rainfall occurrence (Bham)

0

500

1000

1500

2000

2500

3000

3500

4000

0.01

0.01

0.02

0.02

0.03

0.03

0.04

0.05

0.06

0.08

0.10

0.13

0.16

0.20

0.25

0.32

0.40

0.50

0.63

0.79

1.00

1.26

1.58

2.00

2.51

3.16

3.98

5.01

6.31

7.94

10.0

0

12.5

9

15.8

5

19.9

5

25.1

2

31.6

2

39.8

1

50.1

2

63.1

0

79.4

3

100.

00

125.

89

158.

49

199.

53

251.

19

316.

23

Rain intensity (mm/hr)

Occ

urr

en

ce (

min

ute

s)

1 minute rain rates derived from Doppler radar (10 months data)

1 mmh-1

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 11: How low can you go? Retrieval of light precipitation in mid-latitudes

Rainfall occurrence

0

10

20

30

40

50

60

70

80

90

100

0.0

10

.01

0.0

20

.02

0.0

30

.03

0.0

40

.05

0.0

60

.08

0.1

00

.13

0.1

60

.20

0.2

50

.32

0.4

00

.50

0.6

30

.79

1.0

01

.26

1.5

82

.00

2.5

13

.16

3.9

85

.01

6.3

17

.94

10

.00

12

.59

15

.85

19

.95

25

.12

31

.62

39

.81

50

.12

63

.10

79

.43

10

0.0

0

Rain intensity (mm/hr)

Cu

mu

lati

ve

pe

rce

nta

ge

Series1

Series2

Series3

Series4

1 minute rain rates derived from 4 Doppler radars (4x10 months data)

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 12: How low can you go? Retrieval of light precipitation in mid-latitudes

0

10

20

30

40

50

60

70

80

90

100

0.0

10.0

10.0

20.0

20.0

30.0

30.0

40.0

50.0

60.0

80.1

00.1

30.1

60.2

00.2

50.3

20.4

00.5

00.6

30.7

91.0

01.2

61.5

82.0

02.5

13.1

63.9

85.0

16.3

17.9

410.0

012.5

915.8

519.9

525.1

231.6

239.8

150.1

263.1

079.4

3100.0

0

Rain intensity (mm/hr)

Cu

mu

lati

ve

pe

rce

nta

ge

(a

cc

um

ula

tio

n)

Series1

Series2

Series3

Series4

Rainfall accumulation

1 minute rain rates derived from 4 Doppler radars (4x10 months data)

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 13: How low can you go? Retrieval of light precipitation in mid-latitudes

How well are we doing?

Occurrence of rainfall:Cumulative percentage of the occurrence of rainfall by daily totals- significant occurrence < 2 mm/day

Accumulation of rainfall:Mean daily rainfall with contributionby daily rainfall totals- accumulations > 4 mm/day important

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 14: How low can you go? Retrieval of light precipitation in mid-latitudes

Rainfall occurrence and accumulation

Occurrence plots:

Radar ~ 50-60% rain occurrence

ECMWF model ~ 75-85% occurrence

NOGAPS model ~ 65-75% occurrence

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 15: How low can you go? Retrieval of light precipitation in mid-latitudes

Radar ~ 40-50% rain occurrence

Satellite algorithms:

3B42RT ~15-25% occurrence(MPA-RT)

NRLgeo ~25-35% occurrence

CMORPH ~25-60% occurrence

Page 16: How low can you go? Retrieval of light precipitation in mid-latitudes

Radar ~ 40-50% rain occurrence

Satellite algorithms:

3B40-RT ~10-25% occurrence

NRLpmw ~10-25% occurrence

CMORPH ~15-40% occurrence

Page 17: How low can you go? Retrieval of light precipitation in mid-latitudes

Low rainfall identification

How well to algorithms identify light rainfall?

- Difficult to assess since the IPWG products are daily estimates, not instantaneous…

However, the radar data, available every 15 minutes, can be used:

Radar sensitivity to light rain rates is reduced by setting rain rates below a certain threshold to zero – thus matching the algorithms insensitivity.

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 18: How low can you go? Retrieval of light precipitation in mid-latitudes

Effect of removing light rain rates

<0.0 → 0.0 <0.5 → 0.0 <1.0 → 0.0 <2.0 → 0.0

Con

vect

ive

Fro

ntal

Page 19: How low can you go? Retrieval of light precipitation in mid-latitudes

Rain area

20

40

60

80

100

120

140

160

180

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

radar rain threshold

% o

f o

bs

. are

a

amsu

nrlgeo

3b42rt

cmorph

AMSU

NRLGEO

3B42RT

CMORPH

(Radar adjusted to remove low rain rates)

060

914

| 06

090

2

In terms of rain area: algorithms underestimate the area, e.g. CMORPH has a threshold of about 0.5 mm/hr; AMSU ~1.5 mm/hr for stratiform and > 2 mm/hr for convective.

Co

nve

ctiv

e |

Fro

nta

l

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 20: How low can you go? Retrieval of light precipitation in mid-latitudes

Rain totals ratio

0

50

100

150

200

250

300

350

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Radar rain threshold

% o

f o

bs

erv

ed

ra

infa

ll amsu

nrlgeo

3b42rt

cmorph

AMSU

NRLGEO

3B42RT

CMORPH

(Radar adjusted to remove low rain rates)

060

914

| 06

090

2

In terms of rain amount the problem is less acute: some over-estimate rain amounts (while underestimating the rain areas); NRLgeo has a threshold ~1.2 and 0.6 mm/hr.

Co

nve

ctiv

e |

Fro

nta

l

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 21: How low can you go? Retrieval of light precipitation in mid-latitudes

Algorithm performance

Algorithms:• generally underestimate the rain area (0.6-0.9)• are equally split on rain amount (0.6-1.4)

Consequently algorithms overestimate the conditional rainfall

In addition, different rain situations create different problems:

CMORPH ~ same for rain area for both frontal and convective rainfall, but very different for total rainfall

AMSU which gets the rain totals about right, fails to get the rain areas right.

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 22: How low can you go? Retrieval of light precipitation in mid-latitudes

Conclusions

• Most products underestimate the accumulation of rainfall over the European region

• All products underestimate the occurrence of rainfall

- Statistics often confuse the issue (CC/RMSE/bias): more light rain tends to produce poorer statistics

• Instrument noise can be problematic (e.g. AMSR)• Surface screening - potential problems with 'false alarms'

over cold/snow surfaces – plus RFI issues

• Satellite observations are capable of retrieving light rainfall – at least statistically

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006

Page 23: How low can you go? Retrieval of light precipitation in mid-latitudes
Page 24: How low can you go? Retrieval of light precipitation in mid-latitudes

RFI over Europe

Blue = 6.925 GHz Green = 10.65 GHz Red = 18.7 GHz

3rd IPWG workshop, Melbourne, Australia. 23-28 October 2006